Dive Into Python 3 will cover Python 3 and its differences from Python 2. Compared to the original Dive Into Python, it will be about 50% revised and 50% new material. I will publish drafts online as I go. The final book will be published on paper by Apress. The book will remain online under the CC-BY-3.0 license. +
There is a changelog, a feed, and discussion on Reddit. During development, the only way to download it is to clone the Mercurial repository: +
you@localhost:~$ hg clone http://hg.diveintopython3.org/ dip3
Below is the draft table of contents. It is not finalized. Only a few chapters have been written so far. The rest is just stubs and random notes to myself. -
Yes, that is PapayaWhip. All hail PapayaWhip.
chardet to Python 3chardet: a mini-FAQUTF-n with a BOMwindows-1252UTF-n with a BOMwindows-12522to32to3 can'tFalse is invalid syntaxconstantsbytes' object to str implicitlyFalse is invalid syntaxconstantsbytes' object to str implicitlyTentative because most of these have not been ported to Python 3 yet.
2to3has_key() dictionary methodhttp packageurllib packagedbm packagexmlrpc packagehttp packageurllib packagedbm packagexmlrpc packagefilter() global functionmap() global function2to3++❝ Life is pleasant. Death is peaceful. It’s the transition that’s troublesome. ❞
— Isaac Asimov (attributed) +
print statement
+unicode() global function
+long data type
+has_key() dictionary method
+http
+urllib
+dbm
+xmlrpc
+filter() global function
+map() global function
+reduce() global function (3.1+)
+apply() global function
+intern() global function
+exec statement
+execfile statement (3.1+)
+repr literals (backticks)
+try...except statement
+raise statement
+throw statement
+xrange() global function
+raw_input() and input() global functions
+func_* function attributes
+xreadlines() I/O method
+lambda functions with multiple parameters
+next() iterator method
+__nonzero__ special class attribute
+sys.maxint
+callable() global function
+zip() global function
+StandardError() exception
+types module constants
+isinstance() global function (3.1+)
+basestring datatype
+itertools module
+sys.exc_type, sys.exc_value, sys.exc_traceback
+os.getcwdu() function
+set() literals (explicit)
+buffer() global function (explicit)
+Python 3 comes with a utility script called 2to3, which takes your actual Python 2 source code as input and auto-converts as much as it can to Python 3. Case study: porting chardet to Python 3 describes how to run the 2to3 script, then shows some things it can't fix automatically. This appendix documents what it can fix automatically.
+
print statementIn Python 2, print was a statement. Whatever you wanted to print simply followed the print keyword. In Python 3, print() is a function — whatever you want to print is passed to print() like any other function.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +print |
+print() |
| ② | +print 1 |
+print(1) |
| ③ | +print 1, 2 |
+print(1, 2) |
| ④ | +print 1, 2, |
+print(1, 2, end=' ') |
| ⑤ | +print >>sys.stderr, 1, 2, 3 |
+print(1, 2, 3, file=sys.stderr) |
print() without any arguments.
+print() with one argument
+print() with two arguments.
+print statement with a comma, it would print the values separated by spaces, then print a trailing space, then stop without printing a carriage return. In Python 3, the way to do this is to pass end=' ' as a keyword argument to the print() function. The end argument defaults to '\n' (a carriage return), so overriding it will suppress the carriage return after printing the other arguments.
+sys.stderr — by using the >>pipe_name syntax. In Python 3, the way to do this is to pass the pipe in the file keyword argument. The file argument defaults to sys.stdout (standard out), so overriding it will output to a different pipe instead.
+Python 2 had two string types: Unicode strings and non-Unicode strings. Python 3 has one string type: Unicode strings. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +u"PapayaWhip" |
+"PapayaWhip" |
| ① | +ur"PapayaWhip\foo" |
+r"PapayaWhip\foo" |
unicode() global functionPython 2 had two global functions to coerce objects into strings: unicode() to coerce them into Unicode strings, and str() to coerce them into non-Unicode strings. Python 3 has only one string type, Unicode strings, so the str() function is all you need. (The unicode() function no longer exists.)
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | unicode(anything) |
+str(anything) |
+
long data typePython 2 had separate int and long types for non-floating-point numbers. An int could not be any larger than sys.maxint, which varied by platform. Longs were defined by appending an L to the end of the number, and they could be, well, longer than ints. In Python 3, there is only one integer type, called int, which mostly behaves like the long type in Python 2. Further reading: PEP 237: Unifying Long Integers and Integers.
+
Since there are no longer two types, there is no need for special syntax to distinguish them. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +x = 1000000000000L |
+x = 1000000000000 |
| ② | +x = 0xFFFFFFFFFFFFL |
+x = 0xFFFFFFFFFFFF |
| ③ | +long(x) |
+int(x) |
| ④ | +type(x) is long |
+type(x) is int |
| ⑤ | +isinstance(x, long) |
+isinstance(x, int) |
long() function no longer exists, since longs don't exist. To coerce a variable to an integer, use the int() function.
+int, not long.
+isinstance() function to check data types; again, use int, not long, to check for integers.
+Python 2 supported <> as a synonym for !=, the not-equals comparison operator. Python 3 supports the != operator, but not <>.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +if x <> y: |
+if x != y: |
| ② | +if x <> y <> z: |
+if x != y != z: |
has_key() dictionary methodIn Python 2, dictionaries had a has_key() method to test whether the dictionary had a certain key. In Python 3, this method no longer exists. Instead, you need to use the in operator.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +a_dictionary.has_key("PapayaWhip") |
+"PapayaWhip" in a_dictionary |
| ② | +a_dictionary.has_key(x) or a_dictionary.has_key(y) |
+x in a_dictionary or y in a_dictionary |
| ③ | +a_dictionary.has_key(x or y) |
+(x or y) in a_dictionary |
| ④ | +a_dictionary.has_key(x + y) |
+(x + y) in a_dictionary |
| ⑤ | +x + a_dictionary.has_key(y) |
+x + (y in a_dictionary) |
or operator takes precedence over the in operator, so there is no need for parentheses here.
+or takes precedence over in.
+in operator takes precedence over the + operator, so this form needs parentheses too.
+In Python 2, many dictionary methods returned lists. The most frequently used methods were keys(), items(), and values(). In Python 3, all of these methods return dynamic views. In some contexts, this is not a problem. If the method's return value is immediately passed to another function that iterates through the entire sequence, it makes no difference whether the actual type is a list or a view. In other contexts, it matters a great deal. If you were expecting a complete list with individually addressable elements, your code will choke, because views do not support indexing.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +a_dictionary.keys() |
+list(a_dictionary.keys()) |
| ② | +a_dictionary.items() |
+list(a_dictionary.items()) |
| ③ | +a_dictionary.iterkeys() |
+iter(a_dictionary.keys()) |
| ④ | +[i for i in a_dictionary.iterkeys()] |
+[i for i in a_dictionary.keys()] |
| ⑤ | +min(a_dictionary.keys()) |
+no change |
2to3 errs on the side of safety, converting the return value from keys() to a static list with the list() function. This will always work, but it will be less efficient than using a view. You should examine the converted code to see if a list is absolutely necessary, or if a view would do.
