Note

Changes to this document must be approved by the System Architect (RFC-24). To request changes to these standards, please file an RFC.

# Documenting Python APIs with docstrings¶

We use Python docstrings to create reference documentation for our Python APIs. Docstrings are read by developers, interactive Python users, and readers of our online documentation. This page describes how to write these docstrings for LSST DM.

Although this style guide grew out of the Numpydoc Style Guide, and DM docstrings are parsed by Numpydoc, this style guide has small tweaks and clarifications compared to the original Numpydoc Style Guide. Always refer to this guide, rather than others, to learn how to write DM docstrings. If you have any questions, ask in #dm-docs on Slack.

Format reference:

How to format different APIs:

Learn by example:

Treat the guidelines on this page as an extension of the DM Python Style Guide.

## Basic format of docstrings¶

Python docstrings form the __doc__ attributes attached to modules, classes, methods and functions. See PEP 257 for background.

### Docstrings MUST be delimited by triple double quotes¶

Docstrings must be delimited by triple double quotes: """. This allows docstrings to span multiple lines. You may use u""" for unicode, but it’s usually preferable to stick to ASCII.

For consistency, do not use triple single quotes: '''.

### Docstrings SHOULD begin with """ and terminate with """ on its own line¶

The docstring’s summary sentence occurs on the same line as the opening """.

The terminating """ should be on its own line, even for ‘one-line’ docstrings (this is a minor departure from PEP 257). For example, a one-line docstring:

"""Sum numbers in an array.
"""


(Note: one-line docstrings are rarely used for public APIs, see Numpydoc sections in docstrings.)

An example of a multi-paragraph docstring:

"""Sum numbers in an array.

Parameters
----------
values : iterable
Python iterable whose values are summed.

Returns
-------
sum : float
Sum of values.
"""


### Docstrings of methods and functions SHOULD NOT be preceded or followed by a blank line¶

Inside a function or method, there should be no blank lines surrounding the docstring:

def sum(values):
"""Sum numbers in an array.

Parameters
----------
values : iterable
Python iterable whose values are summed.

Returns
-------
sum : float
Sum of values.
"""
pass


### Docstrings of classes SHOULD be followed, but not preceded, by a blank line¶

Like method and function docstrings, the docstring should immediately follow the class definition, without a blank space. However, there should be a single blank line before following code such as class variables or the __init__ method:

class Point(object):
"""A point in a 2D cartesian space.

Parameters
----------
x, y : float
The point's coordinate.
"""

def __init__(x, y):
self.x = x
self.y = y


### Docstring content MUST be indented with the code’s scope¶

For example:

def sum(values):
"""Sum numbers in an array.

Parameters
----------
values : iterable
Python iterable whose values are summed.
"""
pass


Not:

def sum(values):
"""Sum numbers in an array.

Parameters
----------
values : iterable
Python iterable whose values are summed.
"""
pass


## ReStructuredText in docstrings¶

We use reStructuredText to mark up and give semantic meaning to text in docstrings. ReStructuredText is lightweight enough to read in raw form, such as command line terminal printouts, but is also parsed and rendered with our Sphinx-based documentation build system. All of the style guidance for using reStructuredText from our ReStructuredText Style Guide applies in docstrings with a few exceptions defined here.

### No space between headers and paragraphs¶

For docstrings, the Numpydoc standard is to omit any space between a header and the following paragraph.

For example

"""A summary sentence.

Notes
-----
The content of the notes section directly follows the header, with no blank line.
"""


This deviation from the normal style guide is in keeping with Python community idioms and to save vertical space in terminal help printouts.

### Sections are restricted to the Numpydoc section set¶

Sections must be from the set of standard Numpydoc sections (see Numpydoc sections in docstrings). You cannot introduce new section headers, or use the full reStructuredText subsection hierarchy, since these subsections won’t be parsed by the documentation toolchain.

Always use the dash (-) to underline sections. For example:

def myFunction(a):
"""Do something.

Parameters
----------
[...]

Returns
-------
[...]

Notes
-----
[...]
"""


### Simulate subsections with bold text¶

Conventional reStructuredText subsections are not allowed in docstrings, given the previous guideline. However, you may structure long sections with bold text that simulates subsection headers. This technique is useful for the Notes and Examples Numpydoc sections. For example:

def myFunction(a):
"""Do something.

[...]

Examples
--------
**Example 1**

[...]

**Example 2**

[...]
"""


### Line lengths¶

Hard-wrap text in docstrings to match the docstring line length allowed by the coding standard.

### Marking up parameter names¶

The default reStructuredText role in docstrings is :py:obj:. Sphinx automatically generates links when the API names are marked up in single backticks. For example: str or lsst.pipe.base.Struct.

