Python 是否在每个步骤中计算列表理解条件？

Question

In a list comprehension with a condition that has a function call in it, does Python (specifically CPython 3.9.4) call the function each time, or does it calculate the value once and then uses it?

For example if you have:

list_1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
list_2 = [x for x in list_1 if x > np.average(list_1)]

Will Python actually calculate the np.average(list_1) len(list_1) times? So would it be more optimized to write

list_1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
np_avg = np.average(list_1)
list_2 = [x for x in list_1 if x > np_avg]

instead? Or does Python already "know" to just calculate the average beforehand?

Answer 1

Python has to call the function each time. It cannot optimize that part, because successive calls of the function might return different results (for example because of side effects). There is no easy way for Python's compiler to be sure that this can't happen.

Therefore, if you (the programmer) know that the result will always be the same – like in this case – it is probably advisable to calculate the result of the function in advance and use it inside the list comprehension.

Answer 2

Assuming standard CPython - Short answer: Yes. Your second snippet is more efficient.

A function call in the filter part of a list comprehension will be called for each element.

We can test this quite easily with a trivial example:

def f(value):
    """ Allow even values only """ 
    print('function called')
    return value % 2 == 0

mylist = [x for x in range(5) if f(x)]
# 'function called' will be printed 5 times

The above is somewhat equivalent to doing:

mylist = []
for x in range(5):
    if f(x):
        mylist.append(x)

Since you're comparing against the same average each time, you can indeed just calculate it beforehand and use the same value as you did in your second code snippet.