DM Python Style Guide

This is the version 6.0 of the DM Python Coding Standard. The Introduction to DM’s Code Style Guides provides the overarching Coding Standards policy applicable to all DM code.


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


0. Python Version

All DM Python code MUST work with our standard environment

All the Python code written by LSST Data Management must run under the version of Python provided in our standard environment. Any feature available in that version of Python may be used. There is no requirement to support Python 2 or earlier Python 3 versions.

1. PEP 8 is the Baseline Coding Style

Data Management’s Python Coding Style is based on the PEP 8 Style Guide for Python Code with modifications specified in this document.

PEP 8 is used throughout the Python community and should feel familiar to Python developers. DM’s deviations from PEP 8 are primarily motivated by consistency with the DM C++ Style Guide. Additional guidelines are included in this document to address specific requirements of the Data Management System.

Exceptions to PEP 8

The following table summarizes all PEP 8 guidelines that are not followed by the DM Python Style Guide. These exceptions are organized by error codes that may be ignored by the flake8 linter (see Code MAY be validated with flake8).


Closing bracket is missing indentation. This pycodestyle error (via flake8) is not part of PEP 8.


Missing whitespace around arithmetic operator. See Binary operators SHOULD be surrounded by a single space except for [*, /, **, //, %].


Missing whitespace around bitwise or shift operator. See Binary operators SHOULD be surrounded by a single space except for [*, /, **, //, %].

Maximum line length

See Line Length MUST be less than or equal to 110 columns.

The style checker in pycodestyle also provides warnings that can be used to request a specific style that is ambiguous in PEP 8. These codes should be ignored to choose the LSST preferred style:


Line break before binary operator. Disabling this enables W504 that checks that line breaks occur before binary operators. See Wrap lines before binary operators.

Additionally, packages listed in Naming Conventions for Science Pipelines should disable the following rules:


Function name should be lowercase. See Naming Conventions for Science Pipelines.


Argument name should be lowercase. See Naming Conventions for Science Pipelines.


Variable in function should be lowercase. See Naming Conventions for Science Pipelines.


Lowercase imported as non lowercase. See Naming Conventions for Science Pipelines.


Camelcase imported as lowercase. See Naming Conventions for Science Pipelines.


MixedCase variable in class scope. See Naming Conventions for Science Pipelines.


MixedCase variable in global scope. See Naming Conventions for Science Pipelines.

Code MAY be validated with flake8

The flake8 tool may be used to validate Python source code against the portion of PEP 8 adopted by Data Management. Additionally, flake8 statically checks Python for code errors. The separate pep8-naming plugin validates names according to the DM Python Style Guide.


Flake8 validates code against PEP 8 specifications and also allows some configuration for line lengths and cases where PEP8 allows some ambiguity but where we require consistency. This style guide includes additional guidelines that are not automatically linted.

Flake8 installation

Flake8 comes installed with any LSST Science Pipelines installation, and is run automatically when building packages with scons, during manual pytest invocation, and as Github Actions on new Pull Requests. All of our existing packages, and any package created via our template system, come pre-configured with the necessary Flake8 configuration files.

Flake8 configuration files

flake8 can be invoked without arguments when a configuration file is present; existing Science Pipelines packages already include this file. This configuration, included in a setup.cfg file at the root of code repositories, is consistent with the style guide:

max-line-length = 110
max-doc-length = 79
ignore = E133, E226, E228, N802, N803, N806, N812, N813, N815, N816, W503
exclude =

The exclude field lists paths that are not usefully linted by flake8 in DM Stack repositories. Auto-generated Python should not be linted (including bin/ for Stack packages with bin.src/ directories). We also discourage linting modules due to the abundance of PEP 8 exceptions typically involved.

Code MAY be validated with ruff

The ruff tool may be used to validate Python source code against the portion of PEP 8 adopted by Data Management. ruff is significantly faster than flake8 to execute and this can be useful for larger projects. It also is far more configurable with many built-in linting extensions. Sometimes ruff will suggest fixes that can be automated. Use the --fix option to apply the automated fixes, and then examine them before committing them to make sure they look correct.

