Python rest docstrings

Python Docstring Writer (reST)

Write consistent, high-signal docstrings using reST roles for cross-linking. Optimize for: fast scanning in IDE/tooltips, friendly references, minimal redundancy with type hints, and explaining why/behavior rather than restating types.

Conventions: Type aliases/constants → immediate string literal. Classes/functions → PEP 257 docstring. Attributes/TypedDict keys → trailing docstring. Cross-references → reST roles (e.g. :class:`Foo, :meth:`Bar.baz), not plain text. For @overload and :class:typing.Protocol, follow the dedicated sections below.

---

1. Type aliases / constants

Docstring immediately after the assignment. One line when possible. Use double backticks for literals (e.g. "dict", None). Prefer meaning and effects over restating the type.

RowFactory = Literal["tuple", "dict"]
"""Row format for fetch methods: ``"tuple"`` for sequences, ``"dict"`` for column-keyed dicts."""

IsolationLevel = Literal["repeatable read", "serializable"]
"""Supported transaction isolation levels."""

---

2. Classes

First line: short noun phrase. Then: lifecycle, concurrency/transaction semantics, invariants. Don’t list every attribute. Use roles for references. Add exactly one blank line between the docstring and the class definition below the docstring.

@attrs.define(slots=True)
class PostgresClient:
    """Async Postgres client with connection pooling and context-bound transactions.

    Must be :meth:`initialize`d with a DSN before use. Uses context variables to
    share a single connection per logical request, so nested :meth:`transaction`
    blocks reuse the same connection and use savepoints.
    """

---

2.1. :class:typing.Protocol

For :class:typing.Protocol interfaces:

• Document the contract and semantics (what implementers must guarantee), not implementation details.
• Prefer documenting when methods are called, expected side effects, idempotency, ordering, and concurrency guarantees.
• Do not document :raises for protocols unless an exception is a required part of the contract (usually unknown for interfaces).
• Use reST roles for referenced types and callables.
• Don't forget to add ellipsis below the docstring for protocol methods, otherwise the method will be treated as broken.
• For protocol methods with overloads, see section 3.1 @overload.

from typing import Protocol, AsyncContextManager

class AppRuntimePort(Protocol):
    """Application runtime contract for transactional execution.

    Implementations provide a transaction boundary for usecases. Nested
    transactions may be supported via savepoints; callers should not assume a
    specific strategy unless explicitly documented by the implementation.
    """

    def transaction(self) -> AsyncContextManager[None]:
        """Return an async context manager that scopes a transaction.

        The returned context manager starts a transaction on entry and commits or
        rolls back on exit according to implementation policy.
        """
        ...

---

3. Methods and functions

Brief summary + behavioral details. Use reST field lists for params/returns/raises when needed. Explain what it does, what it returns, and edge cases. Document errors only when meaningful (e.g. :raises SomeError: When ...). Add exactly one blank line between the docstring and the function definition below the docstring.

async def fetch_one(self, query: str, *args: Any) -> Mapping[str, Any] | None:
    """Execute a query and return a single row.

    Returns ``None`` when no rows match.

    :param query: SQL query text.
    :param args: Query parameters.
    :returns: A row mapping or ``None``.
    """

---

3.1. @overload

For overloaded callables, docstrings should primarily reflect the semantic differences between overload variants, not just duplicate a shared description.

Many IDEs display the docstring of the selected overload signature. If an overload stub lacks a docstring, callers may see no documentation at all. Therefore, each @overload should have a docstring.

Rules:

• Each @overload stub must have a docstring.
• Prefer documenting the behavior specific to that signature (e.g. return shape, mutation vs new instance, sentinel handling, narrowing).
• Avoid repeating the entire shared description unless necessary.
• If overload semantics cannot be meaningfully distinguished, duplicate the shared docstring verbatim as a fallback.
• The implementation may contain a general docstring, but overload docstrings are the primary source of truth for per-signature guarantees.
  • Don't forget to add ellipsis below the docstring for overloads, otherwise the method will be treated as broken.

from typing import overload, Literal, Self

@overload
def register(self, op: str, *, inplace: Literal[True]) -> None:
    """Register an operation factory and mutate the registry.

    Does not return a value. Raises :exc:`CoreError` if the operation
    is already registered.
    """
    ...

