Advanced Nested List Comprehension Syntax

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you need to use some parentheses:

((x for x in range(10) if x%2==i) for i in range(2))

This didn’t make sense to me, so I
thought it best to try something
simpler first. So I went back to lists
and tried:

[>>> [x for x in range(10) if x%2==i for i in range(2)]
[1, 1, 3, 3, 5, 5, 7, 7, 9, 9]

That worked because a previous list comprehension leaks the i variable to the enclosing scope, and become the i for the current one. Try starting a fresh python interpreter, and that would fail due to NameError. The counter’s leaking behavior has been removed in Python 3.

EDIT:

The equivalent for loop for: 

(x for x in range(10) if x%2==i for i in range(2))

would be:

l = []
for x in range(10):
    if x%2 == i:
        for i in range(2):
            l.append(x)

which also gives a name error.

EDIT2:

the parenthesed version:

((x for x in range(10) if x%2==i) for i in range(2))

is equivalent to:

li = []
for i in range(2):
    lx = []
    for x in range(10):
        if x%2==i:
            lx.append(x)
    li.append(lx)

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