Pdb NameError in generator param and list comprehension

[jayanth]

BPO	35871
Nosy	@xdegaye, @tirkarthi

^{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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assignee = None
closed_at = None
created_at = <Date 2019-02-01.00:25:49.653>
labels = ['interpreter-core', '3.7']
title = 'Pdb NameError in generator param and list comprehension'
updated_at = <Date 2019-02-01.05:21:32.902>
user = 'https://bugs.python.org/jayanth'

bugs.python.org fields:

activity = <Date 2019-02-01.05:21:32.902>
actor = 'jayanth'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2019-02-01.00:25:49.653>
creator = 'jayanth'
dependencies = []
files = []
hgrepos = []
issue_num = 35871
keywords = []
message_count = 3.0
messages = ['334639', '334646', '334647']
nosy_count = 3.0
nosy_names = ['xdegaye', 'jayanth', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue35871'
versions = ['Python 3.7']

[jayanth]

I get a NameError for a variable in the generator param of a function or in a list comprehension. See example below. The variable is available to the program, but not to the interactive Pdb shell.

# Test file:

def main(nn=10):
    xx = list(range(nn))
    breakpoint()
    for ii in range(nn):
        num = sum(xx[jj] for jj in range(nn))
    print(f'xx={xx}')

if __name__ == '__main__':
    main()

$ python3
Python 3.7.2 (default, Jan 13 2019, 12:50:15)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>

$ python3 /tmp/test.py
> /tmp/test.py(5)main()
-> for ii in range(nn):
(Pdb) n
> /tmp/test.py(6)main()
-> num = sum(xx[jj] for jj in range(nn))
(Pdb) sum(xx[jj] for jj in range(nn))
*** NameError: name 'xx' is not defined
(Pdb) [xx[jj] for jj in range(nn)]
*** NameError: name 'xx' is not defined
(Pdb) c
xx=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

FWIW python3 is a homebrew installation. I had the same issue with 3.7.0 as well (also homebrew):
Python 3.7.0 (default, Sep 18 2018, 18:47:22)
[Clang 9.1.0 (clang-902.0.39.2)] on darwin

[tirkarthi]

Thanks for the report. This seems related to bpo-21161 and bpo-26072 that contain explanation about scope where exec is called with the code and patches that fix this issue. As a possible workaround in the issues you can use "interact" command to get to a REPL where you can evaluate and exit back to pdb.

[jayanth]

Thanks for the "interact" tip.

FWIW, I see this issue in 2.7.10 as well. Although the list comprehension works.

$ python
Python 2.7.10 (default, Oct  6 2017, 22:29:07)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ^D

$ python /tmp/test2.py
> /tmp/test2.py(5)main()
-> for ii in range(nn):
(Pdb) n
> /tmp/test2.py(6)main()
-> num = sum(xx[jj] for jj in range(nn))
(Pdb) sum(xx[jj] for jj in range(nn))
*** NameError: global name 'xx' is not defined
(Pdb) [xx[jj] for jj in range(nn)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
(Pdb) c
('xx', [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

# test2.py

def main(nn=10):
    xx = list(range(nn))
    import pdb; pdb.set_trace()
    for ii in range(nn):
        num = sum(xx[jj] for jj in range(nn))
    print('xx', xx)

if __name__ == '__main__':
    main()

[gaogaotiantian]

Fix by PEP 709 + #111094