Ruff installation

Ruff comes installed with any LSST Science Pipelines installation using rubin-env 7.0 or newer. It is run automatically when building packages with scons if a ruff configuration is found.

Ruff configuration files

Science Pipelines packages do not come with a ruff configuration by default and one must be added if you wish to use the linting tool with your package. Once configured SCons will prefer to run this tool over flake8. The configuration for ruff must be added to the pyproject.toml file – this will be the same file used to configure other tools such as black and isort.

exclude = [
ignore = [
line-length = 110
select = [
    "E",  # pycodestyle
    "F",  # pyflakes
    "N",  # pep8-naming
    "W",  # pycodestyle
target-version = "py311"
extend-select = [
    "RUF100", # Warn about unused noqa

max-doc-length = 79

convention = "numpy"

The above is a complete and sufficient specification for a science pipelines package. There is no need to explicitly specify W503 since ruff assumes that by default. If you wish to add more linting rules that are consistent with the style guide these can be specified in the section. For example:

  • D will check docstrings for compliance. The style guide does not yet specify an agreed set of pydocstyle codes to be ignored.

  • UP will check for code from older versions of Python that can be modernized.

  • C4 will check for situations where a list or dict comprehension might be preferable.

  • B enables “bugbear” checks for possible logic problems.

  • SIM can make suggestions for places where your code can be simplified.

There is no requirement to use these but also no restriction from adding them to the pyproject.toml configuration if they are consistent with the general style guide. To temporarily see the effect of a specific extension use the --select option to ruff:

$ ruff --select UP,B .
python/lsst/pipe/tasks/ UP031 [*] Use format specifiers instead of percent format
python/lsst/pipe/tasks/ B905 `zip()` without an explicit `strict=` parameter
[*] nn potentially fixable with the --fix option.

You may temporarily add these configurations to check for possible issues.

Lines that intentionally deviate from DM’s PEP 8 MUST include a noqa comment

Lines of code may intentionally deviate from our application of PEP 8 because of limitations in flake8. In such cases, authors must append a # noqa comment to the line that includes the specific error code being ignored. See the flake8 documentation for details . This prevents the line from triggering false flake8 warnings to other developers, while also linting unexpected errors.

For example, to import a module without using it (to build a namespace, as in a

from .module import AClass  # noqa: F401

Black MAY be used to fix PEP 8 compliance

Many PEP 8 issues in existing code can be fixed with black. If black is used for a repository the codebase should continue to use it to ensure consistency.


When using black there is no control over the whitespace around operators. The benefits of consistency from using black outweigh the reasons for using a bespoke rule and therefore the PEP-8 exceptions noted elsewhere (E133, E226, and E228) can be removed.

autopep8 MAY be used to fix PEP 8 compliance

Many PEP 8 issues in existing code can be fixed with autopep8 version 1.2 or newer:

autopep8 . --in-place --recursive \
    --ignore E133,E226,E228,N802,N803,N806,W503 --max-line-length 110

The . specifies the current directory. Together with --recursive, the full tree of Python files will be processed by autopep8. Alternatively, a single file can be specified in place of ..

autopep8ʼs changes must always be validated before committing.

Style changes must be encapsulated in a distinct commit (see Commits should represent discrete logical changes to the code).


autopep8 only fixes PEP 8 issues and does not address other guidelines listed here.

2. Layout

See also

Documenting Python APIs with docstrings provides guidelines for the layout of docstrings.

Each Python file MUST contain the standard license preamble

A copyright and license block using the standard text MUST be included at the top of each file. This can be written as a Python comment.

# This file is part of {{ cookiecutter.package_name }}.
# Developed for the LSST Data Management System.
# This product includes software developed by the LSST Project
# (
# 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
# it under the terms of the GNU General Public License as published by
# 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
# 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 <>.

