Lecture#4

Iterators how to create

class CustomIterable(object):

def __init__(self, *args, **kwargs):

#initialize your object here

def __iter__(self):

return self

def next(self):

#return some value or raise StopIteration

CustomIterable can now be used in loops

Magic methods

what about this __ methods?

Reserved methods with special meaning

for el in lst:

uses

LstClass.__iter__(lst)

obj = SomeClass(args)

uses

SomeClass.__new__(SomeClass, args) and

SomeClass.__init__(obj, args)

Magic methods

dirty secrets of constructors

__init__ is not a constructor (it’s initializer) although it’s usually to treat it as such

except when subclassing immutables

__new__ is an actual constructor

you need it when subclassing immutables

Magic methods be polite

__str__ is used to emulate str() built-in function

__unicode__ is used to emulate unicode() built-in function

__repr__ is used to get a string representation of object (also `obj` and repr() built-in)

Magic methods be a container

__len__ is used to emulate len()

len(cont) -> cont.__len__()

or ContClass.__len__(cont)

__getitem__ is used to emulate obj[key]

obj[key] -> obj.__getitem__(key)

or ObjClass.__getitem__(obj, key)

Magic methods be a container

__setitem__ is used to emulate obj[key]

=value

obj[key] = value ->

obj.__setitem__(key, value)

or ObjClass.__getitem__(obj, key, value)

Magic methods be a container

__delitem__(self, key) : del obj[key]

__iter__(self) : iter(obj) or in loops

__reversed__(self) : reversed(obj)

__contains__(self, item) : item in obj

Magic methods be smart

__setattr__(self, name, value)

Intercept assignment to attribute name

Be aware of infinite recursion!

__getattribute__(self, name)

__getattr__(self, name)

Intercept access to attribute name

Magic methods be callable

__call__(self, ...)

Intercept call on instance

class SomeClass(object):

def __call__(self, *args, **kwargs): do_something

s = SomeClass() s(1,2,3)

Magic methods all together

Magic methods in Python allow you to create classes, which look and behave like built-in types

See this blog post for general overview

http://www.rafekettler.com/magicmethods.html