I'm exploring python.
If have now lerned from the answers, that when I use *,
a = [1, 2, 3]
b = a*2
c = 2*a
a.__mul__() is called to compute b and a.__rmul__() to compute c (and this is more expensive, since the integer is tried to be called first :))
So, in general:
How can I see which function is called for some shortcut operator like *, and, or +=, ...
Python language reference 3.3.7: Emulating numeric types says this of the __rXXX__ magic methods:
These methods are called to implement the binary arithmetic operations (
+,-,*,/,//,%,divmod(),pow(),**,<<,>>,&,^,|) with reflected (swapped) operands. These functions are only called if the left operand does not support the corresponding operation and the operands are of different types. For instance, to evaluate the expressionx - y, whereyis an instance of a class that has an__rsub__()method,y.__rsub__(x)is called ifx.__sub__(y)returnsNotImplemented.
Thus for your case: at first for a * b Python tries to call a.__mul__(b). If this method does not exist or returns NotImplemented, Python calls b.__rmul__(a). If that returns a value that is not NotImplemented, that shall be the result of multiplication.
That way only one of a or b needs to support multiplication by the other.
In this specific case of multiplying a list (or any sequence) by an integer, it is slightly more efficient to do [1, 2, 3] * 2 than 2 * [1, 2, 3]; both give the same result, but [1, 2, 3] * 2 calls list.__mul__ first:
>>> [1,2,3].__mul__(2)
[1, 2, 3, 1, 2, 3]
The latter calls int.__mul__ first, which, given a list, returns NotImplemented:
>>> (2).__mul__([1,2,3])
NotImplemented
Then Python proceeds to calling [1, 2, 3].__rmul__(2):
>>> [1,2,3].__rmul__(2)
[1, 2, 3, 1, 2, 3]
And of course multiplication is not generally commutative; think about matrix multiplication
__mul__ method and add a print statement inside to see when it is executed, it would be much slower because I overwrite in python? But I don't see the connection. I heard of profiling, which should list all function calls? Would this not list these because they are built-in? Thank you for your patience :)__mul__ is called when the object itself is on the left, and __rmul__ is for when it's on the right. For these particular objects, 2*a has the same result as a*2 because this kind of multiplication is commutative, so it comes out to the same thing, so both docstrings are right.
__mul__ is called. For the object on the right, __rmul__ is called. For cases where it's not commutative, the two methods may be defined differently.