Replace {{ cookiecutter.package_name }} with the package’s name.

Line Length MUST be less than or equal to 110 columns

Limit all lines to a maximum of 110 characters. This conforms to the DM C++ Style Guide (see 4-6).

This differs from the PEP 8 recommendation of 79 characters.

Note that indentation spaces are counted in this line length. This allows editors to be set with fixed line limits. If you find that indentation spaces are consuming a significant fraction of your code line, consider refactoring the code.

Docstring and comment line length MUST be less than or equal to 79 columns

Limit all docstring and comment lines to a maximum of 79 characters.

This differs from the PEP 8 recommendation of 72 characters and the numpydoc recommendation of 75 characters but maintains readability and compatibility with default terminal widths while providing the maximum space.

As for code lines, indentation spaces are counted in this line length to allow editors to be set with fixed line limits.

Python’s implied continuation inside parens, brackets and braces SHOULD be used for wrapped lines

The preferred way of wrapping long lines is by using Python’s implied line continuation inside parentheses, brackets and braces.

If necessary, you can add an extra pair of parentheses around an expression, but sometimes using a backslash looks better. In this example, continuation is naturally implied within the __init__ method argument lists, while both \ and parentheses-based continuations are used in the if statements.

class Rectangle(Blob):
    """Documentation for Rectangle.
    def __init__(self, width, height,
                 color='black', emphasis=None, highlight=0):

        # Discouraged: continuation with '\'
        if width == 0 and height == 0 \
               and color == 'red' and emphasis == 'strong' \
               or highlight > 100:
            raise ValueError("sorry, you lose")

        # Preferred: continuation with parentheses
        if width == 0 and height == 0 and (color == 'red'
                                           or emphasis is None):
            raise ValueError("I don't think so")

        Blob.__init__(self, width, height,
                      color, emphasis, highlight)

Be aware that the continued line must be distinguished from the following lines through indentation. For example, this will generate an E129 error:

if (width == 0
    and height == 0):

Instead, the continued line should be indented:

if (width == 0
        and height == 0):

Consistency with the DM C++ Coding Guide namespaces SHOULD be followed

Consistency with the LSST C++ Coding Standards namespaces exists.


  • from import myFunction is analogous to using lsst::foo::bar::myFunction

  • import as fooBar is analogous to namespace fooBar = lsst::foo::bar

Disallowed in both Coding Standards (except in library initialization contexts):

  • from import * is analogous to using namespace lsst::foo::bar

3. Whitespace

Follow the PEP 8 whitespace style guidelines, with the following adjustments.

The minimum number of parentheses needed for correctness and readability SHOULD be used


a = b(self.config.nSigmaToGrow*sigma + 0.5)

Less readable:

a = b((self.config.nSigmaToGrow*sigma) + 0.5)

Binary operators SHOULD be surrounded by a single space except for [*, /, **, //, %]

Always surround these binary operators with a single space on either side; this helps the user see where one token ends and another begins:

  • assignment (=),

  • augmented assignment (+=, -=, etc.),

  • comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not),

  • Booleans (and, or, not).

Use spaces around these arithmetic operators:

  • addition (+),

  • subtraction (-)

Never surround these binary arithmetic operators with whitespace:

  • multiplication (*),

  • division (/),

  • exponentiation (**),

  • floor division (//),

  • modulus (%). Note that a single space must always surround % when used for string formatting.

For example:

i = i + 1
submitted += 1
x = x*2 - 1
hypot2 = x*x + y*y
c = (a + b)*(a - b)
print('Hello %s' % 'world!')

This deviates from PEP 8, which allows whitespace around these arithmetic operators if they appear alone. Error codes: E226 and E228.

Wrap lines before binary operators

PEP 8 suggests that lines should be broken before binary operators but allows after. For consistency with modern Python conventions and tools such as black, and languages such as SQL, we choose before.

This requires that W503 be disabled in pycodestyle.


