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Special Method Names

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Diving in

FIXME

Basics

__init__ - covered in iterators.html
__repr__ - covered in ordereddict.py
__str__ - covered in fractions.py
__bytes__ (*)
__format__

Rich Comparisons

__lt__ - covered in fractions.py
__le__ - covered in fractions.py
__eq__ - covered in ordereddict.py, fractions.py
__ne__
__gt__ - covered in fractions.py
__ge__ - covered in fractions.py
__bool__ - covered in fractions.py

(__cmp__ is gone)

Custom Attributes

__getattr__
__getattribute__
__setattr__
__delattr__
__dir__

Classes That Act Like Functions

__call__

Classes That Act Like Sequences

FIXME sequence intro

Classes That Act Like Dictionaries

__getitem__
__setitem__ - covered in ordereddict.py
__delitem__ - covered in ordereddict.py
__missing__ (*)

Classes That Act Like Iterators

__iter__ (*) - covered in iterators.html
__next__ (*) - covered in iterators.html
__reversed__ - covered in ordereddict.py

Classes That Act Like Numbers

Using the appropriate special methods, you can define your own classes that act like numbers. That is, you can add them, subtract them, and perform other mathematical operations on them. This is how fractions are implemented — the Fraction class implements these special methods, then you can do things like this:

>>> from fractions import Fraction
>>> x = Fraction(1, 3)
>>> x / 3
Fraction(1, 9)

Here is the comprehensive list of special methods you need to implement a number-like class.

That’s all well and good if x is an instance of a class that implements those methods. But what if it doesn’t implement one of them? Or worse, what if it implements it, but it can’t handle certain kinds of arguments? For example:

>>> from fractions import Fraction
>>> x = Fraction(1, 3)
>>> 1 / x
Fraction(3, 1)

This is not a case of taking a Fraction and dividing it by an integer (as in the previous example). That case was straightforward: x / 3 calls x.__truediv__(3), and the __truedive__() method of the Fraction class handles all the math. But integers don’t “know” how to do arithmetic operations with fractions. So why does this example work?

The answer lies in a second set of arithmetic special methods with reflected operands. Given an arithmetic operation that takes two operands (e.g. x / y), there are two ways to go about it:

1. Tell x to divide itself by y, or
2. Tell y to divide itself into x

The set of special methods above take the first approach: given x / y, they provide a way for x to say “I know how to divide myself by y.” The following set of special methods tackle the second approach: they provide a way for y to say “I know how to be the denominator and divide myself into x.”

But wait! There’s more! If you’re doing “in-place” operations, like x /= 3, there are even more special methods you can define.

Note: for the most part, the in-place operation methods are not required. If you don’t define an in-place method for a particular operation, Python will try the methods. For example, to execute the expression x /= y, Python will:

1. Try calling x.__itruediv__(y). If this method is defined and returns a value other than NotImplemented, we’re done.
2. Try calling x.__truediv__(y). If this method is defined and returns a value other than NotImplemented, the old value of x is discarded and replaced with the return value, just as if you had done x = x / y instead.
3. Try calling y.__rtruediv__(y). If this method is defined and returns a value other than NotImplemented, the old value of x is discarded and replaced with the return value.

So you only need to define in-place methods like the __itruediv__() method if you want to do some special optimization for in-place operands. Otherwise Python will essentially reformulate the in-place operand to use a regular operand + a variable assignment.

There are also a few “unary” mathematical operations you can perform on number-like objects by themselves.

Support For Pickling

see http://docs.python.org/3.0/library/pickle.html:

__copy__ (*) - covered in fractions.py
__deepcopy__ (*) - covered in fractions.py
__getnewargs__ (*)
__getinitargs__ (*)
__getstate__ (*)
__setstate__ (*)
__reduce__ (*) - covered in ordereddict.py, fractions.py
__reduce_ex__ (*)

Classes That Can Be Used in a with Block

__enter__ see http://docs.python.org/3.0/library/stdtypes.html#typecontextmanager
__exit__

relevant excerpt from io.py:

    def __enter__(self) -> "IOBase":  # That's a forward reference
        """Context management protocol.  Returns self."""
        self._checkClosed()
        return self

    def __exit__(self, *args) -> None:
        """Context management protocol.  Calls close()"""
        self.close()

relevant excerpt from http://www.python.org/doc/3.0/reference/datamodel.html#with-statement-context-managers

object.__enter__(self)
  Enter the runtime context related to this object. The with statement will bind this method’s return value to the target(s) specified in the as clause of the statement, if any.
object.__exit__(self, exc_type, exc_value, traceback)
  Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be None.

If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.

Note that __exit__() methods should not reraise the passed-in exception; this is the caller’s responsibility.

Really Esoteric Stuff

__new__ - covered in fractions.py
__del__
__slots__
__hash__ - covered in fractions.py
__get__
__set__
__delete__
__subclasshook__ (*) see http://docs.python.org/3.0/library/abc.html
__instancecheck__ (*) see http://www.ibm.com/developerworks/linux/library/l-python3-2/
__subclasscheck__ (*)

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© 2001–9 Mark Pilgrim