+items() method. 2to3 will do the same thing with the values() method.
+iterkeys() method anymore. Use keys(), and if necessary, convert the view to an iterator with the iter() function.
+2to3 recognizes when the iterkeys() method is used inside a list comprehension, and converts it to the keys() method (without wrapping it in an extra call to iter()). This works because views are iterable.
+2to3 recognizes that the keys() method is immediately passed to a function which iterates through an entire sequence, so there is no need to convert the return value to a list first. The min() function will happily iterate through the view instead. This applies to min(), max(), sum(), list(), tuple(), set(), sorted(), any(), and all().
+Several modules in the Python Standard Library have been renamed. Several other modules which are related to each other have been combined or reorganized to make their association more logical. +
http packageIn Python 3, several related HTTP modules have been combined into a single package, http.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +import httplib |
+import http.client |
| ② | +import Cookie |
+import http.cookies |
| ③ | +import cookielib |
+import http.cookiejar |
| ④ | + |
+import http.server |
http.client module implements a low-level library that can request HTTP resources and interpret HTTP responses.
+http.cookies module provides a Pythonic interface to browser cookies that are sent in a Set-Cookie: HTTP header.
+http.cookiejar module manipulates the actual files on disk that popular web browsers use to store cookies.
+http.server module provides a basic HTTP server.
+urllib packagePython 2 had a rat's nest of overlapping modules to parse, encode, and fetch URLs. In Python 3, these have all been refactored and combined in a single package, urllib.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +import urllib |
+import urllib.request, urllib.parse, urllib.error |
| ② | +import urllib2 |
+import urllib.request, urllib.error |
| ③ | +import urlparse |
+import urllib.parse |
| ④ | +import robotparser |
+import urllib.robotparser |
| ⑤ | + |
+ |
| ⑥ | + |
+ |
urllib module in Python 2 had a variety of functions, including urlopen() for fetching data and splittype(), splithost(), and splituser() for splitting a URL into its constituent parts. These functions have been reorganized more logically within the new urllib package. 2to3 will also change all calls to these functions so they use the new naming scheme.
+urllib2 module in Python 2 has been folded into into the urllib package in Python 3. All your urllib2 favorites — the build_opener() method, Request objects, and HTTPBasicAuthHandler and friends — are still available.
+urllib.parse module in Python 3 contains all the parsing functions from the old urlparse module in Python 2.
+urllib.robotparser module parses robots.txt files.
+FancyURLopener class, which handles HTTP redirects and other status codes, is still available in the new urllib.request module. The urlencode function has moved to urllib.parse.
+Request object is still available in urllib.request, but constants like HTTPError have been moved to urllib.error.
+dbm packageAll the various DBM clones are now in a single package, dbm. If you need a specific variant like GNU DBM, you can import the appropriate module within the dbm package.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | import dbm |
+import dbm.ndbm |
| + | import gdbm |
+import dbm.gnu |
| + | import dbhash |
+import dbm.bsd |
| + | import dumbdbm |
+import dbm.dumb |
| + | |
+import dbm |
+
xmlrpc packageXML-RPC is a lightweight method of performing remote RPC calls over HTTP. The XML-RPC client library and several XML-RPC server implementations are now combined in a single package, xmlrpc.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | import xmlrpclib |
+import xmlrpc.client |
| + | |
+import xmlrpc.server |
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | + |
+import io |
| ② | + |
+import pickle |
| ③ | +import __builtin__ |
+import builtins |
| ④ | +import copy_reg |
+import copyreg |
| ⑤ | +import Queue |
+import queue |
| ⑥ | +import SocketServer |
+import socketserver |
| ⑦ | +import ConfigParser |
+import configparser |
| ⑧ | +import repr |
+import reprlib |
| ⑨ | +import commands |
+import subprocess |
cStringIO as StringIO, and if that failed, to import StringIO instead. Do not do this in Python 3; the io module does it for you. It will find the fastest implementation available and use it automatically.
+pickle module does it for you.
+builtins module contains the global functions, classes, and constants used throughout the Python language. Redefining a function in the builtins module will redefine the global function everywhere. That is exactly as powerful and scary as it sounds.
+copyreg module adds pickle support for custom types defined in C.
+queue module implements a multi-producer, multi-consumer queue.
+socketserver module provides generic base classes for implementing different kinds of socket servers.
+configparser module parses INI-style configuration files.
+reprlib module reimplements the built-in repr() function, but with limits on how many values are represented.
+subprocess module allows you to spawn processes, connect to their pipes, and obtain their return codes.
+A package is a group of related modules that function as a single entity. In Python 2, when modules within a package need to reference each other, you use import foo or from foo import Bar. The Python 2 interpreter first searches within the current package to find foo.py, and then moves on to the other directories in the Python search path (sys.path). Python 3 works a bit differently. Instead of searching the current package, it goes directly to the Python search path. If you want one module within a package to import another module in the same package, you need to explicitly provide the relative path between the two modules.
+
Suppose you had this package, with multiple files in the same directory: +
chardet/ +| ++--__init__.py +| ++--constants.py +| ++--mbcharsetprober.py +| ++--universaldetector.py+
Now suppose that universaldetector.py needs to import the entire constants.py file and one class from mbcharsetprober.py. How do you do it?
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +import constants |
+from . import constants |
| ② | +from mbcharsetprober import MultiByteCharSetProber |
+from .mbcharsetprober import MultiByteCharsetProber |
from . import syntax. The period is actually a relative path from this file (universaldetector.py) to the file you want to import (constants.py). In this case, they are in the same directory, thus the single period. You can also import from the parent directory (from .. import anothermodule) or a subdirectory.
+mbcharsetprober.py is in the same directory as universaldetector.py, so the path is a single period. You can also import form the parent directory (from ..anothermodule import AnotherClass) or a subdirectory.
+filter() global functionIn Python 2, the filter() function returned a list, the result of filtering a sequence through a function that returned True or False for each item in the sequence. In Python 3, the filter() function returns an interator, not a list.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +filter(a_function, a_sequence) |
+list(filter(a_function, a_sequence)) |
| ② | +list(filter(a_function, a_sequence)) |
+no change |
| ③ | +filter(None, a_sequence) |
+[i for i in a_sequence if i] |
| ④ | +for i in filter(None, a_sequence): |
+no change |
| ⑤ | +[i for i in filter(a_function, a_sequence)] |
+no change |
2to3 will wrap a call to filter() with a call to list(), which simply iterates through its argument and returns a real list.
+filter() is already wrapped in list(), 2to3 will do nothing, since the fact that filter() is returning an iterator is irrelevant.