Source code comments should follow PEP 8’s recommendations with the following additional requirements.

Comments MUST always remain up-to-date with code changes

Comments that contradict the code are worse than no comments. Always make a priority of keeping the comments up-to-date when the code changes!

Sentences in comments SHOULD NOT be separated by double spaces

Following PEP 8, comments should be complete sentences.

However, sentences should not be separated by two spaces; a single space is sufficient.

This differs from PEP 8.

Block comments SHOULD reference the code following them and SHOULD be indented to the same level

Block comments generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment).

Paragraphs inside a block comment are separated by a line containing a single #.

To-do comments SHOULD include a Jira issue key

If the commented code is a workaround for a known issue, this rule makes it easier to find and remove the workaround once the issue has been resolved. If the commented code itself is the problem, this rule ensures the issue will be reported on Jira, making it more likely to be fixed in a timely manner.

# TODO: workaround for DM-6789
# TODO: DM-12345 is triggered by this line

5. Documentation Strings (docstrings)

Use Numpydoc to format the content of all docstrings. The page Documenting Python APIs with docstrings authoritatively describes this format. Its guidelines should be treated as an extension of this Python Style Guide.

See also

The ReStructuredText Style Guide—and the RestructuredText Formatting Conventions section in particular—provide guidelines on reStructuredText in general.

Docstrings SHOULD be written for all public modules, functions, classes, and methods

Write docstrings for all public modules, functions, classes, and methods. See Documenting Python APIs with docstrings.

Docstrings are not necessary for non-public methods, but you should have a comment that describes what the method does. This comment should appear after the def line.

6. Naming Conventions

We follow PEP 8ʼs naming conventions, with exceptions listed here. C++ source code included within a Python package SHOULD follow the naming conventions of the Python package for APIs that are to be visible to Python users.

All LSST Python source code is consistent with PEP 8 naming in the following ways:

  • class names are CamelCase with leading uppercase,

  • module variables used as module global constants are UPPERCASE_WITH_UNDERSCORES,

Some packages, for historical reasons, do not fully adhere to PEP 8. These packages, and the associated naming conventions, are described in Naming Conventions for Science Pipelines. Naming style SHOULD be consistent within a top-level package built by Jenkins, or within a distinct service, and it is RECOMMENDED that PEP 8 naming convention be adopted, whilst understanding that it may be difficult to modify existing packages. Consistency within a package is mandatory. Within these stated constraints new packages SHOULD use PEP 8 naming conventions.

Names may be decorated with leading and/or trailing underscores.

Naming Conventions for Science Pipelines

For historical reasons, Science Pipelines code (nominally, all packages included in the lsst_apps metapackage, as well as meas_*, pipe_*, and obs_* and all dependencies), does not completely adhere to PEP 8-style.

PEP 8 style is used in the following cases:

  • class names are CamelCase with leading uppercase,

  • module variables used as module global constants are UPPERCASE_WITH_UNDERSCORES,

but all other names are traditionally camelCase with leading lowercase, and local (class, file, module) consistency in naming is important. In particular:

  • Class Attribute Names MAY be camelCase with leading lowercase (Error code: N803).

  • Module methods (free functions) MAY be camelCase with leading lowercase (Error code: N802)

  • Compound variable names MAY be camelCase with leading lowercase (Error code: N806).

Recognizing that this style is becoming less common in Python at large, new Science Pipelines code MAY also be be written in full PEP 8 style, according to the following guidelines:

  • PEP 8 style (“snake_case”) public names are preferred in “primarily new” code, such as new modules and classes that do not primarily implement an existing camelCase interface. “snake_case” local variables names are similarly preferred in new functions, even functions with a camelCase public API. The strict definition of “primarily new code” is case by case and left to developer judgement.

  • camelCase is still permitted in new code, and is preferred when adding public methods, arguments, and attributes to existing classes with an established camelCase API, or making modifications to existing functions with camelCase local variables.