You cannot use this role to mark up parameters, however. Instead, use the code literal role (double backticks) to mark parameters and return variables in monospace type. For example, the description for format references the should_plot parameter:

Parameters
----------
should_plot : bool
Plot the fit if True.
plot_format : str, optional
Format of the plot when should_plot is True.


## Numpydoc sections in docstrings¶

We organize Python docstrings into sections that appear in a common order. This format is based on the original Numpydoc Style Guide (used by NumPy, SciPy, and Astropy, among other scientific Python packages), though this style guide includes several DM-specific clarifications. These are the sections and their relative order:

1. Short summary
2. Extended summary (optional)
3. Parameters (if applicable; for classes, methods, and functions)
4. Returns or Yields (if applicable; for functions, methods, and generators)
5. Other parameters (if applicable; for classes, methods, and functions)
6. Raises (if applicable)
8. Notes (optional)
9. References (optional)
10. Examples (optional)

For summaries of how these docstring sections are composed in specific contexts, see:

### Short summary¶

All docstrings begin with a one-sentence summary:

def add(a, b):
"""Sum two numbers.

Parameters
----------
a, b : float

Returns
-------
sum : float
The sum of a and b.
"""
return a + b


The summary sentence can wrap across multiple lines (see also Line lengths):

def sumif(sequence, conditional)
"""Sum the numbers in a sequence as long as they pass a
user-provided conditional callback function.

Parameters
----------
sequence : list [float]
A sequence (list, for example) of numbers.
conditional : callable
A callback function that takes a single number as an
argument. The conditional function returns True
if the number passes the conditional, and False
otherwise.

Returns
-------
sum : float
The sum of numbers that meet the conditional.
"""
total = 0
for n in sequence:
if conditional(n):
total += n


Do not write multiple sentences in the Short summary, however. If additional content is needed to clarify the Short summary, consider adding an Extended summary section.

#### On API and parameter names in summary sentences¶

Do not repeat the name of the class, method, function, or attribute in the summary sentence. This information is already clear from the context.

As well, it’s best to not repeat the names of parameters in the summary sentence. The summary sentence is often seen in API listings where individual parameters may be truncated from the displayed API signature. Write the summary in plain English and in a way that is as self-contained as practical (aside from the implicit context of the API name and the name of the parent class). If the English word is the same as the parameter name then it’s fine to use that word in the summary sentence; don’t display the word as a code literal, though.

#### Writing summaries for functions and methods¶

For functions and methods, write in the imperative voice. That is, the summary is treated as a command that the API consumer can give.

Some examples:

• def getMetadata(self):

Returns
-------
metadata : lsst.pipe.base.Struct
"""

• def makeRegistry(doc, configBaseType=Config):
"""Create a Registry.

Parameters
----------
doc : str
Docstring for the created Registry (this is set as the __doc__
attribute of the Registry instance.
configBaseType : lsst.pex.config.Config-type
Base type of config classes in the Registry
(lsst.pex.config.Registry.configBaseType).

Returns
-------
registry : Registry
Registry with __doc__ and ~Registry.configBaseType attributes
set.
"""


#### Writing summaries for classes¶

State what the class is through its summary sentence.

Examples:

• class Job:
"""A container for measurements, blobs, and metadata associated
with a pipeline run.
"""

• class Field:
"""A field in a ~lsst.pex.config.Config class that supports int,
float, complex, bool, and str data types.
"""


• Don’t repeat the class name in the summary sentence.
• Don’t write “This class…”

### Extended summary¶

The extended summary is an optional sentence or short paragraph that clarifies and supports the summary sentence. Taken together with the summary sentence, the summary content in general exists to help users quickly understand the role and scope of the API.

Leave detailed discussions of the API’s features, usage patterns, background theory, and implementation details to the Notes and Examples sections. The Parameters and Returns sections are ideal places to discuss in detail individual parameters and returned values, respectively.

This section’s brevity is critical. The extended summary is proximate to the summary sentence so that the two pieces of content support each other. However, the extended summary also separates the API signature from the Parameters section, which users expect to see close together. As a general guideline, the extended summary should be three sentences or fewer.

### Parameters¶

For functions, methods and classes.

‘Parameters’ is a description of a function or method’s arguments and their respective types. Parameters should be listed in the same order as they appear in the function or method signature.

For example:

def calcDistance(x, y, x0=0., y0=0., **kwargs):
"""Calculate the distance between two points.

Parameters
----------
x : float
X-axis coordinate.
y : float
Y-axis coordinate.
x0 : float, optional
X-axis coordinate for the second point (the origin,
by default).