@overload
def register(self, op: str, *, inplace: Literal[False] = False) -> Self:
    """Register an operation factory and return a new registry.

    Does not mutate the original instance. Raises :exc:`CoreError`
    if the operation is already registered.
    """
    ...

def register(self, op: str, *, inplace: bool = False) -> Self | None:
    """Register an operation factory.

    Dispatches to the appropriate overload behavior based on ``inplace``.
    """

Fallback (no meaningful semantic difference):

@overload
def normalize(value: int) -> int:
    """Normalize a numeric value without changing its meaning."""
    ...

@overload
def normalize(value: str) -> str:
    """Normalize a numeric value without changing its meaning."""
    ...

def normalize(value: int | str) -> int | str:
    """Normalize a value without changing its meaning."""
    ...

---

4. Attributes / fields

Trailing docstring when public or needing clarification. Omit or keep very short for private fields (only if subtle or critical).

min_size: int = 2
"""Minimum number of connections in the pool."""

_ctx_depth: ContextVar[int] = ...
"""Transaction nesting depth used to manage savepoints."""

---

5. TypedDict keys

Class docstring: what the dict represents and where it’s used. Key docstrings: behavior, defaults, interpretation. If a key is optional (e.g. total=False), note what happens when absent.

class TransactionOptions(TypedDict, total=False):
    """Options for :meth:`PostgresClient.transaction`."""

    read_only: bool
    """If true, transaction is read-only."""

    isolation: IsolationLevel
    """Transaction isolation level (e.g. ``"repeatable read"``, ``"serializable"``)."""

---

reST roles (use for cross-references)

Role	Use for
:class:`MyClass	Classes
:meth:`MyClass.method	Methods
:func:`my_function	Functions
:attr:`MyClass.attr	Attributes/properties
:mod:`package.module	Modules
:data:`CONSTANT	Module-level data/constants
:exc:`SomeError	Exceptions

Same-class: :meth:`initialize is fine. Cross-module: use fully-qualified names, e.g. :class:`pkg.mod.Foo.

---

Description length (be concise)

The description explains intent and non-obvious behavior — it is not a place for prose. This budget applies to the summary and body, not to :param:/:returns:/:raises: entries (size those to the actual API surface).

• Default to the one-line summary. For simple or self-explanatory APIs, the summary is the whole docstring; do not add a body.
• Add a body only when it earns its place — non-obvious behavior, side effects, invariants, edge cases, or "why". Keep it to ~1–3 sentences.
• Scale with complexity, not by default. Reserve longer explanations for genuinely complex or tricky code (concurrency, transactions, subtle contracts). Even then, prefer the shortest correct explanation.

---

Formatting (hard requirements)

• Sentence-cased, end with a period. One blank line between definition and docstring.
• Present tense (“Returns …”, “Acquires …”).
• Double backticks for literal values, SQL fragments, flags, env vars.
• Blank line between summary and body. ~88 chars line length when reasonable.
• Do not repeat type hints in the docstring. Add semantics, invariants, side effects, concurrency, performance caveats.

---

Anti-patterns

Attribute — Bad (repeats type): """Timeout as an integer."""
Good: """Timeout in seconds for acquiring a connection from the pool."""

TypedDict — Bad (no role): """Options for PostgresClient.transaction."""
Good: """Options for :meth:`PostgresClient.transaction`."""

---

Checklist before writing

1. User-facing? → Document behavior and edge cases.
2. Type hint already clear? → Docstring adds semantics, not types.
3. Correctness-sensitive? (transactions, concurrency, caching, idempotency) → Must document.
4. Can cross-link? → Use :meth:`... / :class:`....
5. @overload? → Each overload stub must have a docstring; document semantic differences per signature.
6. :class:typing.Protocol? → Document contract/semantics; avoid :raises unless mandated.

---

Minimal templates

Type alias / constant: Thing = ... → """What it represents and how callers should interpret it."""

Class: One-line summary, then key behaviors, lifecycle, invariants, and :meth:`X.foo / :class:`Y references.

Function / method: One-line summary, then details; :param name: Meaning. :returns: Meaning. :raises SomeError: When.

Field: field: Type = default → """Meaning, units, constraints, or why it exists."""

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Compatible with PEP 257, reST, and IDE tooltips. Keep roles even in Markdown-only contexts; they stay readable and beneficial.