docfmt

Reliable docstring formatter for Python

• docfmt
  • Motivation
  • Behavior
    • MyST directives
    • Attribute docstrings
  • Usage
  • Exit codes

Motivation

Docstring formatters that rebuild the file from a token stream are prone to a particular class of bug: edits computed against one version of the token list get applied to another, so behavior depends on how much unrelated code precedes the docstring being formatted. The symptom is formatting that changes when you add or remove an unrelated class elsewhere in the file.

docfmt is built so that cannot happen:

• Docstrings are located by AST structure, never by token heuristics or by matching line contents.
• The file is never regenerated. Formatting produces a set of non-overlapping character-range replacements against the original source; everything outside a docstring is preserved byte-for-byte.
• Results are verified before they are written. Every formatted file is checked for AST equivalence (nothing changed but docstring contents) and for idempotency (format(format(x)) == format(x)). A failure leaves the file untouched and reports an internal error rather than writing a bad result.

Behavior

docfmt normalizes docstring layout and never mutates author text. Adding a trailing period is the one exception, and it is opt-in (--add-summary-period).

Blank lines around docstrings are preserved by default. Normalization is opt-in per position under [tool.docfmt.blank-lines].

Structured content is copied verbatim and never rewrapped: fenced code blocks, MyST directives, doctests, reST directives and field lists, tables, and lists.

MyST directives

MyST directives nest by widening the fence, so fence length is significant:

```{note}
Test note
```

A block closes only on a fence of the same character with at least as many markers and no info string, so a narrower inner fence is content:

````{note}
```{warning}
Test warning
```
````

Inline constructs such as `False`{l=python} are treated as unbreakable atoms, so wrapping never splits an inline code span from its role attribute.

Attribute docstrings

A string statement immediately following an assignment at module or class level is an attribute docstring, including annotation-only attributes:

package: str
"""
Import name of the package.
"""

Usage

docfmt --in-place --recursive src test
docfmt --check src        # exit 1 if anything would change
docfmt --diff src

Configure via [tool.docfmt] in pyproject.toml:

[tool.docfmt]
in-place = true
recursive = true
summary-on-own-line = true

line-length is left unset by default and picked up from [tool.black] line-length when black is configured, falling back to 88.

Blank-line rules live in their own table. Each is a count, or "preserve" to leave the author's spacing alone:

[tool.docfmt.blank-lines]
before-class = "preserve"
after-module = 1
after-class = 1
after-function = "preserve"
after-attribute = 1

Exit codes