Python List Comprehension Execution Order

List Comprehensions in Python look like the code below

[i**2 for i in range(5) if i%2==0]

These tend to be fast enough that we don’t care about what get’s executed when. However let us look at the more complex calculation

[heavy_async_calculation(i) for i in range(5) if heavy_check(i)]

In this situation, we might want to be sure that the async_calculation is triggered as soon as it is found. We could play it safe with a for loop

for i in range(10) :
	if heavy_check(i):
		heavy_async_calculation(i)

However we can check the order of operations in the list comprehension easily:

def f(i):
    print(f"Check {i}%2")
    return i%2==0
 
[print(i) for i in range(10) if f(i)]

creates the output

Check 0%2
0
Check 1%2
Check 2%2
2
Check 3%2
Check 4%2
4
 
[None, None, None]

Confirming that the execution of the print / heavy_async_calculation is done as soon as the check passes