Python modules are objects and have several useful attributes. You can use this to easily test your modules as you write them.
- Here's an example that uses the if __name__ trick.
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Some quick observations before you get to the good stuff. First, parentheses are not required around the if expression. Second, the if statement ends with a colon, and is followed by indented code.
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- | Like C, Python uses == for comparison and = for assignment. Unlike C, Python does not support in-line assignment, so there's no chance of accidentally assigning the value you thought you were comparing.
-So why is this particular if statement a trick? Modules are objects, and all modules have a built-in attribute __name__. A module's __name__ depends on how you're using the module. If you import the module, then __name__ is the module's filename, without a directory path or file extension. But you can also run the module directly as a standalone
-program, in which case __name__ will be a special default value, __main__.
- >>> import odbchelper
->>> odbchelper.__name__
-'odbchelper' Knowing this, you can design a test suite for your module within the module itself by putting it in this if statement. When you run the module directly, __name__ is __main__, so the test suite executes. When you import the module, __name__ is something else, so the test suite is ignored. This makes it easier to develop and debug new modules before integrating
-them into a larger program.
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- | On MacPython, there is an additional step to make the if __name__ trick work. Pop up the module's options menu by clicking the black triangle in the upper-right corner of the window, and
- make sure Run as __main__ is checked.
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- Further Reading on Importing Modules
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- 3.4. Declaring variables
- Now that you know something about dictionaries, tuples, and lists (oh my!), let's get back to the sample program from Chapter 2, odbchelper.py.
- Python has local and global variables like most other languages, but it has no explicit variable declarations. Variables spring
- into existence by being assigned a value, and they are automatically destroyed when they go out of scope.
- Example 3.17. Defining the myParams Variable
-if __name__ == "__main__":
- myParams = {"server":"mpilgrim", \
- "database":"master", \
- "uid":"sa", \
- "pwd":"secret" \
- }
Notice the indentation. An if statement is a code block and needs to be indented just like a function.
- Also notice that the variable assignment is one command split over several lines, with a backslash (“\”) serving as a line-continuation marker.
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- | When a command is split among several lines with the line-continuation marker (“\”), the continued lines can be indented in any manner; Python's normally stringent indentation rules do not apply. If your Python IDE auto-indents the continued line, you should probably accept its default unless you have a burning reason not to.
-Strictly speaking, expressions in parentheses, straight brackets, or curly braces (like defining a dictionary) can be split into multiple lines with or without the line continuation character (“\”). I like to include the backslash even when it's not required because I think it makes the code easier to read, but that's
-a matter of style.
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-[unbound variable exception example was here]
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- 3.4.2. Assigning Multiple Values at Once
-One of the cooler programming shortcuts in Python is using sequences to assign multiple values at once.
- Example 3.19. Assigning multiple values at once>>> v = ('a', 'b', 'e')
->>> (x, y, z) = v ①
->>> x
-'a'
->>> y
-'b'
->>> z
-'e'
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-- v is a tuple of three elements, and
(x, y, z) is a tuple of three variables. Assigning one to the other assigns each of the values of v to each of the variables, in order.
-This has all sorts of uses. I often want to assign names to a range of values. In C, you would use enum and manually list each constant and its associated value, which seems especially tedious when the values are consecutive.
- In Python, you can use the built-in range function with multi-variable assignment to quickly assign consecutive values.
- Example 3.20. Assigning Consecutive Values>>> range(7) ①
-[0, 1, 2, 3, 4, 5, 6]
->>> (MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY) = range(7) ②
->>> MONDAY ③
-0
->>> TUESDAY
-1
->>> SUNDAY
-6
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-- The built-in
range function returns a list of integers. In its simplest form, it takes an upper limit and returns a zero-based list counting
- up to but not including the upper limit. (If you like, you can pass other parameters to specify a base other than 0 and a step other than 1. You can print range.__doc__ for details.)
- - MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, and SUNDAY are the variables you're defining. (This example came from the
calendar module, a fun little module that prints calendars, like the UNIX program cal. The calendar module defines integer constants for days of the week.)
- - Now each variable has its value: MONDAY is
0, TUESDAY is 1, and so forth.
-You can also use multi-variable assignment to build functions that return multiple values, simply by returning a tuple of
- all the values. The caller can treat it as a tuple, or assign the values to individual variables. Many standard Python libraries do this, including the os module, which you'll discuss in Chapter 6.
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- Further Reading on Variables
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Example 6.12. Introducing sys.modules>>> import sys ①
>>> print '\n'.join(sys.modules.keys()) ②
win32api
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