Dunder methods

__slot__

https://towardsdatascience.com/understand-slots-in-python-e3081ef5196d

__doc__

Show the docstring inside triple quotes """...""".

# mymodule.py
"""This is the module docstring."""

def f(x):
    """This is the function docstring."""
    return 2 * x


>>> import mymodule
>>> mymodule.__doc__
'This is the module docstring.'
>>> mymodule.f.__doc__
'This is the function docstring.'

__str__ and __repr__

class Point:

def __init__(self, x, y):

self.x, self.y = x, y

def __repr__(self):

return f’Point(x={self.x}, y={self.y})’

p = Point(3, 4) print(p) Point(x=3, y=4)

class Point:

def __init__(self, x, y):

self.x, self.y = x, y

def __repr__(self):

return f’Point(x={self.x}, y={self.y})’

def __str__(self):

return f’Point(x={self.x}, y={self.y}) from __str__’

p = Point(3, 4) print(p) Point(x=3, y=4) from __str__

Here __str__ override __repr__.

str(p) ‘Point(x=3, y=4) from __str__’

repr(p) ‘Point(x=3, y=4)’

print(eval(repr(p))) Point(x=3, y=4) from __str__

eval and repr are conjugate. repr shows the argument’s type info(quote of str).

Attributs related

__getattribute__

Check lookup chain on this document. All attribute/attr related dunder methods(__getattribute__, __setattr__, __getattr__ and __delattr__) are related to attributes lookup by . convention not [].

__setattr__

>>> # this example uses __setattr__ to dynamically change attribute value to uppercase
>>> class Frob:
...    def __setattr__(self, name, value):
...        self.__dict__[name] = value.upper()
...
>>> f = Frob()
>>> f.bamf = "bamf"
>>> f.bamf
'BAMF'

__getattr__

Gets called when the item is not found via __getattribute__

__delattr__

class foo():

def __delattr__(self, name):

print(name) super().__delattr__(name)

__getitem__

Get called when look up by [] like a dict.

Three dots(Ellipsis)

class foo(list):
  def __getitem__(self, arg):
    if arg == ...:
      print('abc')

>>> f = foo()
>>> f[...]
abc

f = foo() f.xyz = 123

del f.xyz f.xyz AttributeError: ‘foo’ object has no attribute ‘xyz’

__next__ and __iter__

• __iter__ return the iterable object(the whole things, list, tuple or generator).

• __next__ return one element.

An iterable object is an object that implements __iter__, which is expected to return an iterator object. An iterator is an object that implements __next__, which is expected to return the next element of the iterable object that returned it, and raise a StopIteration exception when no more elements are available. In the simplest case the iterable will implement __next__ itself and return self in __iter__.

Example:

import random

class RandomIterable:
    def __iter__(self):
        return self
    def __next__(self):
        if random.choice(["go", "go", "stop"]) == "stop":
            raise StopIteration  # signals "the end"
        return 1

Example:

def __iter__(self):
    yield 5
    yield from some_list

__contains__

__subclasshook__

__format__

__reduce__

__reduce_ex__

__class__

__init_subclass__

Comparison methods

__hash__

Called by built-in function hash() and for object comparison. Usually it defined with __eq__ together.

object.__eq__(self, other) object.__lt__(self, other) object.__le__(self, other) object.__ne__(self, other) object.__gt__(self, other) object.__ge__(self, other)