Descriptions can have multiple paragraphs, and lists:

- First list item.
- Second list item.
y0 : float, optional
Y-axis coordinate for the second point (the origin,
by default).
**kwargs
calcExternalApi.
"""


Formatting tips:

• Each parameter is declared with a line formatted as {name} : {type} that is justified to the docstring.
• A single space is required before and after the colon (:).
• The name corresponds to the variable name in the function or method’s arguments.
• The type is discussed in Describing parameter types).
• The description is indented by four spaces relative to the docstring and appears without a preceding blank line. Descriptions can have multiple lines, and even multiple paragraphs, lists, and definition lists. Ensure that all of the content in a description is aligned with respect to the first line of the description content. See the x0 parameter in the example, above.
• Normally parameters are documented consecutively, without blank lines between.

#### Describing parameter types¶

Be as precise as possible when describing parameter types. The type description is free-form text, making it possible to list several supported types or indicate nuances. Complex and lengthy type descriptions can be partially moved to the parameter’s description field. The following sections will help you deal with the different kinds of types commonly seen:

##### Concrete types¶

Wrap concrete types in backticks (in docstrings, single backticks are equivalent to :py:obj:) to make a link to either an internal API or an external API that is supported by intersphinx. This works for both built-in types and most importable objects:

Parameters
----------
filename : str
[...]
n : int
[...]
verbose : bool
[...]
items : list or tuple
[...]
magnitudes : numpy.ndarray
[...]
struct : lsst.pipe.base.Struct
[...]


In general, provide the full namespace to the object, such as lsst.pipe.base.Struct. It may be possible to reference objects in the same namespace as the current module without any namespace prefix. Always check the compiled documentation site to ensure the link worked.

##### Choices¶

When a parameter can only assume one of a fixed set of values, those choices can be listed in braces:

order : {'C', 'F', 'A'}
[...]

##### Sequence types¶

When a type is a sequence container (like a list or tuple), you can describe the type of the contents using a PEP 484-like syntax. For example:

mags : list [float]
A sequence of magnitudes.


The type inside the brackets is the type of each item.

Warning

Ensure that you place a space between the container type and the bracket that wraps the item type. Don’t do:

list[float]


list [float]


Leaving out the space results in a parsing error.

If the type of the sequence’s items isn’t known, leave out the item typing and explain with the description:

items : list
A sequence of items, which can be any type.

##### Dictionary types¶

If the types of the keys and values are well-known, document the type of a dict parameter using a PEP 484-like syntax:

measurements : dict [str, astropy.quantity.Quantity]
The keys are names of metrics and values are measurements
as astropy.quantity.Quantity instances, which combine
both value and unit information.


Use the description to explain what the keys and values, since that information often isn’t completely obvious from types alone.

Warning

Ensure that you place a space between the container type and the bracket that wraps the key and value type. Don’t do:

dict[str, str]


dict [str, str]


Leaving out the space results in a parsing error.

If the types of keys and values are completely unknown, simply describe the type as dict and explain in the description.

If the keys are well-known, document the keys using a pattern similar to Struct types, by documenting each key-value pair using a definition list:

settings : dict
Settings dictionary with keys:

"color"
Hex colour code (str).
"size"
Point area in pixels (float).
"complicatedKey"
A key with a complicated description. Like any definition list item,
the content can be wrapped. You can include lists inside the item as well:

- The first item of the list. Lorem ipsum dolor sit amet, consectetur

Proin nulla magna, egestas quis nisi id, dictum mollis diam. Duis lorem
eros, tempor egestas ligula eget, dapibus posuere justo.

- The second item of the list.

You can also include multiple paragraphs in a key's description. Ensure that
all content is aligned with the opening content line.


Notice how the keys are shown by enclosing the quoted strings in double backticks to clarify that the keys are str types.

##### Struct types¶

lsst.pipe.base.Struct parameters are documented similarly to dictionaries with known keys. Describe each attribute of a Struct parameter as a definition list item:

coord : lsst.pipe.base.Struct
Coordinate as a struct with attributes:

x
x-axis coordinate (int).
y
y-axis coordinate (int).
z
z-axis coordinate (int). Nam ut ligula tristique, consequat risus vel,
sodales tellus. Sed sit amet vehicula felis, placerat pharetra nunc:

- Morbi commodo euismod faucibus.
- Fusce quis tortor et ex tincidunt dapibus quis ac lorem. Morbi quis
tellus suscipit quam elementum euismod.

Morbi vehicula facilisis diam ac volutpat. Proin suscipit mi ac ullamcorper
vulputate. Nullam aliquet iaculis aliquam.

