Question

Read a question on stack overflow sometime back with the following syntax

In [1]: [lambda: x for x in range(5)][0]()
Out[1]: 4
In [2]: [lambda: x for x in range(5)][2]()
Out[2]: 4

But i am having a hard time to understand why exactly the output of this comes as 4,
my understanding is it always gives the last value of the list as output,

In [4]: [lambda: x for x in [1,5,7,3]][0]()
Out[4]: 3

but still not convinced how does this syntax ends up with the last value.

Would be very glad if i can get a proper explanation for this syntax

Answer 1

This isn’t really about either list comprehensions or lambdas. It’s about the scoping rules in Python. Let’s rewrite the list comprehension into an equivalent loop:

funcs = []
for x in range(5):
    def f(): return x
    funcs.append(f)
funcs[0]() # returns 4

Here, we can see that we successively construct functions and store them in a list. When one of these functions is called, all that happens is the value of x is looked up and returned. But x is a variable whose value changes, so the final value of 4 is what is always returned. You could even change the value of x after the loop, e.g.,

x = 32 
funcs[2]() # returns 32

To get the behavior you expected, Python would need to scope the for contents as a block; it doesn’t. Usually, this isn’t a problem, and is easy enough to work around.