PyTorch Docstring Writing Guide

This skill describes how to write docstrings for functions and methods in the PyTorch project, following the conventions in torch/_tensor_docs.py and torch/nn/functional.py.

General Principles

• Use raw strings (r"""...""") for all docstrings to avoid issues with LaTeX/math backslashes
• Follow Sphinx/reStructuredText (reST) format for documentation
• Be concise but complete - include all essential information
• Always include examples when possible
• Use cross-references to related functions/classes

Docstring Structure

1. Function Signature (First Line)

Start with the function signature showing all parameters:

r"""function_name(param1, param2, *, kwarg1=default1, kwarg2=default2) -> ReturnType

Notes:

• Include the function name
• Show positional and keyword-only arguments (use * separator)
• Include default values
• Show return type annotation
• This line should NOT end with a period

2. Brief Description

Provide a one-line description of what the function does:

r"""conv2d(input, weight, bias=None, stride=1, padding=0, dilation=1, groups=1) -> Tensor

Applies a 2D convolution over an input image composed of several input
planes.

3. Mathematical Formulas (if applicable)

Use Sphinx math directives for mathematical expressions:

.. math::
    \text{Softmax}(x_{i}) = \frac{\exp(x_i)}{\sum_j \exp(x_j)}

Or inline math: :math:\x^2``

4. Cross-References

Link to related classes and functions using Sphinx roles:

• :class:\~torch.nn.ModuleName`` - Link to a class
• :func:\torch.function_name`` - Link to a function
• :meth:\~Tensor.method_name`` - Link to a method
• :attr:\attribute_name`` - Reference an attribute
• The ~ prefix shows only the last component (e.g., Conv2d instead of torch.nn.Conv2d)

Example:

See :class:`~torch.nn.Conv2d` for details and output shape.

5. Notes and Warnings

Use admonitions for important information:

.. note::
    This function doesn't work directly with NLLLoss,
    which expects the Log to be computed between the Softmax and itself.
    Use log_softmax instead (it's faster and has better numerical properties).

.. warning::
    :func:`new_tensor` always copies :attr:`data`. If you have a Tensor
    ``data`` and want to avoid a copy, use :func:`torch.Tensor.requires_grad_`
    or :func:`torch.Tensor.detach`.

6. Args Section

Document all parameters with type annotations and descriptions:

Args:
    input (Tensor): input tensor of shape :math:`(\text{minibatch} , \text{in\_channels} , iH , iW)`
    weight (Tensor): filters of shape :math:`(\text{out\_channels} , kH , kW)`
    bias (Tensor, optional): optional bias tensor of shape :math:`(\text{out\_channels})`. Default: ``None``
    stride (int or tuple): the stride of the convolving kernel. Can be a single number or a
      tuple `(sH, sW)`. Default: 1

Formatting rules:

• Parameter name in lowercase
• Type in parentheses: (Type), (Type, optional) for optional parameters
• Description follows the type
• For optional parameters, include "Default: value" at the end
• Use double backticks for inline code: ``None``
• Indent continuation lines by 2 spaces

7. Keyword Args Section (if applicable)

Sometimes keyword arguments are documented separately:

Keyword args:
    dtype (:class:`torch.dtype`, optional): the desired type of returned tensor.
        Default: if None, same :class:`torch.dtype` as this tensor.
    device (:class:`torch.device`, optional): the desired device of returned tensor.
        Default: if None, same :class:`torch.device` as this tensor.
    requires_grad (bool, optional): If autograd should record operations on the
        returned tensor. Default: ``False``.

8. Returns Section (if needed)

Document the return value:

Returns:
    Tensor: Sampled tensor of same shape as `logits` from the Gumbel-Softmax distribution.
        If ``hard=True``, the returned samples will be one-hot, otherwise they will
        be probability distributions that sum to 1 across `dim`.

Or simply include it in the function signature line if obvious from context.

9. Examples Section

Always include examples when possible:

Examples::

    >>> inputs = torch.randn(33, 16, 30)
    >>> filters = torch.randn(20, 16, 5)
    >>> F.conv1d(inputs, filters)

    >>> # With square kernels and equal stride
    >>> filters = torch.randn(8, 4, 3, 3)
    >>> inputs = torch.randn(1, 4, 5, 5)
    >>> F.conv2d(inputs, filters, padding=1)

Formatting rules:

• Use Examples:: with double colon
• Use >>> prompt for Python code
• Include comments with # when helpful
• Show actual output when it helps understanding (indent without >>>)

10. External References

Link to papers or external documentation:

.. _Link Name:
    https://arxiv.org/abs/1611.00712

Reference them in text: See `Link Name`_

Method Types

Native Python Functions

For regular Python functions, use a standard docstring:

def relu(input: Tensor, inplace: bool = False) -> Tensor:
    r"""relu(input, inplace=False) -> Tensor

    Applies the rectified linear unit function element-wise. See
    :class:`~torch.nn.ReLU` for more details.
    """
    # implementation

C-Bound Functions (using add_docstr)

For C-bound functions, use _add_docstr:

conv1d = _add_docstr(
    torch.conv1d,
    r"""
conv1d(input, weight, bias=None, stride=1, padding=0, dilation=1, groups=1) -> Tensor

Applies a 1D convolution over an input signal composed of several input
planes.

