feat: Add description field to columns

[Oliver Borchert (borchero)]

Motivation

Implements #323 (comment) and closes #132.

Why introduce a description parameter?

Unfortunately, attribute docstrings cannot be extracted at runtime (PEP 224 was rejected). Hence, this is the only way to make use of the description at runtime, e.g. to pass to pydantic field descriptions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b339296) to head (ff4f2f8).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #342   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           56        56           
  Lines         3404      3410    +6     
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+ Hits          3404      3410    +6     

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[Copilot]

Pull request overview

This PR introduces an optional description field for column definitions so that human-readable column documentation is available at runtime (notably for propagation into pydantic field/schema descriptions).

Changes:

• Add description: str | None to Column and thread it through concrete column constructors.
• Include description in pydantic.Field(...) kwargs so it appears in generated JSON schema.
• Add tests validating description storage, immutability via with_description, and pydantic propagation.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/columns/test_description.py	Adds coverage for the new description attribute and pydantic schema propagation.
dataframely/columns/_base.py	Adds description storage, pydantic field kwargs propagation, and with_description().
dataframely/columns/any.py	Accepts and forwards description for Any columns.
dataframely/columns/array.py	Accepts and forwards description for Array columns.
dataframely/columns/categorical.py	Accepts and forwards description for Categorical columns.
dataframely/columns/datetime.py	Accepts and forwards description for datetime-related columns.
dataframely/columns/decimal.py	Accepts and forwards description for Decimal columns.
dataframely/columns/enum.py	Accepts and forwards description for Enum columns.
dataframely/columns/float.py	Accepts and forwards description for float columns.
dataframely/columns/integer.py	Accepts and forwards description for integer columns.
dataframely/columns/list.py	Accepts and forwards description for List columns.
dataframely/columns/object.py	Accepts and forwards description for Object columns.
dataframely/columns/string.py	Accepts and forwards description for String columns.
dataframely/columns/struct.py	Accepts and forwards description for Struct columns.