+filter(None, ...), 2to3 will transform the call into a semantically equivalent list comprehension.
+for loops, which iterate through the entire sequence anyway, no changes are necessary.
+filter() returns an iterator as if it returns a list.
+map() global functionIn much the same way as filter(), the map() function now returns an iterator. (In Python 2, it returned a list.)
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +map(a_function, 'PapayaWhip') |
+list(map(a_function, 'PapayaWhip')) |
| ② | +map(None, 'PapayaWhip') |
+list('PapayaWhip') |
| ③ | +map(lambda x: x+1, range(42)) |
+[x+1 for x in range(42)] |
| ④ | +for i in map(a_function, a_sequence): |
+no change |
| ⑤ | +[i for i in map(a_function, a_sequence)] |
+no change |
filter(), in the most basic case, 2to3 will wrap a call to map() with a call to list().
+map(None, ...), the identity function, 2to3 will convert it to an equivalent call to list().
+map() is a lambda function, 2to3 will convert it to an equivalent list comprehension.
+for loops, which iterate through the entire sequence anyway, no changes are necessary.
+map() returns an iterator as if it returns a list.
+reduce() global function (3.1+)In Python 3, the reduce() function has been removed from the global namespace and placed in the functools module.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | reduce(a, b, c) |
+ |
++☞ +
The version of
2to3that shipped with Python 3.0 would not fix thereduce()function automatically. The fix first appeared in the2to3script that shipped with Python 3.1. +
apply() global functionPython 2 had a global function called apply(), which took a function f and a list [a, b, c] and returned f(a, b, c). In Python 3, the apply() function no longer exists. Instead, there is a new function calling syntax that allows you to pass a list and have Python apply the list as the function's arguments.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +apply(a_function, a_list_of_args) |
+a_function(*a_list_of_args) |
| ② | +apply(a_function, a_list_of_args, a_dictionary_of_named_args) |
+a_function(*a_list_of_args, **a_dictionary_of_named_args) |
| ③ | +apply(a_function, a_list_of_args + z) |
+a_function(*a_list_of_args + z) |
| ④ | +apply(aModule.a_function, a_list_of_args) |
+aModule.a_function(*a_list_of_args) |
[a, b, c]) by prepending the list with an asterisk (*). This is exactly equivalent to the old apply() function in Python 2.
+apply() function could actually take three parameters: a function, a list of arguments, and a dictionary of named arguments. In Python 3, you can accomplish the same thing by prepending the list of arguments with an asterisk (*) and the dictionary of named arguments with two asterisks (**).
++ operator, used here for list concatenation, takes precedence over the * operator, so there is no need for extra parentheses around a_list_of_args + z.
+2to3 script is smart enough to convert complex apply() calls, including calling functions within imported modules.
+intern() global functionIn Python 2, you could call the intern() function on a string to intern it as a performance optimization. In Python 3, the intern() function has been moved to the sys module.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | intern(aString) |
+sys.intern(aString) |
+
exec statementJust as the print statement became a function in Python 3, so too has the exec statement. The exec() function takes a string which contains arbitrary Python code and executes it as if it were just another statement or expression.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +exec codeString |
+exec(codeString) |
| ② | +exec codeString in a_global_namespace |
+exec(codeString, a_global_namespace) |
| ③ | +exec codeString in a_global_namespace, a_local_namespace |
+exec(codeString, a_global_namespace, a_local_namespace) |
2to3 script simply encloses the code-as-a-string in parentheses, since exec() is now a function instead of a statement.
+exec statement could take a namespace, a private environment of globals in which the code-as-a-string would be executed. Python 3 can also do this; just pass the namespace as the second argument to the exec() function.
+exec statement could also take a local namespace (like the variables defined within a function). In Python 3, the exec() function can do that too.
+execfile statement (3.1+)Like the old exec statement, the old execfile statement will execute strings as if they were Python code. Where exec took a string, execfile took a filename. In Python 3, the execfile statement has been eliminated. If you really need to take a file of Python code and execute it (but you're not willing to simply import it), you can accomplish the same thing by opening the file, reading its contents, calling the global compile() function to force the Python interpreter to compile the code, and then call the new exec() function.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | execfile("a_filename") |
+exec(compile(open("a_filename").read(), "a_filename", "exec")) |
++☞ +
The version of
2to3that shipped with Python 3.0 would not fix theexecfilestatement automatically. The fix first appeared in the2to3script that shipped with Python 3.1. +
repr literals (backticks)In Python 2, there was a special syntax of wrapping any object in backticks (like `x`) to get a representation of the object. In Python 3, this capability still exists, but you can no longer use backticks to get it. Instead, use the global repr() function.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +`x` |
+repr(x) |
| ② | +`"PapayaWhip" + `2`` |
+repr("PapayaWhip" + repr(2)) |
repr() function works on everything.
+2to3 tool is smart enough to convert this into nested calls to repr().
+try...except statementThe syntax for catching exceptions has changed slightly between Python 2 and 3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | + |
+ |
| ② | + |
+ |
| ③ | + |
+no change |
| ④ | + |
+no change |
as.
+as keyword also works for catching multiple types of exceptions at once.
+++☞ +
You should never use a fallback to catch all exceptions when importing modules (or most other times). Doing so will catch things like
KeyboardInterrupt(if the user pressed Ctrl-C to interrupt the program) and can make it more difficult to debug errors. +
raise statementFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +raise MyException, "error message" |
+raise MyException("error message") |
| ② | +raise MyException, "error message", a_traceback |
+raise MyException("error message").with_traceback(a_traceback) |
| ③ | +raise "error message" |
+unsupported |
throw statementFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +aGenerator.throw(MyException) |
+no change |
| ② | +aGenerator.throw(MyException, "error message") |
+aGenerator.throw(MyException("error message")) |
| ③ | +aGenerator.throw("error message") |
+unsupported |
xrange() global functionIn Python 2, there were two ways to get a range of numbers: range(), which returned a list, and xrange(), which returned an iterator. In Python 3, range() returns an iterator, and xrange() doesn't exist.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +xrange(10) |
+range(10) |
| ② | +a_list = range(10) |
+a_list = list(range(10)) |
| ③ | +[i for i in xrange(10)] |
+[i for i in range(10)] |
| ④ | +for i in range(10): |
+no change |
| ⑤ | +sum(range(10)) |
+no change |
2to3 script will simply convert xrange() to range().
+range(), the 2to3 script does not know whether you needed a list, or whether an iterator would do. It errs on the side of caution and coerces the return value into a list by calling the list() function.
+xrange() function was inside a list comprehension, there is no need to coerce the result to a list, since the list comprehension will work just fine with an iterator.
+for loop will work just fine with an iterator, so there is no need to change anything here.
+sum() function will also work with an iterator, so 2to3 makes no changes here either. Like dictionary methods that return views instead of lists, this applies to min(), max(), sum(), list(), tuple(), set(), sorted(), any(), and all().
