Using __mul__

[Thomas Philips]

To compute the product of a list of numbers, I tried entering

reduce(__mul__,[1,2,3])
Traceback (most recent call last):
File "<pyshell#0>", line 1, in -toplevel-
reduce(__mul__,[1,2,3])
NameError: name '__mul__' is not defined

I can get the answer I want by defining a function that returns the
product of two numbers: def product(x,y): return x*y

and then using it: reduce(product,[1,2,3])

6

But why does my first approach not work? __mul__ is a valid method of
type int. Is there an obvious flaw in my logic?

Thomas Philips

[Sam Jervis]

Thomas Philips wrote:

To compute the product of a list of numbers, I tried entering

reduce(__mul__,[1,2,3])

Traceback (most recent call last):
File "<pyshell#0>", line 1, in -toplevel-
reduce(__mul__,[1,2,3])
NameError: name '__mul__' is not defined

I can get the answer I want by defining a function that returns the
product of two numbers:
def product(x,y):
return x*y

and then using it:
reduce(product,[1,2,3])

6

But why does my first approach not work? __mul__ is a valid method of
type int. Is there an obvious flaw in my logic?

Thomas Philips

The answer is in your question. __mul__ is a valid method of type int,
but __mul__ on its own is an undefined name. The following code works:

reduce(int.__mul__, [1, 2, 3])

Sam

[Peter Otten]

Sam Jervis wrote:

Thomas Philips wrote:

To compute the product of a list of numbers, I tried entering

>reduce(__mul__,[1,2,3])
[...]

The answer is in your question. __mul__ is a valid method of type int,
but __mul__ on its own is an undefined name. The following code works:

reduce(int.__mul__, [1, 2, 3])

Or use operator.mul if you cannot guarantee that all numbers are integers:
reduce(int.__mul__, [1.0, 2, 3]) Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: descriptor '__mul__' requires a 'int' object but received a
'float' reduce(operator.mul, [1.0, 2, 3])

6.0

Peter