##### Array types¶

For Numpy arrays, try to include the dimensionality:

coords : numpy.ndarray, (N, 2)
[...]
flags : numpy.ndarray, (N,)
[...]
image : numpy.ndarray, (Ny, Nx)
[...]


Choose conventional variables or labels to describe dimensions, like N for the number of sources or Nx, Ny for rectangular dimensions.

##### Callable types¶

For callback functions, describe the type as callable:

likelihood : callable
Likelihood function that takes two positional arguments:

- x: current parameter (float).
- extra_args: additional arguments (dict).


#### Optional parameters¶

For keyword arguments with useful defaults, add optional to the type specification:

x : int, optional


Optional keyword parameters have default values, which are automatically documented as part of the function or method’s signature. You can also explain defaults in the description:

x : int, optional
Description of parameter x (the default is -1, which implies summation
over all axes).


#### Additional positional and keyword arguments¶

Functions and methods can take additional positional (*args) and keyword arguments (*kwargs):

def demoFunction(namedArg, *args, flag=False, **kwargs):
"""Demonstrate documentation for additional keyword and
positional arguments.

Parameters
----------
namedArg : str
A named argument that is documented like always.
*args : str

Notice how the type is singular since the user is expected to pass individual
str arguments, even though the function itself sees args as an iterable
of str objects).
flag : bool
A regular keyword argument.
**kwargs
Additional keyword arguments passed to otherApi.

Usually kwargs are used to pass parameters to other functions and
methods. If that is the case, be sure to mention (and link) the
API or APIs that receive the keyword arguments.

If kwargs are being used to generate a dict, use the description to
document the use of the keys and the types of the values.
"""


Order *args and **kwargs as they appear in the signature.

#### Shorthand¶

When two or more consecutive input parameters have exactly the same type, shape and description, they can be combined:

x1, x2 : array-like
Input arrays, description of x1, x2.


### Returns¶

For functions and methods.

‘Returns’ is an explanation of the returned values and their types, in the same format as ‘Parameters’. See the Parameters secton and Describing parameter types for guidelines on how to describe specific types of yielded values.

#### Basic example¶

If a sequence of values is returned, each value may be separately listed, in order:

def getCoord(self):
"""Get the point's pixel coordinate.

Returns
-------
x : int
X-axis pixel coordinate.
y : int
Y-axis pixel coordinate.
"""
return self._x, self._y


#### Naming return variables¶

Note that the names of the returned variables do not necessarily correspond to the names of variables. Simply choose a variable-like name that is clear.

Rename attributes or private names:

def getDistance(self):
"""Get the distance.

Returns
-------
x : float
Distance, in units of pixels.
"""
return self._x


Name variables that don’t have a name within the function scope:

def getDistance(self, x, y):
"""Compute the distance of the point to an (x, y) coordinate.

[...]

Returns
-------
distance : float
Distance, in units of pixels.
"""
return np.hypot(self._x - x, self._y - y)


If the variable has a name in the scope that is useful, feel free to use that:

def getDistance(self, x, y):
"""Compute the distance of the point to an (x, y) coordinate.

[...]

Returns
-------
distance : float
Distance, in units of pixels.
"""
distance = np.hypot(self._x - x, self._y - y)
return distance


Warning

The original Numpydoc Style Guide suggests a short-hand syntax that avoids naming a returned value:

def getDistance(self, x, y):
"""Compute the distance of the point to an (x, y) coordinate.

[...]

Returns
-------
float
Distance, in units of pixels.
"""
return np.hypot(self._x - x, self._y - y)


Don’t use this syntax because it does not render properly.

### Yields¶

For generators.

‘Yields’ is used identically to ‘Returns’, but for generators. See the Parameters secton and Describing parameter types for guidelines on how to describe specific types of yielded values.

Describe the yielded values as singular items yielded from each step, rather than as a sequence of items yielded from all iteration steps.

For example:

def items(self):
"""Iterate over items in the container.

Yields
------
key : str
An item's key.
value : obj
An item's value.
"""
for key, value in self._data.items():
yield key, value


### Other parameters¶

For classes, methods and functions.

‘Other Parameters’ is an optional section used to describe infrequently used parameters. It should only be used if a function has a large number of keyword parameters, to prevent cluttering the Parameters section. In practice, this section is seldom used.

### Raises¶

For classes, methods and functions.

‘Raises’ is an optional section for describing the exceptions that can be raised. You usually cannot document all possible exceptions that might get raised by the entire call stack. Instead, focus on:

• Exceptions that are commonly raised.
• Exceptions that are unique (custom exceptions, in particular).
• Exceptions that are important to using an API.

The ‘Raises’ section looks like this:

Raises
------
IOError
Raised if the input file cannot be read.
TypeError
Raised if parameter example is an invalid type.


Don’t wrap each exception’s name with backticks, as we do when describing types in Parameters and Returns). No namespace prefix is needed when referring to exceptions in the same module as the API. Providing the full namespace is often a good idea, though.

The description text is indented by four spaces from the docstring’s left justification. Like the description fields for Parameters and Returns, the description can consist of multiple paragraphs and lists.

Stylistically, write the first sentence of each description in the form:

Raised if [insert circumstance].


Use the ‘See also’ section to link to related APIs that the user may not be aware of, or may not easily discover from other parts of the docstring. Here are some good uses of the ‘See also’ section:

• If a function wraps another function, you may want to reference the lower-level function.
• If a function is typically used with another API, you can reference that API.
• If there is a family of closely related APIs, you might link to others in the family so a user can compare and choose between them easily.

As an example, for a function such as numpy.cos, we would have:

See also
--------
sin
tan


Numpydoc assumes that the contents of the ‘See also’ section are API names, so don’t wrap each name with backticks, as we do when describing types in Parameters and Returns). No namespace prefix is needed when referring to functions in the same module. Providing the full namespace is always safe, though, and provides clarity to fellow developers:

See also
--------
numpy.sin
numpy.tan


### Notes¶

Notes is an optional section that provides additional conceptual information about the API. Some things to include in a Notes section:

• Discussions of features, going beyond the level of the summary sentence and extended summary.
• Usage patterns, like how code is expected to use this API, or how this API is intended to be used in relation to other APIs.
• Background theory. For example, if the API implements an algorithm, you can fully document the algorithm here.
• Implementation details and limitations, if those details affect the user’s experience. Purely internal details should be written as regular code comments.

Specific how-tos, tutorials, and examples go in the Examples section instead of Notes. The Notes section is dedicated to conceptual documentation.

The Parameters, Returns and Yields sections are the best places to describe specific input and output variables in detail. The Notes section can still reference these variables by name (see Marking up parameter names), and discuss how they work at a big-picture level.

Most reStructuredText formatting is allowed in the Notes section, including:

When using images, remember that many developers and users will be reading the docstring in its raw source form. Images should add information, but the docstring should still be useful and complete without them.

Subsections are not allowed in a Notes section (or any other section). You can simulate subsections with bold text. See also ReStructuredText in docstrings for restrictions.

### References¶

References cited in the ‘Notes’ section are listed here. For example, if you cited an article using the syntax [1]_, include its reference as follows:

References
----------
.. [1] O. McNoleg, "The integration of GIS, remote sensing,
expert systems and adaptive co-kriging for environmental habitat
modelling of the Highland Haggis using object-oriented, fuzzy-logic
and neural-network techniques," Computers & Geosciences, vol. 22,
pp. 585-588, 1996.


Web pages should be referenced with regular inline links.

References are meant to augment the docstring, but should not be required to understand it. References are numbered, starting from one, in the order in which they are cited.

Note

In the future we may support bibtex-based references instead instead of explicitly writing bibliographies in docstrings.

### Examples¶

‘Examples’ is an optional section for usage examples written in the doctest format. These examples do not replace unit tests, but are intended to be tested to ensure documentation and code are consistent. While optional, this section is useful for users and developers alike.

When multiple examples are provided, they should be separated by blank lines. Comments explaining the examples should have blank lines both above and below them:

Examples
--------
A simple example:

3

Comment explaining the second example:

array([4, 6])


For tests with a result that is random or platform-dependent, mark the output as such:

Examples
--------
An example marked as creating a random result:

>>> np.random.rand(2)
array([ 0.35773152,  0.38568979])  #random


It is not necessary to use the doctest markup <BLANKLINE> to indicate empty lines in the output.

Subsections are not allowed in an Examples section (or any other section). You can simulate subsections with bold text. See also ReStructuredText in docstrings for restrictions.

## Documenting modules¶

### Sections in module docstrings¶

Module docstrings contain the following sections:

1. Short summary
2. Extended summary (optional)

Note

Module docstrings aren’t featured heavily in the documentation we generate and publish with Sphinx. Avoid putting important end-user documentation in module docstrings. Instead, write introductory and overview documentation in the module’s user guide (the doc/ directories of Stack packages).

Module docstrings can still be useful for developer-oriented notes, though.

### Placement of module docstrings¶

Module-level docstrings must be placed as close to the top of the Python file as possible: below any #!/usr/bin/env python and license statements, but above imports. See also: Standard code order SHOULD be followed.

Module docstrings should not be indented. For example:

#
# This file is part of dm_dev_guide.
#
# Developed for the LSST Data Management System.
# This product includes software developed by the LSST Project
# (http://www.lsst.org).
# See the COPYRIGHT file at the top-level directory of this distribution
# for details of code ownership.
#
# This program is free software: you can redistribute it and/or modify
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
"""Summary of MyModule.

Extended discussion of my module.
"""

import lsst.afw.table as afwTable
# [...]


## Documenting classes¶

Class docstrings are placed directly after the class definition, and serve to document both the class as a whole and the arguments passed to the __init__ constructor.

### Sections in class docstrings¶

Class docstrings contain the following sections:

1. Short summary
2. Extended summary (optional)
3. Parameters (if applicable)
4. Other parameters (if applicable)
5. Raises (if applicable)
7. Notes (optional)
8. References (optional)
9. Examples (optional)

### Placement of class docstrings¶

Class docstrings must be placed directly below the declaration, and indented according to the code scope:

class MyClass(object):
"""Summary of MyClass.

Parameters
----------
a : str
Documentation for the a parameter.
"""

def __init__(self, a):
pass


The __init__ method never has a docstring since the class docstring documents the constructor.

### Examples of class docstrings¶

Here’s an example of a more comprehensive class docstring with Short Summary, Parameters, Raises, See Also, and Examples sections:

class SkyCoordinate(object):
"""Equatorial coordinate on the sky as Right Ascension and Declination.

Parameters
----------
ra : float
Right ascension (degrees).
dec : float
Declination (degrees).
frame : {'icrs', 'fk5'}, optional
Coordinate frame.

Raises
------
ValueError
Raised when input angles are outside range.

--------
lsst.example.GalacticCoordinate

Examples
--------
To define the coordinate of the M31 galaxy:

>>> m31_coord = SkyCoordinate(10.683333333, 41.269166667)
SkyCoordinate(10.683333333, 41.269166667, frame='icrs')
"""

def __init__(self, ra, dec, frame='icrs'):
pass


## Documenting methods and functions¶

### Sections in method and function docstring sections¶

Method and function docstrings contain the following sections:

1. Short summary
2. Extended summary (optional)
3. Parameters (if applicable)
4. Returns or Yields (if applicable)
5. Other parameters (if applicable)
6. Raises (if applicable)
8. Notes (optional)
9. References (optional)
10. Examples (optional)

### Placement of module and function docstrings¶

Class, method, and function docstrings must be placed directly below the declaration, and indented according to the code scope:

class MyClass(object):
"""Summary of MyClass.

Extended discussion of MyClass.
"""

def __init__(self):
pass

def myMethod(self):
"""Summary of method.

Extended Discussion of myMethod.
"""
pass

def my_function():
"""Summary of my_function.

Extended discussion of my_function.
"""
pass


Again, the class docstring takes the place of a docstring for the __init__ method. __init__ methods don’t have docstrings.

### Dunder methods¶

Special “dunder” methods on classes only need to have docstrings if they are doing anything non-standard. For example, if a __getslice__ method cannot take negative indices, that should be noted. But if __ge__ returns true if self is greater than or equal to the argument, that need not be documented.

### Examples of method and function docstrings¶

Here’s an example function:

def check_unit(self, quantity):
"""Check that a ~astropy.units.Quantity has equivalent units to
this metric.

Parameters
----------
quantity : astropy.units.Quantity
Quantity to be tested.

Returns
-------
is_equivalent : bool
True if the units are equivalent, meaning that the quantity
can be presented in the units of this metric. False if not.

--------
astropy.units.is_equivalent

Examples
--------
Check that a quantity in arcseconds is compatible with a metric defined in arcminutes:

>>> import astropy.units as u
>>> from lsst.verify import Metric
>>> metric = Metric('example.test', 'Example', u.arcminute)
>>> metric.check_units(1.*u.arcsecond)
True

But mags are not a compatible unit:

>>> metric.check_units(21.*u.mag)
False
"""
if not quantity.unit.is_equivalent(self.unit):
return False
else:
return True


## Documenting constants and class attributes¶

### Sections in constant and class attribute docstrings¶

Constants in modules and attributes in classes are all documented similarly. At a minimum, they should have a summary line that includes the type. They can also have a more complete structure with these sections:

1. Short Summary
2. Extended Summary (optional)
3. Notes (optional)
4. References (optional)
5. Examples (optional)

### Placement of constant and class attribute docstrings¶

Docstrings for module-level variables and class attributes appear directly below their first declaration. For example:

MAX_ITER = 10
"""Maximum number of iterations (int).
"""

class MyClass(object):
"""Example class for documenting attributes.
"""

x = None
"""Description of x attribute.
"""


### Examples of constant and class attribute docstrings¶

#### Minimal constant or attribute example¶

Include the attribute or constant’s type in parentheses at the end of the summary line:

NAME = 'LSST'
"""Name of the project (str)."""


#### Multi-section docstrings¶

Multi-section docstrings keep the type information in the summary line. For example:

PA1_DESIGN = 5. * u.mmag
"""PA1 design specification (astropy.units.Quantity).

Notes
-----
The PA1 metric [1]_ is defined so that the rms of the unresolved source
magnitude distribution around the mean value (repeatability) will not
exceed PA1 millimag (median distribution for a large number of sources).

References
----------
.. [1] Z. Ivezic and the LSST Science Collaboration. 2011, LSST Science
Requirements Document, LPM-17, URL https://ls.st/LPM-17
"""


#### Attributes set in __init__ methods¶

In many classes, public attributes are set in the __init__ method. The best way to document these public attributes is by declaring the attribute at the class level and including a docstring with that declaration:

class Metric(object):
"""Verification metric.

Parameters
----------
name : str
Name of the metric.
unit : astropy.units.Unit
Units of the metric.
package : str, optional
Name of the package where this metric is defined.
"""

name = None
"""Name of the metric (str).
"""

unit = None
"""Units of the metric (astropy.units.Unit).
"""

def __init__(self, name, unit, package=None):
self.name = name
self.unit = unit
self._package = package


Notice that the parameters to the __init__ method are documented separately from the class attributes (highlighted).

## Documenting class properties¶

Properties are documented like class attributes rather than methods. After all, properties are designed to appear to the user like simple attributes.

For example:

class Measurement(object):

@property
def quantity(self):
"""The measurement quantity (astropy.units.Quantity).
"""
# ...

@quantity.setter
def quantity(self, q):
# ...

@property
def unit(self):
"""Units of the measurement (astropy.units.Unit, read-only).
"""
# ...


Note:

• Do not use the Returns section in the property’s docstring. Instead, include type information in the summary, as is done for class attributes.
• Only document the property’s “getter” method, not the “setter” (if present).
• If a property does not have a “setter” method, include the words read-only after the type information.

## Complete example module¶

#
# This file is part of dm_dev_guide.
#
# Developed for the LSST Data Management System.
# This product includes software developed by the LSST Project
# (http://www.lsst.org).
# See the COPYRIGHT file at the top-level directory of this distribution
# for details of code ownership.
#
# This program is free software: you can redistribute it and/or modify
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
"""Example Python module with Numpydoc-formatted docstrings.

This module demonstrates documentation written according to LSST DM's
guidelines for Documenting Python APIs with Docstrings_.

Notes
-----
Docstrings have well-specified sections. This is the Notes section. Permitted
sections are listed in Numpydoc Sections in Docstrings_. You can't add
arbitrary sections since they won't be parsed.

Usually we don't write extensive module docstrings. Focus the module docstring
on information that a Stack developer needs to know when working inside that
module. Module *users* typically won't see module docstrings (instead they will
read module documentation topics written in the package's doc/ directory).

.. _Documenting Python APIs with Docstrings:
https://developer.lsst.io/python/numpydoc.html
.. _Numpydoc Sections in Docstrings:
https://developer.lsst.io/python/numpydoc.html#py-docstring-sections
"""

__all__ = ('MODULE_LEVEL_VARIABLE', 'moduleLevelFunction', 'exampleGenerator',
'ExampleClass', 'ExampleError')

MODULE_LEVEL_VARIABLE = 12345
"""Module level variable documented inline (int).

The module variable's type is specified in the short summary, as shown above.
Module variables (constants) can have extended descriptions, like this
paragraph. For a complete list of sections permitted in constant docstrings see
Documenting Constants and Class Attributes_.

.. _Documenting Constants and Class Attributes:
https://developer.lsst.io/docs/py_docs.html#py-docstring-attribute-constants-structure
"""

def moduleLevelFunction(param1, *args, param2=None, **kwargs):
"""Test that two parameters are not equal.

This is an example of a function docstring. Function parameters are
documented in the Parameters section. See *Notes* for the format
specification.

Parameters
----------
param1 : int
The first parameter. Note how the type is marked up with backticks.
This marks int as an API object so that Sphinx will attempt to
link to its reference documentation. You can do this for custom types
as well. You'll see an example in the Returns_ documentation.
*args
Additional positional arguments. If the type is known, it can be
included like usual. Leave out the type if it is not known.
param2 : str, optional
Optional arguments (those with defaults) always include the word
optional after the type info. See the Parameters_ section
documentation for details.
**kwargs
Arbitrary keyword arguments. If you do accept **kwargs, make sure
you link to documentation that describes what keywords are accepted,
or list the keyword arguments as a definition list:

key1:
Description of key1 (int).
key2
Description of key2 (str).

Returns
-------
success : bool
True if successful, False otherwise.

The return type is not optional. The Returns section may span
multiple lines and paragraphs. Following lines should be indented to
match the first line of the description.

The Returns section supports any reStructuredText formatting,
including literal blocks::

{
'param1': param1,
'param2': param2
}

See the Returns_ section documentation for details.

Raises
------
AttributeError
Raised if <insert situation>.
ValueError
Raised if <insert situation>. See the Raises_ section documentation
for details.

Notes
-----
The Notes section is where you can write about the usage patterns and
background for the API. The bulk of the conceptual documentation goes
here, instead of in the extended summary. Save example code for the
"Examples" section.

If *args or **kwargs are accepted, they should be listed as
*args and **kwargs.

The format for a parameter is::

name : type
Description.

The description may span multiple lines. Following lines
should be indented to match the first line of the description.

Multiple paragraphs are supported in parameter descriptions.

--------
exampleGenerator
ExampleClass

Examples
--------
If possible, include an API usage example using the doctest format:

>>> moduleLevelFunction('Hello', param2='World')
True

See the Examples_ section reference for details.

.. _Parameters:
https://developer.lsst.io/python/numpydoc.html#py-docstring-parameters
.. _Returns:
https://developer.lsst.io/python/numpydoc.html#py-docstring-returns
.. _Raises:
https://developer.lsst.io/python/numpydoc.html#py-docstring-raises
.. _Examples:
https://developer.lsst.io/python/numpydoc.html#py-docstring-examples
"""
if param1 == param2:
raise ValueError('param1 may not be equal to param2')
return True

def exampleGenerator(n):
"""Generate an increasing sequence of numbers from 0 to a given limit.

Generators have a Yields section instead of a Returns section.

Parameters
----------
n : int
The upper limit of the range to generate, from 0 to n - 1.

Yields
------
number : int
The next number in the range of 0 to n - 1.

--------
moduleLevelFunction

Examples
--------
Examples should be written in doctest format, and should illustrate how to
use the function:

>>> print([i for i in example_generator(4)])
[0, 1, 2, 3]
"""
for i in range(n):
yield i

class ExampleClass(object):
"""An example class for demonstrating docstrings for classes, methods, and
attributes in the Numpydoc format.

Parameters
----------
param1 : str
Description of param1.
param2 : list of str
Description of param2.
param3 : int, optional
Description of param3.
"""

attr1 = None
"""Description of attr1 (str).
"""

attr2 = None
"""Description of attr2 (list of str).
"""

attr3 = None
"""Description of attr3 (int).
"""

attr4 = None
"""Description of attr4 (list of str).
"""

def __init__(self, param1, param2, param3=None):
self.attr1 = param1
self.attr2 = param2
self.attr3 = param3

self.attr4 = ['attr4']

@property
"""Properties are documented in their getter method (str, read-only).
"""

@property
"""Properties with both a getter and setter are documented in their
getter method (list of str).

If the setter method contains notable behavior, it should be mentioned
here as well.
"""
return self.attr4

self.attr4 = value

def exampleMethod(self, param1, param2):
"""Test that a situation is true.

Parameters
----------
param1 : obj
The first parameter.
param2 : obj
The second parameter.

Returns
-------
success : bool
True if successful, False otherwise.

Notes
-----
Class methods are similar to regular functions. Always use the
imperative mood when writing the one-sentence summary of a method or
function.

Do not include the self parameter in the Parameters section.
"""
return True

def __special__(self):
"""Do something special.

Documentation for a special method.

Notes
-----
Special members are any methods or attributes that start with and end
with a double underscore.

At the moment, special members with docstrings are not published in
the HTML documentation.

You can still write docstrings for them, though.
"""
pass

def _private(self):
"""Do something internally.

By default private members are not included in the HTML docs either.

Notes
-----
underscore and are *not* special. By default they are not included in
the output.

However, you should still provide docstrings for private members to
document code for internal developers.
"""
pass

class ExampleError(Exception):
"""An example exception.

Parameters
----------
msg : str
Human readable string describing the exception.
code : int, optional
Numeric error code.

Notes
-----
Exceptions are documented in the same manner as other classes.

Do not include the self parameter in the Parameters section.
"""

msg = None
"""Human readable string describing the exception (str).
"""

code = None
"""Numeric error code (int).
"""

def __init__(self, msg, code=None):
self.msg = msg
self.code = code


## Acknowledgements¶

These docstring guidelines are derived/adapted from the Numpydoc and Astropy documentation. The example module is adapted from the Napoleon documentation.