+raw_input() and input() global functionsFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +raw_input() |
+input() |
| ② | +raw_input("prompt") |
+input("prompt") |
| ③ | +input() |
+eval(input()) |
| ④ | +input("prompt") |
+eval(input("prompt")) |
func_* function attributesFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +a_function.func_closure |
+a_function.__closure__ |
| ② | +a_function.func_doc |
+a_function.__doc__ |
| ③ | +a_function.func_name |
+a_function.__name__ |
| ④ | +a_function.func_defaults |
+a_function.__defaults__ |
| ⑤ | +a_function.func_code |
+a_function.__code__ |
| ⑥ | +a_function.func_globals |
+a_function.__globals__ |
| ⑦ | +a_function.func_dict |
+a_function.__dict__ |
xreadlines() I/O methodIn Python 2, file objects had an xreadlines() method which returned an iterator that would read the file one line at a time. This was useful in for loops, among other places. In fact, it was so useful, later versions of Python 2 [FIXME exact version?] added the capability to file objects themselves.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +for line in a_file.xreadlines(): |
+for line in a_file: |
| ② | +for line in a_file.xreadlines(5): |
+no change |
xreadlines() with no arguments, 2to3 will convert it to just the file object. In Python 3, this will accomplish the same thing: read the file one line at a time and execute the body of the for loop.
+xreadlines() with an argument (the number of lines to read at a time), keep doing that. It still works in Python 3, and 2to3 will not change it.
+lambda functions with multiple parametersFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +lambda (x,): x + f(x) |
+lambda x1: x1[0] + f(x1[1]) |
| ② | +lambda (x, y): x + f(y) |
+lambda x_y: x_y[0] + f(x_y[1]) |
| ③ | +lambda (x, (y, z)): x + y + z |
+lambda x_y_z: x_y_z[0] + x_y_z[1][0] + x_y_z[1][1] |
FIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +aClassInstance.aClassMethod.im_func |
+aClassInstance.aClassMethod.__func__ |
| ② | +aClassInstance.aClassMethod.im_self |
+aClassInstance.aClassMethod.__self__ |
| ③ | +aClassInstance.aClassMethod.im_class |
+aClassInstance.aClassMethod.self.__class__ |
next() iterator methodIn Python 2, iterators had a next() method which returned the next item in the sequence. That's still true in Python 3, but there is now also a global next() function that takes an iterator as an argument.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +anIterator.next() |
+next(anIterator) |
| ② | +a_function_that_returns_an_iterator().next() |
+next(a_function_that_returns_an_iterator()) |
| ③ | + |
+ |
| ④ | + |
+no change |
| ⑤ | + |
+ |
next() method, you now pass the iterator itself to the global next() function.
+next() function. (The 2to3 script is smart enough to convert this properly.)
+__next__() special method.
+next() that takes one or more arguments, 2to3 will not touch it. This class can not be used as an iterator, because its next() method takes arguments.
+next() function. In this case, you need to call the iterator's special __next()__ method to get the next item in the sequence. (Alternatively, you could also refactor the code so the local variable wasn't named next, but 2to3 will not do that for you automatically.)
+__nonzero__ special class attributeFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | + |
+ |
| ② | + |
+no change |
The syntax for defining base 8 (octal) numbers has changed slightly between Python 2 and Python 3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | x = 0755 |
+x = 0o755 |
+
sys.maxintDue to the integration of the long and int types, the sys.maxint constant is no longer accurate. Because the value may still be useful in determining platform-specific capabilities, it has been retained but renamed as sys.maxsize.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +from sys import maxint |
+from sys import maxsize |
| ② | +a_function(sys.maxint) |
+a_function(sys.maxsize) |
maxint becomes maxsize.
+sys.maxint becomes sys.maxsize.
+callable() global functionIn Python 2, you could check whether an object was callable (like a function) with the global callable() function. In Python 3, this global function has been eliminated. To check whether an object is callable, check for the existence of the __call__() special method.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | callable(anything) |
+hasattr(anything, "__call__") |
+
zip() global functionFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +zip(a, b, c) |
+list(zip(a, b, c)) |
| ② | +d.join(zip(a, b, c)) |
+no change |
StandardError() exceptionFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +x = StandardError() |
+x = Exception() |
| ② | +x = StandardError(a, b, c) |
+x = Exception(a, b, c) |
types module constantsThe types module contains a variety of constants to help you determine the type of an object. In Python 2, it contained constants for all primitive types like dict and int. In Python 3, these constants have been eliminated; just use the primitive type name instead.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | types.StringType |
+bytes |
| + | types.DictType |
+dict |
| + | types.IntType |
+int |
| + | types.LongType |
+int |
| + | types.ListType |
+list |
| + | types.NoneType |
+type(None) |
+
isinstance() global function (3.1+)The isinstance() function checks whether an object is an instance of a particular class or type. In Python 2, you could pass a tuple of types, and isinstance() would return True if the object was any of those types. In Python 3, you can still do this, but passing the same type twice is deprecated.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | isinstance(x, (int, float, int)) |
+isinstance(x, (int, float)) |
++☞ +
The version of
2to3that shipped with Python 3.0 would not fix these cases ofisinstance()automatically. The fix first appeared in the2to3script that shipped with Python 3.1. +
basestring datatypePython 2 had two string types: Unicode and non-Unicode. But there was also another type, basestring. It was an abstract type, a superclass for both the str and unicode types. It couldn't be called or instantiated directly, but you could pass it to the global isinstance() function to check whether an object was either a Unicode or non-Unicode string. In Python 3, there is only one string type, so basestring has no reason to exist.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | isinstance(x, basestring) |
+isinstance(x, str) |
+
itertools moduleFIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +itertools.izip(a, b) |
+zip(a, b) |
| ② | +itertools.imap(a, b) |
+map(a, b) |
| ③ | +itertools.ifilter(a, b) |
+filter(a, b) |
| ④ | +itertools.ifilterfalse(a, b) |
+filterfalse(a, b) |
| ⑤ | +from itertools import imap, izip, foo |
+from itertools import foo |
sys.exc_type, sys.exc_value, sys.exc_tracebackPython 2 had three variables in the sys module that you could access while an exception was being handled: sys.exc_type, sys.exc_value, sys.exc_traceback. (Actually, these date all the way back to Python 1.) Ever since Python 1.5, these variables have been deprecated in favor of sys.exc_info, which is a tuple that contains all three values. In Python 3, these individual variables have finally gone away; you must use sys.exc_info.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +sys.exc_type |
+sys.exc_info()[0] |
| ② | +sys.exc_value |
+sys.exc_info()[1] |
| ③ | +sys.exc_traceback |
+sys.exc_info()[2] |
+
FIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | [i for i in 1, 2] |
+[i for i in (1, 2)] |
+
os.getcwdu() functionPython 2 had a function named os.getcwd(), which returned the current working directory as a (non-Unicode) string. Because modern file systems can handle directory names in any character encoding, Python 2.3 introduced os.getcwdu(). The os.getcwdu() function returned the current working directory as a Unicode string. In Python 3, there is only one string type, so os.getcwd() is all you need.
