Python: understanding lambda expressions with two variables

Question

While looking up concise ways to calculate the product of a list, I was a little confused by this response. Here's the code in question:

reduce(lambda x,y:x*y,[3,4,5])

Can someone explain why this works? I'm familiar with using lambda expressions involving one variable (e.g. when using a custom compare function for sorting) but am confused why and how this works.

Could you be more specific? There's nothing magical about lambda: this is the same as def f(x,y): return x*y and then reduce(f, [3,4,5]). So is it really the behaviour of reduce which is confusing you? — DSM
– DSM, Commented May 3, 2014 at 23:36
@DSM: I was confused about f(x,y) works to compute the product (why we need y). I think @unutbu has explained this well with an example from the documentation. — rookie
– rookie, Commented May 3, 2014 at 23:41
You should replace lambdas with simple for-loop in most cases, they are much friendlier to read and there's no performance disadvantage. You don't need reduce() either - artima.com/weblogs/viewpost.jsp?thread=98196 — CodeManX
– CodeManX, Commented May 3, 2014 at 23:46
possible duplicate of What's the Python function like sum() but for multiplication? — CodeManX
– CodeManX, Commented May 3, 2014 at 23:55

unutbu · Accepted Answer · 2014-05-03 23:37:27Z

11

lambda x,y: x*y is an anonymous function otherwise equivalent to

def foo(x, y):
    return x*y

To understand its use in reduce(lambda ...), consider the example from the docs:

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5).

Similarly, reduce(lambda x,y:x*y,[3,4,5]) calculates ((3*4)*5).

answered May 3, 2014 at 23:37

unutbu

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