ConfigParser AttributeError: 'NoneType' object has no attribute 'append'

[mpf82]

Bug report

Checklist

• I am confident this is a bug in CPython, not a bug in a third-party project
• I have searched the CPython issue tracker, and am confident this bug has not been reported before

A clear and concise description of the bug

When trying to parse an INI with an indented key that follows a no-value key, an AttributeError is raised

import configparser
lines = [
  '[SECT]\n',
  'KEY1\n',
  ' KEY2 = VAL2\n', # note the Space before the key!
]
conf = configparser.ConfigParser(
  comment_prefixes ="",
  allow_no_value   =True,
  strict           =False,
  delimiters       =( '=', ),
  interpolation    =None,
)
conf.read_file( lines, "test" )
print( conf.__dict__[ '_sections' ] )

File "C:\Python311\Lib\configparser.py", line 1076, in _read      
    cursect[optname].append(value)
    ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'append'

The reason is that ConfigParser assumes the second key is a continuation line and therefore further assumes that cursect[optname] is initialized in the following check:

            if (cursect is not None and optname and
                cur_indent_level > indent_level):
                cursect[optname].append(value)

Suggested fix: add a check for cursect[optname] is not None:

            if (cursect is not None and optname and
                cursect[optname] is not None and
                cur_indent_level > indent_level):
                cursect[optname].append(value)

With this check added, the print will output:

{'SECT': {'key1': None, 'key2': 'VAL2'}}

If the suggested fix is not acceptable, please consider to add a check and maybe raise a ParsingError, but making an assumption about cursect[optname] and let it raise an AttributeError is just not a good thing, IMHO.

Your environment

• CPython versions tested on: 3.7, 3.9, 3.11.2 (Python 3.11.2 (tags/v3.11.2:878ead1, Feb 7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)] on win32)
• Operating system and architecture: Windows 10 Pro, 64 bit

Linked PRs

• gh-107625: configparser: Raise ParsingError if a key contains empty value #107651

[terryjreedy]

Behavior verified with 3.12.0b4 on Win10. I agree that configparser has a bug. I also think that a continuation line for a no-value option is a user bug. So I would a line to the structure section saying so and raise a ParsingError with a message something like "No-value option {opname} cannot be continued".

Line 1084, cursect[optname] = None executes when optval is not None (line 1079) is false. Since this appears to be the only such setting, I think cursect[optname].append(value) can be replaced with if...else raise....

Adding = to the KEY1 line results in KEY1 initially getting a blank value "" that is continued, resulting in value '\nKEY2 = VAL2'

[Agent-Hellboy]

I have added a PR as you suggested @terryjreedy

[encukou]

Thank you for the fix!

Since this only changes the error type (and message), I'll not backport. Let me know if you think I should.