List Comprehension output different than nested for loops?

Question

I really am at a loss as to why this list comprehension is generating different output to what I thought the equivalent with nested for loops. Can anyone shed any insight?

The list comprehension:

[(n ** 5, m ** 3) for n in range(3) for m in range(5) if m % 2 == 0]

Results in the following:

[(0, 0), (0, 8), (0, 64), (1, 0), (1, 8), (1, 64), (32, 0), (32, 8), (32, 64)]

However, what I believe is the equivalent nested For loop:

L = []
for n in range(3):
    for m in range(5):
        if m % 2 == 0:
            n = n**5
            m = m**3
            vals = (n,m)
            L.append(vals)
L

Results in the following:

[(0, 0),
 (0, 8),
 (0, 64),
 (1, 0),
 (1, 8),
 (1, 64),
 (32, 0),
 (33554432, 8),
 (42535295865117307932921825928971026432, 64)]

Why is there a difference in the output between the two?

Answer 1

Your nested loop is not equivalent to the list comprehension, because you have altered n in the inner loop:

n = n**5

For any value other than 0 or 1 this increases n exponentially. So for the last iteration of the outer loop n = 2 is set, but then you assign 2 ** 5 = 32 to n , and then 32 ** 5 = 33554432 .

Use a different name:

L = []
for n in range(3):
    for m in range(5):
        if m % 2 == 0:
            n_power_5 = n**5
            m_power_3 = m**3
            vals = (n_power_5, m_power_3)
            L.append(vals)

or just don't use intermediate variables at all:

for n in range(3):
    for m in range(5):
        if m % 2 == 0:
            L.append((n**5, m**3))