+
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| + | os.getcwdu() |
+os.getcwd() |
+
FIXME intro +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | + |
+ |
| ② | + |
+ |
set() literals (explicit)FIXME intro +
++☞ +
The
2to3script will not fixset()literals by default. To enable this fix, specify -f set_literal on the command line when you call2to3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +set([1, 2, 3]) |
+{1, 2, 3} |
| ② | +set((1, 2, 3)) |
+{1, 2, 3} |
| ③ | +set([i for i in a_sequence]) |
+{i for i in a_sequence} |
buffer() global function (explicit)FIXME intro +
++☞ +
The
2to3script will not fix thebuffer()function by default. To enable this fix, specify -f buffer on the command line when you call2to3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +x = buffer(y) |
+x = memoryview(y) |
FIXME intro +
++☞ +
The
2to3script will not fix whitespace around commas by default. To enable this fix, specify -f wscomma on the command line when you call2to3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | +a ,b |
+a, b |
| ② | +{a :b} |
+{a: b} |
FIXME intro +
++☞ +
The
2to3script will not fix common idioms by default. To enable this fix, specify -f idioms on the command line when you call2to3. +
| Notes | +Python 2 | +Python 3 | +
|---|---|---|
| ① | + |
+ |
| ② | +type(x) == T |
+isinstance(x, T) |
| ③ | +type(x) is T |
+isinstance(x, T) |
| ④ | + |
+ |
FIXME: once the rest of the book is written, this appendix should contain copious links back to any chapter or section that touches on these features. +
© 2001-4, 2009 ℳark Pilgrim, CC-BY-3.0 - -
-2to3-- -❝ Life is pleasant. Death is peaceful. It's the transition that's troublesome. ❞
— Isaac Asimov (attributed) -
print statement
-unicode() global function
-long data type
-has_key() dictionary method
-http package
- urllib package
- dbm package
- xmlrpc package
- filter() global function
-map() global function
-reduce() global function (3.1+)
-apply() global function
-intern() global function
-exec statement
-execfile statement (3.1+)
-repr literals (backticks)
-try...except statement
-raise statement
-throw statement
-xrange() global function
-raw_input() and input() global functions
-func_* function attributes
-xreadlines() I/O method
-lambda functions with multiple parameters
-next() iterator method
-__nonzero__ special class attribute
-sys.maxint
-callable() global function
-zip() global function
-StandardError() exception
-types module constants
-isinstance() global function (3.1+)
-basestring datatype
-itertools module
-sys.exc_type, sys.exc_value, sys.exc_traceback
-os.getcwdu() function
-set() literals (explicit)
-buffer() global function (explicit)
-Python 3 comes with a utility script called 2to3, which takes your actual Python 2 source code as input and auto-converts as much as it can to Python 3. Case study: porting chardet to Python 3 describes how to run the 2to3 script, then shows some things it can't fix automatically. This appendix documents what it can fix automatically.
-
-
print statementIn Python 2, print was a statement -- whatever you wanted to print simply followed the print keyword. In Python 3, print() is a function -- whatever you want to print is passed to print() like any other function.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -print |
-print() |
-
| ② | -print 1 |
-print(1) |
-
| ③ | -print 1, 2 |
-print(1, 2) |
-
| ④ | -print 1, 2, |
-print(1, 2, end=' ') |
-
| ⑤ | -print >>sys.stderr, 1, 2, 3 |
-print(1, 2, 3, file=sys.stderr) |
-
print() without any arguments.
-print() with one argument
-print() with two arguments.
-print statement with a comma, it would print the values separated by spaces, then print a trailing space, then stop without printing a carriage return. In Python 3, the way to do this is to pass end=' ' as a keyword argument to the print() function. The end argument defaults to '\n' (a carriage return), so overriding it will suppress the carriage return after printing the other arguments.
-sys.stderr -- by using the >>pipe_name syntax. In Python 3, the way to do this is to pass the pipe in the file keyword argument. The file argument defaults to sys.stdout (standard out), so overriding it will output to a different pipe instead.
-Python 2 had two string types: Unicode strings and non-Unicode strings. Python 3 has one string type: Unicode strings. - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -u"PapayaWhip" |
-"PapayaWhip" |
-
| ① | -ur"PapayaWhip\foo" |
-r"PapayaWhip\foo" |
-
unicode() global functionFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -unicode(anything) |
-str(anything) |
-
long data typePython 2 had separate int and long types for non-floating-point numbers. An int could not be any larger than sys.maxint, which varied by platform. Longs were defined by appending an L to the end of the number, and they could be, well, longer than ints. In Python 3, there is only one integer type, called int, which mostly behaves like the long type in Python 2.
-
-
Since there are no longer two types, there is no need for special syntax to distinguish them. - -
Further reading: PEP 237: Unifying Long Integers and Integers. - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -x = 1000000000000L |
-x = 1000000000000 |
-
| ② | -x = 0xFFFFFFFFFFFFL |
-x = 0xFFFFFFFFFFFF |
-
| ③ | -long(x) |
-int(x) |
-
| ④ | -type(x) is long |
-type(x) is int |
-
| ⑤ | -isinstance(x, long) |
-isinstance(x, int) |
-
long() function no longer exists, since longs don't exist. To coerce a variable to an integer, use the int() function.
-int, not long.
-isinstance() function to check data types; again, use int, not long, to check for integers.
-Python 2 supported <> as a synonym for !=, the not-equals comparison operator. Python 3 supports the != operator, but not <>.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -if x <> y: |
-if x != y: |
-
| ② | -if x <> y <> z: |
-if x != y != z: |
-
has_key() dictionary methodIn Python 2, dictionaries had a has_key() method to test whether the dictionary had a certain key. In Python 3, this method no longer exists. Instead, you need to use the in operator.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -a_dictionary.has_key("PapayaWhip") |
-"PapayaWhip" in a_dictionary |
-
| ② | -a_dictionary.has_key(x) or a_dictionary.has_key(y) |
-x in a_dictionary or y in a_dictionary |
-
| ③ | -a_dictionary.has_key(x or y) |
-(x or y) in a_dictionary |
-
| ④ | -a_dictionary.has_key(x + y) |
-(x + y) in a_dictionary |
-
| ⑤ | -x + a_dictionary.has_key(y) |
-x + (y in a_dictionary) |
-
or operator takes precedence over the in operator, so there is no need for parentheses here.
-or takes precedence over in.
-in operator takes precedence over the + operator, so this form needs parentheses too.
