operators - What is the difference between Python's __add__ and __concat__? -

the list of python's standard operators includes both __add__(a, b) , __concat__(a, b). both of them invoked a + b. question is, difference between them? there scenario 1 used rather other? there reason define both on single object?

here's documentation found methods mentioned in.

edit: adding weirdness documentation:

finally, sequence types should implement addition (meaning concatenation) , multiplication (meaning repetition) defining methods __add__(), __radd__(), __iadd__(), __mul__(), __rmul__() , __imul__() described below; should not define __coerce__() or other numerical operators.

if check source operator module (add, concat), find these definitions functions:

def add(a, b):     "same + b."     return + b  def concat(a, b):     "same + b, , b sequences."     if not hasattr(a, '__getitem__'):         msg = "'%s' object can't concatenated" % type(a).__name__         raise typeerror(msg)     return + b 

so there no difference except concat requires sequence type. both functions use + operator effect depends on types add.

in general, using operator module not useful of time. module used when need pass function performs operation, example functional functions map, filter, or reduce. usually, can use + operator directly.

as underscore functions (__add__ , __concat__), these just aliases:

__add__ = add __concat__ = concat 

but of course not related special methods used overload operators custom types. functions match same name special methods, make them appear similar. note there no special __concat__ method on objects though.

implementing __add__ on custom type affect how operator module functions work, example:

>>> class example:         def __init__ (self, x):             self.x = x         def __repr__ (self):             return 'example({})'.format(self.x)         def __add__ (self, other):             return example(self.x + other.x)  >>> = example(2) >>> b = example(4) >>> operator.add(a, b) example(6) >>> + b example(6) 

as can see, operator.add use implementation of special method example.__add__; reason implementation of operator.add uses + operator (which behavior explicitely defined special __add__ method).