fix(flit): don't emit null values when importing a flit project

[shuvamk]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

The problem

Two optional keys in [tool.flit.*] make pdm import abort. In both cases the importer
builds a value of None, which tomlkit cannot represent, so the import fails and nothing
is written to pyproject.toml.

1. A modern PEP 621 flit project that sets only exclude in [tool.flit.sdist]

The project being imported — no [tool.flit.metadata] at all, this is the layout
flit_core ≥ 3.2 recommends:

[build-system]
requires = ["flit_core >=3.2,<4"]
build-backend = "flit_core.buildapi"

[project]
name = "mymod"
version = "0.1.0"
description = "A modern flit project"
authors = [{name = "Test Author", email = "t@example.com"}]
requires-python = ">=3.9"
dependencies = ["requests>=2.6"]

[tool.flit.sdist]
exclude = ["doc/*.html"]

check_fingerprint only looks for a tool.flit table, so a bare pdm import auto-detects
this as flit — no -f needed:

=== pdm import (bare, auto-detect) with flit.py from: main ===
[ConvertError]: Unable to convert an object of <class 'NoneType'> to a TOML item
WARNING: Add '-v' to see the detailed traceback
--- resulting [tool.pdm.build] ---
(absent)

=== pdm import (bare, auto-detect) with flit.py from: thispr ===
Changes are written to pyproject.toml.
--- resulting [tool.pdm.build] ---
[tool.pdm.build]
excludes = ["doc/*.html"]

Setting only one of the two keys is valid: flit_core reads them independently, each
defaulting to [] — flit_core/config.py:153-161, flit_core 3.12.0:

loaded_cfg.sdist_include_patterns = _check_glob_patterns(
    dtool['sdist'].get('include', []), 'include'
)
exclude = [
    "**/__pycache__",
    "**.pyc",
] + dtool['sdist'].get('exclude', [])
loaded_cfg.sdist_exclude_patterns = _check_glob_patterns(
    exclude, 'exclude'
)

2. A legacy-layout project with an author name and no email

[tool.flit.metadata]
module = "mymod"
author = "Test Author"

$ pdm import -f flit ../pyproject.toml
[MetaConvertError]:
name: Unable to convert an object of <class 'NoneType'> to a TOML item

Also valid: author-email is in flit_core's metadata_allowed_fields but
metadata_required_fields is only {module, author} — flit_core/config.py:51-54,
flit_core 3.12.0:

metadata_required_fields = {
    'module',
    'author',
}

Same for maintainer without maintainer-email.

input	expected	main @ b3cb02ed
PEP 621 project, [tool.flit.sdist] with only exclude	[tool.pdm.build] excludes = [...]	ConvertError, import aborts, nothing written
PEP 621 project, [tool.flit.sdist] with only include	[tool.pdm.build] includes = [...]	ConvertError, import aborts, nothing written
author without author-email	authors = [{name = "Test Author"}]	MetaConvertError, import aborts
maintainer without maintainer-email	maintainers = [{name = "..."}]	MetaConvertError, import aborts

The cause

Both sites write the optional key unconditionally.

src/pdm/formats/flit.py:38-40:

name = metadata.pop(type_)
email = metadata.pop(f"{type_}-email", None)
return cast(list[str], array_of_inline_tables([{"name": name, "email": email}]))

src/pdm/formats/flit.py:140-142:

self.settings.setdefault("build", {}).update(
    {"excludes": value.get("exclude"), "includes": value.get("include")}
)

When the source omits the key, email — respectively the missing get() — is None.
tomlkit.inline_table().update(...) raises immediately for the author case; the sdist
case survives conversion and blows up when [tool.pdm.build] is written in
import_cmd.do_import.

The fix

Only set the key when the source provides it. This is what the Poetry importer already
does for a missing email: parse_name_email filters out the None groups of the
name/email regex match (src/pdm/formats/poetry.py:116-126).

The excludes-before-includes insertion order is deliberately preserved, so a project
that sets both keys — the common case, and what the flit-demo fixture uses —
generates a byte-identical file. Running pdm import -f flit on
tests/fixtures/projects/flit-demo/pyproject.toml under both source versions:

### main vs this PR ###
no differences
5edda2664b636d03c3f7488ab55939fedf3d0daf37769ae83c40a05114a29ae7  out_main/pyproject.toml
5edda2664b636d03c3f7488ab55939fedf3d0daf37769ae83c40a05114a29ae7  out_thispr/pyproject.toml

Tests

Two new tests in tests/test_formats.py, next to test_convert_flit:

• test_convert_flit_author_and_maintainer_without_email
• test_convert_flit_sdist_with_one_of_include_and_exclude, parametrised over
  include-only and exclude-only

The sdist fixture carries an author-email on purpose, so it fails for the sdist reason
and not the author one — the two halves are proven independently.

I verified all three fail without the fix. Reverting only src/pdm/formats/flit.py to
origin/main and keeping the tests:

    def test_convert_flit_author_and_maintainer_without_email(project, tmp_path):
>       result, _ = flit.convert(project, pyproject_file, None)
>           raise MetaConvertError(errors, data=self._data, settings=self.settings)
E           pdm.formats.base.MetaConvertError:
E           name: Unable to convert an object of <class 'NoneType'> to a TOML item

    def test_convert_flit_sdist_with_one_of_include_and_exclude(project, tmp_path, sdist_table, expected_build):
>       assert settings["build"] == expected_build
E       AssertionError: assert {'excludes': ...es': ['doc/']} == {'includes': ['doc/']}
E         Left contains 1 more item:
E         {'excludes': None}
E         Full diff:
E           {
E         +     'excludes': None,
E               'includes': [
E                   'doc/',
E               ],
E           }

    def test_convert_flit_sdist_with_one_of_include_and_exclude(project, tmp_path, sdist_table, expected_build):
>       assert settings["build"] == expected_build
E       AssertionError: assert {'excludes': ...cludes': None} == {'excludes': ['doc/*.html']}
E         Left contains 1 more item:
E         {'includes': None}
E         Full diff:
E           {
E               'excludes': [
E                   'doc/*.html',
E               ],
E         +     'includes': None,
E           }

3 failed, 21 deselected in 0.93s

Restoring the file: 4 passed, 20 deselected in 0.74s (the three new ones plus the
pre-existing test_convert_flit).

Beyond the unit tests I ran the real CLI against both inputs in a scratch project; the
transcripts above are that run.

What I ran

command	main @ b3cb02ed	this PR
pytest -n auto -m "not integration and not network" -q	1292 passed, 1 skipped	1295 passed, 1 skipped
prek run --all-files	ruff / ruff format / codespell / mypy all Passed	all Passed

The one skip is tests/test_utils.py:171 (Windows-only) on both. I did not run the
integration/network marked tests or the tox matrix.

---

Disclosure: I used an AI coding assistant (Claude Code) to find this and to help write
the fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.28%. Comparing base (b3cb02e) to head (717856f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3832      +/-   ##
==========================================
+ Coverage   88.16%   88.28%   +0.12%     
==========================================
  Files         121      121              
  Lines       13221    13227       +6     
  Branches     2246     2249       +3     
==========================================
+ Hits        11656    11678      +22     
+ Misses        982      975       -7     
+ Partials      583      574       -9     

Flag	Coverage Δ
unittests	88.17% <100.00%> (+0.12%)	⬆️

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