-
-
Notifications
You must be signed in to change notification settings - Fork 30k
New issue
Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.
Already on GitHub? Sign in to your account
Weird behavior with generators with self-referencing output. #57804
Comments
The following self-referencing generator has incorrect output: def ab_combinations():
#'', 'a', 'b', 'aa', 'ab', 'ba', 'bb', 'aaa', ...
def _deferred_output():
yield ""
tees = tee(output)
#This definition works fine: '', 'a', 'b', 'aa', 'ab', 'ba', ...
l = [(item+"a" for item in tees[0]), (item+"b" for item in tees[1])]
#This definition results in: '', 'b', 'b', 'bb', 'bb', 'bb', ...
#l = [(item+label for item in t) for t, label in zip(tees,"ab")]
while True:
for g in l:
yield next(g)
|
This is expected, and is due to the late binding of the "label" variable in the "item+label" expression. Look at the example below: >>> l = [lambda item: item + label for label in "ab"]
>>> f1, f2 = l
>>> print f1(''), f2('')
b b For the lambda function, 'label' is a free variable, whose value is fetched from the global environment at runtime. But at the time the second time is executed, 'label' has only one value, the last one from the for loop, and both functions only see 'b'. Now to fix your code, if 'ab' can be of arbitrary length, I can't find a simpler way without yet another inline function: l = [(lambda lbl:(item + lbl for item in t))(label) for t, label in zip(tees,"ab")] 'lbl' is still a free variable, but now 'lbl' comes from the lambda function, and there is one *different* *function* per iteration of the loop; so they are in fact distinct 'lbl' variables, and the generator expressions will correctly see different labels. |
Oh, I was sure it had to do with binding issues. I just couldn't put my finger on it because the behavior seemed so counterintuitive. |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
Show more details
GitHub fields:
bugs.python.org fields:
The text was updated successfully, but these errors were encountered: