Docstring Style Guide#

General Guidelines#

Documentation is one of the most highly valued aspects of an open source project. Documentation teaches users and contributors how to use a project, how to contribute, and the standards of conduct within an open source community. But according to GitHub’s Open Source Survey, incomplete or confusing documentation is the most commonly encountered problem in open source. This style guide aims to change that.

The purpose of this style guide is to provide the SymPy community with a set of style and formatting guidelines that can be utilized and followed when writing SymPy documentation. Adhering to the guidelines offered in this style guide will bring greater consistency and clarity to SymPy’s documentation, supporting its mission to become a full-featured, open source computer algebra system (CAS).

The SymPy documentation found at docs.sympy.org is generated from docstrings in the source code and dedicated narrative documentation files in the doc/src directory. Both are written in reStructuredText format extended by Sphinx.

The documentation contained in the doc/src directory and the docstrings embedded in the Python source code are processed by Sphinx and various Sphinx extensions. This means that the documentation source format is specified by the documentation processing tools. The SymPy Documentation Style Guide provides both the essential elements for writing SymPy documentation as well as any deviations in style we specify relative to these documentation processing tools. The following lists the processing tools:

Everything supported by the above processing tools is available for use in the SymPy documentation, but this style guide supersedes any recommendations made in the above documents. Note that we do not follow PEP 257 or the www.python.org documentation recommendations.

If you are contributing to SymPy for the first time, please read our Introduction to Contributing page as well as this guide.

Types of Documentation#

There are four main locations where SymPy’s documentation can be found:

SymPy Website https://sympy.org

The SymPy website’s primary function is to advertise the software to users and developers. It also serves as an initial location to point viewers to other relevant resources on the web. The SymPy website has basic information on SymPy and how to obtain it, as well as examples to advertise it to users, but it does not have technical documentation. The source files are located in the SymPy webpage directory. Appropriate items for the website are:

  • General descriptions of what SymPy and the SymPy community are

  • Explanations/demonstrations of major software features

  • Listings of other major software that uses SymPy

  • Getting started info for users (download and install instructions)

  • Getting started info for developers

  • Where users can get help and support on using SymPy

  • News about SymPy

SymPy Documentation https://docs.sympy.org

This is the main place where users go to learn how to use SymPy. It contains a tutorial for SymPy as well as technical documentation for all of the modules. The source files are hosted in the main SymPy repository in the doc directory at and are built using the Sphinx site generator and uploaded to the docs.sympy.org site automatically. There are two primary types of pages that are generated from different source files in the docs directory:

  • Narrative Pages: reStructuredText files that correspond to manually written documentation pages not present in the Python source code. Examples are the tutorial RST files. In general, if your documentation is not API documentation it belongs in a narrative page.

  • API Documentation Pages: reStructuredText files that contain directives that generate the Application Programming Interface documentation. These are automatically generated from the SymPy Python source code.

SymPy Source Code https://github.com/sympy/sympy

Most functions and classes have documentation written inside it in the form of a docstring, which explains the function and includes examples called doctests. The purpose of these docstrings are to explain the API of that class or function. The doctests examples are tested as part of the test suite, so that we know that they always produce the output that they say that they do. Here is an example docstring. Most docstrings are also automatically included in the Sphinx documentation above, so that they appear on the SymPy Documentation website. Here is that same docstring on the SymPy website. The docstrings are formatted in a specific way so that Sphinx can render them correctly for the docs website. The SymPy sources all contain sparse technical documentation in the form of source code comments, although this does not generally constitute anything substantial and is not displayed on the documentation website.

SymPy Wiki https://github.com/sympy/sympy/wiki

The SymPy Wiki can be edited by anyone without review. It contains various types of documentation, including:

Narrative Documentation Guidelines#

Extensive documentation, or documentation that is not centered around an API reference, should be written as a narrative document in the Sphinx docs (located in the doc/src directory). The narrative documents do not reside in the Python source files, but as standalone restructured files in the doc/src directory. SymPy’s narrative documentation is defined as the collective documents, tutorials, and guides that teach users how to use SymPy. Reference documentation should go in the docstrings and be pulled into the RST with autodoc. The RST itself should only have narrative style documentation that is not a reference for a single specific function.

Documentation using Markdown#

Narrative documentation can be written using either Restructured Text (.rst) or Markdown (.md). Markdown documentation uses MyST. See this guide for more information on how to write documents in MArkdown. Markdown is only supported for narrative documentation. Docstrings should continue to use RST syntax. Any part of this style guide that is not specific to RST syntax should still apply to Markdown documents.

Best Practices for Writing Documentation#

Please follow these formatting, style, and tone preferences when writing documentation.

Formatting Preferences#

In order for math and code to render correctly on the SymPy website, please follow these formatting guidelines.

Math#

Text that is surrounded by dollar signs $ _ $ will be rendered as LaTeX math. Any text that is meant to appear as LaTeX math should be written as $math$. In the HTML version of the docs, MathJax will render the math.

Example

The Bessel $J$ function of order $\nu$ is defined to be the function
satisfying Bessel’s differential equation.

LaTeX Recommendations#

  • If a docstring has any LaTeX, be sure to make it “raw.” See the Docstring Formatting section for details.

  • If you are not sure how to render something, you can use the SymPy latex() function. But be sure to strip out the unimportant parts (the bullet points below).