-In Python 2, many dictionary methods returned lists. The most frequently used methods were keys(), items(), and values(). In Python 3, all of these methods return dynamic views. In some contexts, this is not a problem. If the method's return value is immediately passed to another function that iterates through the entire sequence, it makes no difference whether the actual type is a list or a view. In other contexts, it matters a great deal. If you were expecting a complete list with individually addressable elements, your code will choke, because views do not support indexing.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -a_dictionary.keys() |
-list(a_dictionary.keys()) |
-
| ② | -a_dictionary.items() |
-list(a_dictionary.items()) |
-
| ③ | -a_dictionary.iterkeys() |
-iter(a_dictionary.keys()) |
-
| ④ | -[i for i in a_dictionary.iterkeys()] |
-[i for i in a_dictionary.keys()] |
-
| ⑤ | -min(a_dictionary.keys()) |
-no change | -
2to3 errs on the side of safety, converting the return value from keys() to a static list with the list() function. This will always work, but it will be less efficient than using a view. You should examine the converted code to see if a list is absolutely necessary, or if a view would do.
-items() method. 2to3 will do the same thing with the values() method.
-iterkeys() method anymore. Use keys(), and if necessary, convert the view to an iterator with the iter() function.
-2to3 recognizes when the iterkeys() method is used inside a list comprehension, and converts it to the keys() method (without wrapping it in an extra call to iter()). This works because views are iterable.
-2to3 recognizes that the keys() method is immediately passed to a function which iterates through an entire sequence, so there is no need to convert the return value to a list first. The min() function will happily iterate through the view instead. This applies to min(), max(), sum(), list(), tuple(), set(), sorted(), any(), and all().
-Several modules in the Python Standard Library have been renamed. Several other modules which are related to each other have been combined or reorganized to make their association more logical. - -
http packageIn Python 3, several related HTTP modules have been combined into a single package, http.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -import httplib |
-import http.client |
-
| ② | -import Cookie |
-import http.cookies |
-
| ③ | -import cookielib |
-import http.cookiejar |
-
| ④ | - |
-import http.server |
-
http.client module implements a low-level library that can request HTTP resources and interpret HTTP responses.
-http.cookies module provides a Pythonic interface to "cookies" that are sent in a Set-Cookie: HTTP header.
-http.cookiejar module manipulates the actual files on disk that popular web browsers use to store cookies.
-http.server module provides a basic HTTP server.
-urllib packagePython 2 had a rat's nest of overlapping modules to parse, encode, and fetch URLs. In Python 3, these have all been refactored and combined in a single package, urllib.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -import urllib |
-import urllib.request, urllib.parse, urllib.error |
-
| ② | -import urllib2 |
-import urllib.request, urllib.error |
-
| ③ | -import urlparse |
-import urllib.parse |
-
| ④ | -import robotparser |
-import urllib.robotparser |
-
| ⑤ | - |
- |
-
| ⑥ | - |
- |
-
urllib module in Python 2 had a variety of functions, including urlopen() for fetching data and splittype(), splithost(), and splituser() for splitting a URL into its constituent parts. These functions have been reorganized more logically within the new urllib package. 2to3 will also change all calls to these functions so they use the new naming scheme.
-urllib2 module in Python 2 has been folded into into the urllib package in Python 3. All your urllib2 favorites -- the build_opener() method, Request objects, and HTTPBasicAuthHandler and friends -- are still available.
-urllib.parse module in Python 3 contains all the parsing functions from the old urlparse module in Python 2.
-urllib.robotparser module parses robots.txt files.
-FancyURLopener class, which handles HTTP redirects and other status codes, is still available in the new urllib.request module. The urlencode function has moved to urllib.parse.
-Request object is still available in urllib.request, but constants like HTTPError have been moved to urllib.error.
-dbm packageAll the various DBM clones are now in a single package, dbm. If you need a specific variant like GNU DBM, you can import the appropriate module within the dbm package.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | import dbm |
-import dbm.ndbm |
-
| - | import gdbm |
-import dbm.gnu |
-
| - | import dbhash |
-import dbm.bsd |
-
| - | import dumbdbm |
-import dbm.dumb |
-
| - | |
-import dbm |
-
- -
xmlrpc packageXML-RPC is a lightweight method of performing remote RPC calls over HTTP. The XML-RPC client library and several XML-RPC server implementations are now combined in a single package, xmlrpc.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | import xmlrpclib |
-import xmlrpc.client |
-
| - | |
-import xmlrpc.server |
-
- -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | - |
-import io |
-
| ② | - |
-import pickle |
-
| ③ | -import __builtin__ |
-import builtins |
-
| ④ | -import copy_reg |
-import copyreg |
-
| ⑤ | -import Queue |
-import queue |
-
| ⑥ | -import SocketServer |
-import socketserver |
-
| ⑦ | -import ConfigParser |
-import configparser |
-
| ⑧ | -import repr |
-import reprlib |
-
| ⑨ | -import commands |
-import subprocess |
-
cStringIO as StringIO, and if that failed, to import StringIO instead. Do not do this in Python 3; the io module does it for you. It will find the fastest implementation available and use it automatically.
-pickle module does it for you.
-builtins module contains the "global" functions, classes, and constants used throughout the Python language. Redefining a function in the builtins module will redefine the "global" function everywhere. That is exactly as powerful and scary as it sounds.
-copyreg module adds pickle support for custom types defined in C.
-queue module implements a multi-producer, multi-consumer queue.
-socketserver module provides generic base classes for implementing different kinds of socket servers.
-configparser module parses INI-style configuration files.
-reprlib module reimplements the built-in repr() function, but with limits on how many values are represented.
-subprocess module allows you to spawn processes, connect to their pipes, and obtain their return codes.
-A package is a group of related modules that function as a single entity. In Python 2, when modules within a package need to reference each other, you use import foo or from foo import Bar. The Python 2 interpreter first searches within the current package to find foo.py, and then moves on to the other directories in the Python search path (sys.path). Python 3 works a bit differently. Instead of searching the current package, it goes directly to the Python search path. If you want one module within a package to import another module in the same package, you need to explicitly provide the relative path between the two modules.
-
-
Suppose you had this package, with multiple files in the same directory: - -
chardet/ -| -+--__init__.py -| -+--constants.py -| -+--mbcharsetprober.py -| -+--universaldetector.py- -
Now suppose that universaldetector.py needs to import the entire constants.py file and one class from mbcharsetprober.py. How do you do it?
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -import constants |
-from . import constants |
-
| ② | -from mbcharsetprober import MultiByteCharSetProber |
-from .mbcharsetprober import MultiByteCharsetProber |
-
from . import syntax. The period is actually a relative path from this file (universaldetector.py) to the file you want to import (constants.py). In this case, they are in the same directory, thus the single period. You can also import from the parent directory (from .. import anothermodule) or a subdirectory.
-mbcharsetprober.py is in the same directory as universaldetector.py, so the path is a single period. You can also import form the parent directory (from ..anothermodule import AnotherClass) or a subdirectory.