See :class:`~torch.nn.Conv1d` for details and output shape.

Args:
    input: input tensor of shape :math:`(\text{minibatch} , \text{in\_channels} , iW)`
    weight: filters of shape :math:`(\text{out\_channels} , kW)`
    ...
""",
)

In-Place Variants

For in-place operations (ending with _), reference the original:

add_docstr_all(
    "abs_",
    r"""
abs_() -> Tensor

In-place version of :meth:`~Tensor.abs`
""",
)

Alias Functions

For aliases, simply reference the original:

add_docstr_all(
    "absolute",
    r"""
absolute() -> Tensor

Alias for :func:`abs`
""",
)

Common Patterns

Shape Documentation

Use LaTeX math notation for tensor shapes:

:math:`(\text{minibatch} , \text{in\_channels} , iH , iW)`

Reusable Argument Definitions

For commonly used arguments, define them once and reuse:

common_args = parse_kwargs(
    """
    dtype (:class:`torch.dtype`, optional): the desired type of returned tensor.
        Default: if None, same as this tensor.
"""
)

# Then use with .format():
r"""
...

Keyword args:
    {dtype}
    {device}
""".format(**common_args)

Template Insertion

Insert reproducibility notes or other common text:

r"""
{tf32_note}

{cudnn_reproducibility_note}
""".format(**reproducibility_notes, **tf32_notes)

Complete Example

Here's a complete example showing all elements:

def gumbel_softmax(
    logits: Tensor,
    tau: float = 1,
    hard: bool = False,
    eps: float = 1e-10,
    dim: int = -1,
) -> Tensor:
    r"""
    Sample from the Gumbel-Softmax distribution and optionally discretize.

    Args:
        logits (Tensor): `[..., num_features]` unnormalized log probabilities
        tau (float): non-negative scalar temperature
        hard (bool): if ``True``, the returned samples will be discretized as one-hot vectors,
              but will be differentiated as if it is the soft sample in autograd. Default: ``False``
        dim (int): A dimension along which softmax will be computed. Default: -1

    Returns:
        Tensor: Sampled tensor of same shape as `logits` from the Gumbel-Softmax distribution.
            If ``hard=True``, the returned samples will be one-hot, otherwise they will
            be probability distributions that sum to 1 across `dim`.

    .. note::
        This function is here for legacy reasons, may be removed from nn.Functional in the future.

    Examples::
        >>> logits = torch.randn(20, 32)
        >>> # Sample soft categorical using reparametrization trick:
        >>> F.gumbel_softmax(logits, tau=1, hard=False)
        >>> # Sample hard categorical using "Straight-through" trick:
        >>> F.gumbel_softmax(logits, tau=1, hard=True)

    .. _Link 1:
        https://arxiv.org/abs/1611.00712
    """
    # implementation

Quick Checklist

When writing a PyTorch docstring, ensure:

• Use raw string (r""")
• Include function signature on first line
• Provide brief description
• Document all parameters in Args section with types
• Include default values for optional parameters
• Use Sphinx cross-references (:func:, :class:, :meth:)
• Add mathematical formulas if applicable
• Include at least one example in Examples section
• Add warnings/notes for important caveats
• Link to related module class with :class:
• Use proper math notation for tensor shapes
• Follow consistent formatting and indentation

Common Sphinx Roles Reference

• :class:\~torch.nn.Module`` - Class reference
• :func:\torch.function`` - Function reference
• :meth:\~Tensor.method`` - Method reference
• :attr:\attribute`` - Attribute reference
• :math:\equation`` - Inline math
• :ref:\label`` - Internal reference
• ``code`` - Inline code (use double backticks)

Additional Notes

• Indentation: Use 4 spaces for code, 2 spaces for continuation of parameter descriptions
• Line length: Try to keep lines under 100 characters when possible
• Periods: End sentences with periods, but not the signature line
• Backticks: Use double backticks for code: ``True`` ``None`` ``False``
• Types: Common types are Tensor, int, float, bool, str, tuple, list, etc.