  • Avoid unnecessary \left and \right (but be sure to use them when they are required).

  • Avoid unnecessary {}. (For example, write x^2 instead of x^{2}.)

  • Use whitespace in a way that makes the equation easiest to read.

  • Always check the final rendering to make sure it looks the way you expect it to.

  • The HTML documentation build will not fail if there is invalid math, but rather it will show as an error on the page. However, the PDF build, which is run on GitHub Actions on pull requests, will fail. If the LaTeX PDF build fails on CI, there is likely an issue with LaTeX math somewhere.

Examples

Correct:

\int \sin(x)\,dx

Incorrect:

\int \sin{\left( x\right)}\, dx

For more in-depth resources on how to write math in LaTeX, see:

Code#

Text that should be printed verbatim, such as code, should be surrounded by a set of double backticks like this.

Example

To use this class, define the ``_rewrite()`` and ``_expand()`` methods.

Sometimes a variable will be the same in both math and code, and can even appear in the same paragraph, making it difficult to know if it should be formatted as math or code. If the sentence in question is discussing mathematics, then LaTeX should be used, but if the sentence is discussing the SymPy implementation specifically, then code should be used.

In general, the rule of thumb is to consider if the variable in question were something that rendered differently in code and in math. For example, the Greek letter α would be written as alpha in code and $\alpha$ in LaTeX. The reason being that $\alpha$ cannot be used in contexts referring to Python code because it is not valid Python, and conversely alpha would be incorrect in a math context because it does not render as the Greek letter (α).

Example

class loggamma(Function):
    r"""
    The ``loggamma`` function implements the logarithm of the gamma
    function (i.e, $\log\Gamma(x)$).

    """

Variables listed in the parameters after the function name should, in written text, be italicized using Sphinx emphasis with asterisks like *this*.

Example

def stirling(n, k, d=None, kind=2, signed=False):
    """
    ...

    The first kind of Stirling number counts the number of permutations of
    *n* distinct items that have *k* cycles; the second kind counts the
    ways in which *n* distinct items can be partitioned into *k* parts.
    If *d* is given, the "reduced Stirling number of the second kind" is
    returned: $S^{d}(n, k) = S(n - d + 1, k - d + 1)$ with $n \ge k \ge d$.
    This counts the ways to partition $n$ consecutive integers into $k$
    groups with no pairwise difference less than $d$.

    """

Note that in the above example, the first instances of n and k are referring to the input parameters of the function stirling. Because they are Python variables but also parameters listed by themselves, they are formatted as parameters in italics. The last instances of \(n\) and \(k\) are talking about mathematical expressions, so they are formatted as math.

If a variable is code, but is also a parameter written by itself, the parameter formatting takes precedence and it should be rendered in italics. However, if a parameter appears in a larger code expression it should be within double backticks to be rendered as code. If a variable is only code and not a parameter as well, it should be in double backticks to be rendered as code.

Please note that references to other functions in SymPy are handled differently from parameters or code. If something is referencing another function in SymPy, the cross-reference reStructuredText syntax should be used. See the section on Cross-Referencing for more information.

Headings#

Section headings in reStructuredText files are created by underlining (and optionally overlining) the section title with a punctuation character at least as long as the text.

Normally, there are no heading levels assigned to certain characters as the structure is determined from the succession of headings. However, for SymPy’s documentation, here is a suggested convention:

=== with overline: title (top level heading)

=== heading 1

--- heading 2

^^^ heading 3

~~~ heading 4

""" heading 5

Style Preferences#

Spelling and Punctuation#

All narrative writing in SymPy follows American spelling and punctuation standards. For example, “color” is preferred over “colour” and commas should be placed inside of quotation marks.

Examples

If the ``line_color`` aesthetic is a function of arity 1, then the coloring
is a function of the x value of a point.

The term "unrestricted necklace," or "bracelet," is used to imply an object
that can be turned over or a sequence that can be reversed.

If there is any ambiguity about the spelling of a word, such as in the case of a function named after a person, refer to the spelling of the actual SymPy function.

For example, Chebyshev polynomials are named after Pafnuty Lvovich Tchebychev, whose name is sometimes transliterated from Russian to be spelled with a “T,” but in SymPy it should always be spelled “Chebyshev” to refer to the SymPy function.

Example

class chebyshevt(OrthogonalPolynomial):
    r"""
    Chebyshev polynomial of the first kind, $T_n(x)$
    ...

    """

Capitalization#

Title case capitalization is preferred in all SymPy headings.

Example

What is Symbolic Computation?
-----------------------------

Tone Preferences#

Across SymPy documentation, please write in:

  • The present tense (e.g., In the following section, we are going to learn…)

  • The first-person inclusive plural (e.g., We did this the long way, but now we can try it the short way…)

  • Use the generic pronoun “you” instead of “one.” Or use “the reader” or “the user.” (e.g., You can access this function by… The user can then access this function by…)

  • Use the gender-neutral pronoun “they” instead of “he” or “she.” (e.g., A good docstring tells the user exactly what they need to know.)

Avoid extraneous or belittling words such as “obviously,” “easily,” “simply,” “just,” or “straightforward.”

Avoid unwelcoming or judgement-based phrases like “That is wrong.” Instead use friendly and inclusive language like “A common mistake is…”

Avoid extraneous phrases like, “we just have to do one more thing.”