Error referencing local variables in dict comprehensions inside class definitions

[AlexMashianov]

BPO	34517
Nosy	@mdickinson, @serhiy-storchaka
Superseder	bpo-3692: improper scope in list comprehension, when used in class declaration

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-08-27.09:30:55.284>
created_at = <Date 2018-08-27.08:42:26.368>
labels = ['interpreter-core', 'type-bug']
title = 'Error referencing local variables in dict comprehensions inside class definitions'
updated_at = <Date 2018-08-27.10:01:47.440>
user = 'https://bugs.python.org/AlexMashianov'

bugs.python.org fields:

activity = <Date 2018-08-27.10:01:47.440>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2018-08-27.09:30:55.284>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2018-08-27.08:42:26.368>
creator = 'Alex Mashianov'
dependencies = []
files = []
hgrepos = []
issue_num = 34517
keywords = []
message_count = 4.0
messages = ['324156', '324158', '324159', '324162']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'serhiy.storchaka', 'Alex Mashianov']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '3692'
type = 'behavior'
url = 'https://bugs.python.org/issue34517'
versions = ['Python 3.6']

[AlexMashianov]

This code:
class A:
a = 1
b = {str(x): x for x in range(5) if x != a}

Produces following error:
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 3, in A
  File "<input>", line 3, in <dictcomp>
NameError: name 'a' is not defined

Which i think it shouldn't produce. Issue occurs only in dict comprehensions inside class definitions referencing class attributes(which are in local scope during definition).

[AlexMashianov]

Update: any class definition comprehension referencing local variables inside filtering block produces this error.

[serhiy-storchaka]

This is a duplicate of bpo-3692.

[mdickinson]

[I wrote this before Serhiy posted his reply; sending it anyway, in case the doc links are useful.]

This is documented, by-design behaviour. Essentially name resolution skips class scopes.

See https://docs.python.org/3/reference/executionmodel.html#resolution-of-names, and particularly the example at the end of that section, which is very similar to your example.

See also: https://docs.python.org/3/reference/expressions.html#displays-for-lists-sets-and-dictionaries, and in particular this part:

However, aside from the iterable expression in the leftmost for clause,
the comprehension is executed in a separate implicitly nested scope.
This ensures that names assigned to in the target list don’t “leak” into
the enclosing scope.

That "leftmost for clause" bit explains why, in the following example, the first comprehension is evaluated successfully, but the second raises a NameError.

>>> class A:
...     a = 3
...     b = [x+y for x in range(a) for y in range(3)]  # okay
...     c = [x+y for x in range(3) for y in range(a)]  # raises