-filter() global functionIn Python 2, the filter() function returned a list, the result of "filtering" a sequence through a function that returned True or False for each item in the sequence. In Python 3, the filter() function returns an interator, not a list.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -filter(a_function, a_sequence) |
-list(filter(a_function, a_sequence)) |
-
| ② | -list(filter(a_function, a_sequence)) |
-no change | -
| ③ | -filter(None, a_sequence) |
-[i for i in a_sequence if i] |
-
| ④ | -for i in filter(None, a_sequence): |
-no change | -
| ⑤ | -[i for i in filter(a_function, a_sequence)] |
-no change | -
2to3 will wrap a call to filter() with a call to list(), which simply iterates through its argument and returns a real list.
-filter() is already wrapped in list(), 2to3 will do nothing, since the fact that filter() is returning an iterator is irrelevant.
-filter(None, ...), 2to3 will transform the call into a semantically equivalent list comprehension.
-for loops, which iterate through the entire sequence anyway, no changes are necessary.
-filter() returns an iterator as if it returns a list.
-map() global functionIn much the same way as filter(), the map() function now returns an iterator. (In Python 2, it returned a list.)
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -map(a_function, 'PapayaWhip') |
-list(map(a_function, 'PapayaWhip')) |
-
| ② | -map(None, 'PapayaWhip') |
-list('PapayaWhip') |
-
| ③ | -map(lambda x: x+1, range(42)) |
-[x+1 for x in range(42)] |
-
| ④ | -for i in map(a_function, a_sequence): |
-no change | -
| ⑤ | -[i for i in map(a_function, a_sequence)] |
-no change | -
filter(), in the most basic case, 2to3 will wrap a call to map() with a call to list().
-map(None, ...), the identity function, 2to3 will convert it to an equivalent call to list().
-map() is a lambda function, 2to3 will convert it to an equivalent list comprehension.
-for loops, which iterate through the entire sequence anyway, no changes are necessary.
-map() returns an iterator as if it returns a list.
-reduce() global function (3.1+)In Python 3, the reduce() function has been removed from the global namespace and placed in the functools module.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | reduce(a, b, c) |
- |
-
-- -☞ -
The version of
2to3that shipped with Python 3.0 would not fix thereduce()function automatically. The fix first appeared in the2to3script that shipped with Python 3.1. -
apply() global functionPython 2 had a global function called apply(), which took a function f and a list [a, b, c] and returned f(a, b, c). In Python 3, the apply() function no longer exists. Instead, there is a new function calling syntax that allows you to pass a list and have Python apply the list as the function's arguments.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -apply(a_function, a_list_of_args) |
-a_function(*a_list_of_args) |
-
| ② | -apply(a_function, a_list_of_args, a_dictionary_of_named_args) |
-a_function(*a_list_of_args, **a_dictionary_of_named_args) |
-
| ③ | -apply(a_function, a_list_of_args + z) |
-a_function(*a_list_of_args + z) |
-
| ④ | -apply(aModule.a_function, a_list_of_args) |
-aModule.a_function(*a_list_of_args) |
-
[a, b, c]) by prepending the list with an asterisk (*). This is exactly equivalent to the old apply() function in Python 2.
-apply() function could actually take three parameters: a function, a list of arguments, and a dictionary of named arguments. In Python 3, you can accomplish the same thing by prepending the list of arguments with an asterisk (*) and the dictionary of named arguments with two asterisks (**).
-+ operator, used here for list concatenation, takes precedence over the * operator, so there is no need for extra parentheses around a_list_of_args + z.
-2to3 script is smart enough to convert complex apply() calls, including calling functions within imported modules.
-intern() global functionIn Python 2, you could call the intern() function on a string to intern it as a performance optimization. In Python 3, the intern() function has been moved to the sys module.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | intern(aString) |
-sys.intern(aString) |
-
- -
exec statementJust as the print statement became a function in Python 3, so too has the exec statement. The exec() function takes a string which contains arbitrary Python code and executes it as if it were just another statement or expression.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -exec codeString |
-exec(codeString) |
-
| ② | -exec codeString in a_global_namespace |
-exec(codeString, a_global_namespace) |
-
| ③ | -exec codeString in a_global_namespace, a_local_namespace |
-exec(codeString, a_global_namespace, a_local_namespace) |
-
2to3 script simply encloses the code-as-a-string in parentheses, since exec() is now a function instead of a statement.
-exec statement could take a namespace, a private environment of globals in which the code-as-a-string would be executed. Python 3 can also do this; just pass the namespace as the second argument to the exec() function.
-exec statement could also take a local namespace (like the variables defined within a function). In Python 3, the exec() function can do that too.
-execfile statement (3.1+)Like the old exec statement, the old execfile statement will execute strings as if they were Python code. Where exec took a string, execfile took a filename. In Python 3, the execfile statement has been eliminated. If you really need to take a file of Python code and execute it (but you're not willing to simply import it), you can accomplish the same thing by opening the file, reading its contents, calling the global compile() function to force the Python interpreter to compile the code, and then call the new exec() function.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | execfile("a_filename") |
-exec(compile(open("a_filename").read(), "a_filename", "exec")) |
-
-- -☞ -
The version of
2to3that shipped with Python 3.0 would not fix theexecfilestatement automatically. The fix first appeared in the2to3script that shipped with Python 3.1. -
repr literals (backticks)In Python 2, there was a special syntax of wrapping any object in backticks (like `x`) to get a representation of the object. In Python 3, this capability still exists, but you can no longer use backticks to get it. Instead, use the global repr() function.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -`x` |
-repr(x) |
-
| ② | -`"PapayaWhip" + `2`` |
-repr("PapayaWhip" + repr(2)) |
-
repr() function works on everything.
-2to3 tool is smart enough to convert this into nested calls to repr().
-try...except statementThe syntax for catching exceptions has changed slightly between Python 2 and 3. - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | - |
- |
-
| ② | - |
- |
-
| ③ | - |
-no change | -
| ④ | - |
-no change | -
as.
-as keyword also works for catching multiple types of exceptions at once.
--- -☞ -
You should never use a fallback to catch all exceptions when importing modules (or most other times). Doing so will catch things like
KeyboardInterrupt(if the user pressed Ctrl-C to interrupt the program) and can make it more difficult to debug errors. -
raise statementFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -raise MyException, "error message" |
-raise MyException("error message") |
-
| ② | -raise MyException, "error message", a_traceback |
-raise MyException("error message").with_traceback(a_traceback) |
-
| ③ | -raise "error message" |
-unsupported | -
throw statementFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -aGenerator.throw(MyException) |
-no change | -
| ② | -aGenerator.throw(MyException, "error message") |
-aGenerator.throw(MyException("error message")) |
-
| ③ | -aGenerator.throw("error message") |
-unsupported | -
xrange() global functionIn Python 2, there were two ways to get a range of numbers: range(), which returned a list, and xrange(), which returned an iterator. In Python 3, range() returns an iterator, and xrange() doesn't exist.
-
-
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -xrange(10) |
-range(10) |
-
| ② | -a_list = range(10) |
-a_list = list(range(10)) |
-
| ③ | -[i for i in xrange(10)] |
-[i for i in range(10)] |
-
| ④ | -for i in range(10): |
-no change | -
| ⑤ | -sum(range(10)) |
-no change | -
2to3 script will simply convert xrange() to range().
-range(), the 2to3 script does not know whether you needed a list, or whether an iterator would do. It errs on the side of caution and coerces the return value into a list by calling the list() function.
-xrange() function was inside a list comprehension, there is no need to coerce the result to a list, since the list comprehension will work just fine with an iterator.
-for loop will work just fine with an iterator, so there is no need to change anything here.
-sum() function will also work with an iterator, so 2to3 makes no changes here either. Like dictionary methods that return views instead of lists, this applies to min(), max(), sum(), list(), tuple(), set(), sorted(), any(), and all().
-raw_input() and input() global functionsFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -raw_input() |
-input() |
-
| ② | -raw_input("prompt") |
-input("prompt") |
-
| ③ | -input() |
-eval(input()) |
-
| ④ | -input("prompt") |
-eval(input("prompt")) |
-
func_* function attributesFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -a_function.func_closure |
-a_function.__closure__ |
-
| ② | -a_function.func_doc |
-a_function.__doc__ |
-
| ③ | -a_function.func_name |
-a_function.__name__ |
-
| ④ | -a_function.func_defaults |
-a_function.__defaults__ |
-
| ⑤ | -a_function.func_code |
-a_function.__code__ |
-
| ⑥ | -a_function.func_globals |
-a_function.__globals__ |
-
| ⑦ | -a_function.func_dict |
-a_function.__dict__ |
-
xreadlines() I/O methodFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -for line in a_file.xreadlines(): |
-for line in a_file: |
-
| ② | -for line in a_file.xreadlines(5): |
-no change | -
lambda functions with multiple parametersFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -lambda (x,): x + f(x) |
-lambda x1: x1[0] + f(x1[1]) |
-
| ② | -lambda (x, y): x + f(y) |
-lambda x_y: x_y[0] + f(x_y[1]) |
-
| ③ | -lambda (x, (y, z)): x + y + z |
-lambda x_y_z: x_y_z[0] + x_y_z[1][0] + x_y_z[1][1] |
-
FIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -aClassInstance.aClassMethod.im_func |
-aClassInstance.aClassMethod.__func__ |
-
| ② | -aClassInstance.aClassMethod.im_self |
-aClassInstance.aClassMethod.__self__ |
-
| ③ | -aClassInstance.aClassMethod.im_class |
-aClassInstance.aClassMethod.self.__class__ |
-
next() iterator methodFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -anIterator.next() |
-next(anIterator) |
-
| ② | -a_function_that_returns_an_iterator().next() |
-next(a_function_that_returns_an_iterator()) |
-
| ③ | - |
- |
-
| ④ | - |
-no change | -
| ⑤ | - |
- |
-
__nonzero__ special class attributeFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | - |
- |
-
| ② | - |
-no change | -
FIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ② | -x = 0755 |
-x = 0o755 |
-
sys.maxintFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -from sys import maxint |
-from sys import maxsize |
-
| ② | - |
- |
-
callable() global functionFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -callable(anything) |
-hasattr(anything, "__call__") |
-
zip() global functionFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -zip(a, b, c) |
-list(zip(a, b, c)) |
-
| ② | -d.join(zip(a, b, c)) |
-no change | -
StandardError() exceptionFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -x = StandardError() |
-x = Exception() |
-
| ② | -x = StandardError(a, b, c) |
-x = Exception(a, b, c) |
-
types module constantsFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | types.StringType |
-bytes |
-
| - | types.DictType |
-dict |
-
| - | types.IntType |
-int |
-
| - | types.LongType |
-int |
-
| - | types.ListType |
-list |
-
| - | types.NoneType |
-type(None) |
-
- -
isinstance() global function (3.1+)FIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -isinstance(x, (int, float, int)) |
-isinstance(x, (int, float)) |
-
-- -☞ -
The version of
2to3that shipped with Python 3.0 would not fix these cases ofisinstance()automatically. The fix first appeared in the2to3script that shipped with Python 3.1. -
basestring datatypeFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | isinstance(x, basestring) |
-isinstance(x, str) |
-
- -
itertools moduleFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -itertools.izip(a, b) |
-zip(a, b) |
-
| ② | -itertools.imap(a, b) |
-map(a, b) |
-
| ③ | -itertools.ifilter(a, b) |
-filter(a, b) |
-
| ④ | -itertools.ifilterfalse(a, b) |
-filterfalse(a, b) |
-
| ⑤ | -from itertools import imap, izip, foo |
-from itertools import foo |
-
sys.exc_type, sys.exc_value, sys.exc_tracebackFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -sys.exc_type |
-sys.exc_info()[0] |
-
| ② | -sys.exc_value |
-sys.exc_info()[1] |
-
| ③ | -sys.exc_traceback |
-sys.exc_info()[2] |
-
- -
FIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | [i for i in 1, 2] |
-[i for i in (1, 2)] |
-
- -
os.getcwdu() functionFIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| - | os.getcwdu() |
-os.getcwd() |
-
- -
FIXME intro - -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | - |
- |
-
| ② | - |
- |
-
set() literals (explicit)FIXME intro - -
-- -☞ -
The
2to3script will not fixset()literals by default. To enable this fix, specify-f set_literalon the command line when you call2to3. -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -set([1, 2, 3]) |
-{1, 2, 3} |
-
| ② | -set((1, 2, 3)) |
-{1, 2, 3} |
-
| ③ | -set([i for i in a_sequence]) |
-{i for i in a_sequence} |
-
buffer() global function (explicit)FIXME intro - -
-- -☞ -
The
2to3script will not fix thebuffer()function by default. To enable this fix, specify-f bufferon the command line when you call2to3. -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -x = buffer(y) |
-x = memoryview(y) |
-
FIXME intro - -
-- -☞ -
The
2to3script will not fix whitespace around commas by default. To enable this fix, specify-f wscommaon the command line when you call2to3. -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | -a ,b |
-a, b |
-
| ② | -{a :b} |
-{a: b} |
-
FIXME intro - -
-- -☞ -
The
2to3script will not fix common idioms by default. To enable this fix, specify-f idiomson the command line when you call2to3. -
| Notes | -Python 2 | -Python 3 | -
|---|---|---|
| ① | - |
- |
-
| ② | -type(x) == T |
-isinstance(x, T) |
-
| ③ | -type(x) is T |
-isinstance(x, T) |
-
| ④ | - |
- |
-
FIXME: once the rest of the book is written, this appendix should contain copious links back to any chapter or section that touches on these features. - - -