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:

Answer 1

This is happening because zip(*) call resulted in evaluation of the outer generator and this outer returned two more generators.

(c[k], print(c)) for k in unique_keys)

The evaluation of outer generator moved c to the second dict: {'a': 2, 'b':2}.

Now when we are evaluating these generators individually they look for c somewhere, and as its value is now {'a': 2, 'b':2} you get the output as [(2, 2), (2, 2)].

Demo:

>>> def my_zip(*args):
...     print(args)
...     for arg in args:
...         print (list(arg))
...
... my_zip(*((c[k] for k in unique_keys) for c in all_configs))
...

Output:

# We have two generators now, means it has looped through `all_configs`.
(. at 0x104415c50>, . at 0x10416b1a8>)
[2, 2]
[2, 2]

---

The list-comprehension on the other hand evaluates right away and can fetch the value of current value of c not its last value.

---

How to force it use the correct value of c?

Use a inner function and generator function. The inner function can help us remember c's value using default argument.

>>> def solve():
...     for c in all_configs:
...         def func(c=c):
...             return (c[k] for k in unique_keys)
...         yield func()
...

>>>

>>> list(zip(*solve()))
[(1, 2), (3, 2)]