python - Generator Comprehension different output from list comprehension?

Question

I get different output when using a list comprehension versus a generator comprehension. Is this expected behavior or a bug?

Consider the following setup:

all_configs = [
    {'a': 1, 'b':3},
    {'a': 2, 'b':2}
]
unique_keys = ['a','b']

If I then run the following code, I get:

print(list(zip(*( [c[k] for k in unique_keys] for c in all_configs))))
>>> [(1, 2), (3, 2)]
# note the ( vs [
print(list(zip(*( (c[k] for k in unique_keys) for c in all_configs))))
>>> [(2, 2), (2, 2)]

This is on python 3.6.0:

Python 3.6.0 (default, Dec 24 2016, 08:01:42)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin

Answer 1

In a list comprehension, expressions are evaluated eagerly. In a generator expression, they are only looked up as needed.

Thus, as the generator expression iterates over for c in all_configs, it refers to c[k] but only looks up c after the loop is done, so it only uses the latest value for both tuples. By contrast, the list comprehension is evaluated immediately, so it creates a tuple with the first value of c and another tuple with the second value of c.

Consider this small example:

>>> r = range(3)
>>> i = 0
>>> a = [i for _ in r]
>>> b = (i for _ in r)
>>> i = 3
>>> print(*a)
0 0 0
>>> print(*b)
3 3 3

When creating a, the interpreter created that list immediately, looking up the value of i as soon as it was evaluated. When creating b, the interpreter just set up that generator and didn't actually iterate over it and look up the value of i. The print calls told the interpreter to evaluate those objects. a already existed as a full list in memory with the old value of i, but b was evaluated at that point, and when it looked up the value of i, it found the new value.