Question

Say I have

a = [1,2,3]
b = [foo(x) for x in a]

Now b is something like [ < function at 0x00F91540>, …], how do I get python to evaluate b into it’s actual values?

–EDIT–

Sorry for the confusion, my frustration is that Python doesn’t give me the actual values in b, hence when I try to do something like:

b[0] + 1

it gives me the below error:

TypeError: unsupported operand type(s) for +: 'function' and 'int'

Answer 1

Define your function outside the comprehension:

def foo(x):
    return x + 1

a = [1,2,3]
b = [foo(x) for x in a]

Now, b == [2,3,4]. Or, as others have pointed out, just use the function body in the list comprehension:

[x + 1 for x in a]

…although this assumes that you can reproduce the function body in this way (which you won’t be able to do for more complicated functions).

Note that the result of a lambda expression is the same as typing the name of a function, eg. lambda x: x+1 and foo represent the same thing: a function which needs a parameter given to it and returns a result. For the sake of clarification, this is how you’d “use” a lambda in a list comprehension:

[(lambda y: y+1)(x) for x in a]

…but seriously, don’t do this 😉