Special methods

https://docs.python.org/3/reference/datamodel.html

INTRODUCTION

A class can implement certain operations that are invoked by special syntax by defining methods with special names. This is Python’s approach to 'operator overloading', allowing classes to define their own behavior with respect to language operators.

__new__   called to create a new instance of class

__init__   called after the instance has been created (a constructor)

__del__   Called when the instance is about to be destroyed (a finalizer, a destructor).
    Note that 'del x' decrements the reference count for x by one,
    and 'x.__del__()' is only called when x’s reference count reaches zero.

__str__   Called by str() and the built-in functions format()
    and print() to compute the 'informal' string representation of an object.
    The return value must be a string object.

__repr__   called by repr(), must return a string

__format__   called by format() to produce a 'formatted' string representation

__bytes__   called by bytes() to compute a byte-string representation

__hash__   called by hash(), should return an integer
    If x == y then always hash(x) == hash(y).

__cmp__   called by cmp() in Py2, should return -1 or 0 or 1
cmp(1, 4) returns -1
cmp(3, 3) returns 0
cmp(5, 3) returns 1

We can define our cmp() function in Py3:
cmp = lambda x, y: (x > y) - (x < y)

Rich comparison methods.

__lt__   called by x < y
__gt__   called by x > y
__eq__   called by x == y
__le__   called by x <= y
__ge__   called by x >= y
__ne__   called by x != y
assert (x < y) == (y > x)   # may be used by Python
assert (x <= y) == (y >= x)   # may be used by Python
assert (x == y) == not (x != y)   # not always True!

PEP 207
min() and list.sort() operations only use the < operator (less).
max() only use the > operator.
'in' and 'not in' operators and dictionary lookup only use the == operator.
The reflexivity rules are assumed by Python.

__bool__   called by bool() to implement truth value testing, should return False or True

__nonzero__   truth value testing in Py2
__bool__ = __nonzero__   compatibility

Emulating container types.

__len__   called by len(), should return the length of the container;
    if len(x) == 0 and __bool__ is not defined then x is considered False

__getitem__(self, key)   called to implement evaluation of 'X[key]'

__setitem__(self, key, value)   called to implement assignment 'X[key] = value'

__delitem__(self, key)   called to implement deletion 'del X[key]'

__iter__   called when an iterator is required for a container

__reversed__   called by reversed() to implement reverse iteration

__contains__(self, item)   called to implement membership test operators
    'item in X' and 'item not in X'

__copy__   shallow copy, Y = copy.copy(X)

__deepcopy__   deep copy, Y = copy.deepcopy(X, memo)

Emulating numeric types.

__add__   called by x + y
__sub__   called by x - y
__mul__   called by x * y
__mod__   called by x % y (remainder)
__divmod__   called by divmod(x, y), return (a // b, a % b) for integers
__pow__   called by pow(x, y) and x ** y

__floordiv__   called by x // y (quotient)
__truediv__   called by x / y (Py3)
__div__   called by x / y (Py2)
__div__ = __truediv__   compatibility (not always good)

__pos__   called by +x
__neg__   called by -x
__abs__   called by abs(x)
__invert__   called by ~x
__complex__  called by complex(x)
__int__   called by int(x)
__long__   called by long(x) (Py2)
__float__   called by float(x)