  • Under no circumstances should the Python side of a pybind11-wrapped C++ function use a different naming convention than the C++ function. C++ naming conventions and the guidelines for applying them in new/old code are no different from those of Python, but it is never permitted to just change things at the pybind11 level; either both C++ and Python or neither should be changed.

Changing existing names from camelCase to snake_case is generally discouraged as unnecessary churn, and should only be done in highly localized code (e.g. individual function bodies) that already being extensively modified. Name changes to public interfaces are of course API changes, and must go through the usual RFC and deprecation procedure.

Modules which contain class definitions SHOULD be named after the class name

Modules which contain class definitions should be named after the class name (one module per class).

User defined names SHOULD NOT shadow python built-in functions

Names which shadow a python built-in function may cause confusion for readers of the code. Creating a more specific identifier is suggested to avoid collisions. For example, in the case of filter, filter_name may be appropriate; for filter objects, something like filter_obj might be appropriate.

Names l (lowercase: el), O (uppercase: oh), I (uppercase: eye) MUST be avoided

Never use these characters as single character variable names:

  • l (lowercase letter el),

  • O (uppercase letter oh), or

  • I (uppercase letter eye).

In some fonts, these characters are indistinguishable from the numerals one and zero. When tempted to use l, use L instead.


This matches the PEP 8 standard but is repeated here for emphasis.

Always use cls for the first argument to metaclass instance methods

For regular classes self is used, but for class methods and hence also for metaclass instance methods, cls should be used instead.


This is consistent with the naming conventions in PEP 8 as indicated explicitly by upstream.

Import statements SHOULD follow a standard pattern

You may choose either to write module import statements so that they import the fully-qualified module name:


or you may use as to assign a short name to the module:

import as fooBar

Short names are typically formed by dropping lsst. and combining the rest of the name using camel-case. They should be consistent between Python and C++ (see C++ Style Guide rule 5-42).

When working with an established part of the codebase — editing an existing file, or working within an existing package — the import style should be consistent with the existing code.

7. Source Files & Modules

A Python source file name SHOULD be camelCase-with-leading-lowercase or snake_case, and end in ‘.py’

A module containing a single class should be a camelCase-with-leading-lowercase transliteration of the class’s name (if the code within primarily adheres to the older, camelCase version of Science Pipelines naming conventions) or a snake_case version of the class’s name (if the code within primarily adheres to the full PEP 8 naming conventions).

Test files must have the form test_{description}.py for compatibility with Pytest. The name of a test case should be descriptive without the need for a trailing numeral to distinguish one test case from another.

This rule does not apply to executable script files, for which both no extension and a ‘.py’ extension are acceptable. Script files should always be minimal (ideally a single non-import statement), and delegate any actual logic to importable code. This maximizes usability from other Python code (including test code) and makes it much easier to include script interfaces in documentation. It also means the impact of having no extension on tools that rely on the file extension should be negligible. Legacy scripts that do contain signficant logic should have a ‘.py’ script to support this tooling.

ASCII Encoding MUST be used for new code

Always use ASCII for new Python code.

  • Do not include a coding comment (as described in PEP 263) for ASCII files.

  • Existing code using Latin-1 encoding (a.k.a. ISO-8859-1) is acceptable so long as it has a proper coding comment. All other code must be converted to ASCII or Latin-1 except for 3rd party packages used “as is.”

Standard code order SHOULD be followed

Within a module, follow the order:

  1. Shebang line, #! /usr/bin/env python (only for executable scripts)

  2. Module-level comments (such as the license statement)

  3. Module-level docstring

  4. __all__ = [...] statement, if present

  5. Imports

  6. Private module variables (names start with underscore)

  7. Private module functions and classes (names start with underscore)

  8. Public module variables

  9. Public functions and classes

8. Classes

See also

Designing for Inheritance in PEP 8 describes naming conventions related to public and private class APIs.

