locally imported modules are unaccessible in lambdas in pdb

[YuriKanivetsky]

BPO	32806
Nosy	@nitishch

^{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 = None
created_at = <Date 2018-02-09.09:47:46.402>
labels = ['type-bug']
title = 'locally imported modules are unaccessible in lambdas in pdb'
updated_at = <Date 2018-02-11.20:46:57.026>
user = 'https://bugs.python.org/YuriKanivetsky'

bugs.python.org fields:

activity = <Date 2018-02-11.20:46:57.026>
actor = 'nitishch'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-02-09.09:47:46.402>
creator = 'Yuri Kanivetsky'
dependencies = []
files = []
hgrepos = []
issue_num = 32806
keywords = []
message_count = 3.0
messages = ['311871', '312010', '312012']
nosy_count = 2.0
nosy_names = ['nitishch', 'Yuri Kanivetsky']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32806'
versions = ['Python 3.6']

[YuriKanivetsky]

Consider the following script:

 # import pdb; pdb.set_trace()
 # import re
 def f():
 import re
 print((lambda: re.findall('a', 'aaa'))())
 import pdb; pdb.set_trace()
 print('test')
 f()

When you run it and try to evaluate (lambda: re.findall('a', 'aaa'))(), you get:

['a', 'a', 'a']
> /home/yuri/_/1.py(7)f()
-> print('test')
(Pdb) (lambda: re.findall('a', 'aaa'))()
*** NameError: name 're' is not defined
(Pdb) import re
(Pdb) (lambda: re.findall('a', 'aaa'))()
*** NameError: name 're' is not defined
(Pdb) 

With the commented out breakpoint it works:

> /home/yuri/_/a.py(3)<module>()
-> def f():
(Pdb) import re
(Pdb) (lambda: re.findall('a', 'aaa'))()
['a', 'a', 'a']
(Pdb) 

Also it works with uncommented global import and second breakpoint:

['a', 'a', 'a']
> /srv/http/sl/makosh/a.py(7)f()
-> print('test')
(Pdb) (lambda: re.findall('a', 'aaa'))()
['a', 'a', 'a']
(Pdb) 

From what I can see the issue occurs when there's no re in globals argument here: https://github.com/python/cpython/blob/v3.6.4/Lib/pdb.py#L376

I've run into it when trying to grep some object's attribute names, like:

!list(filter(lambda x: re.search('class', x), dir(__name__)))

[nitishch]

This can be traced back to the following issue:

>>> c = compile("(lambda: re.findall('a', 'aaa'))()", "<stdin>", "single")
>>> import re as rea
>>> exec(c, None, {'re': rea})
NameError: name 're' is not defined.

Seeing disassembly of the compiled code, it used LOAD_GLOBAL to get re. This seems to be the problem.

[nitishch]

Sorry. I didn't finish my last message. Is the behaviour described in that message a bug? If yes, that would explain the original behaviour. If no, why not?

[gaogaotiantian]

Fixed by #111094