super MAY be used to call parent class methods

If you are overriding a method from a parent class, use super to call the parent class’s method. For example:

class B(object):
    def method(self, arg): = arg

class C(B):
    def method(self, arg):


Using super ensures a consistent Method Resolution Order, and prevents inherited methods from being called multiple times. In Python 3, super does not require naming the class that it is part of, making its use simpler and removing a maintenance issue.

super() and Multiple Inheritance

In the presence of multiple inheritance (two or more parents, e.g. class C(A, B)), the trickiest issue with the use of super is that the class author generally doesn’t know a priori which overridden method will be called in what order. In particular, this means that the calling signature (arguments) for all versions of a method must be compatible. As a result, there are a few argument-related caveats about the use of super in multiple inheritance hierarchies:

  • Only pass super the exact arguments you received.

  • When you use it on methods whose acceptable arguments can be altered on a subclass via addition of more optional arguments, always accept *args, **kwargs, and call super like super().currentmethod(arg1, arg2, ..., *args, **kwargs). If you don’t do this, document that addition of optional arguments in subclasses is forbidden.

  • Do not use positional arguments in __init__ or __new__. Instead, use keyword args in the declarations, always call them using keywords, and always pass all keywords on, e.g. super().__init__(**kwargs).

To use super with multiple inheritance, all base classes in Python’s Method Resolution Order need to use super; otherwise the calling chain gets interrupted. If your class may be used in multiple inheritance, ensure that all relevant classes use super including documenting requirements for subclasses.

For more details, see the super documentation, the astropy coding guide, and this article from Raymond Hettinger.

9. Comparisons

is and is not SHOULD only be used for determining if two variables point to same object

Use is or is not only for the case that you need to know that two variables point to the exact same object.

To test for equality in value, use == or != instead.

is and is not SHOULD be used when comparing to None

There are two reasons:

  1. is None works with NumPy arrays, whereas == None does not;

  2. is None is idiomatic.

This is also consistent with PEP 8, which states:

Comparisons to singletons like None should always be done with is or is not, never the equality operators.

For sequences, (str, list, tuple), use the fact that empty sequences are False.


if not seq:

if seq:


if len(seq):

if not len(seq):

10. Idiomatic Python

Strive to write idiomatic Python. Writing Python with accepted patterns makes your code easier for others to understand and often prevents bugs.

Fluent Python by Luciano Ramalho is an excellent guide to writing idiomatic Python.

Idiomatic Python also reduces technical debt. For more information see the online book Supporting Python 3 by Lennart Regebro.

A mutable object MUST NOT be used as a keyword argument default

Never use a mutable object as default value for a keyword argument in a function or method.

When used a mutable is used as a default keyword argument, the default can change from one call to another leading to unexpected behavior. This issue can be avoided by only using immutable types as defaults.

For example, rather than provide an empty list as a default:

def proclist(alist=[]):

this function should create a new list in its internal scope:

def proclist(alist=None):
    if alist is None:
        alist = []

Context managers (with) SHOULD be used for resource allocation

Use the with statement to simplify resource allocation.

For example to be sure a file will be closed when you are done with it:

with open('/etc/passwd', 'r') as f:
    for line in f:

Avoid dict.keys() when iterating over keys or checking membership

For iterating over keys, iterate over the dictionary itself, e.g.:

for x in mydict:

To test for inclusion use in:

if key in myDict:

This is preferred over keys. Use keys when storing the keys for later access:

k = list(mydict.keys())

where list ensures that a view or iterator is not being retained.

The subprocess module SHOULD be used to spawn processes

Use the subprocess module to spawn processes.

lambda SHOULD NOT be used

Avoid the use of lambda. You can almost always write clearer code by using a named function or using the functools module to wrap a function.

The set type SHOULD be used for unordered collections

Use the set type for unordered collections of objects.

The argparse module SHOULD be used for command-line scripts

Use the argparse module for command-line scripts.

Command line tasks for pipelines should use lsst.pipe.base.ArgumentParser instead.

Iterators and generators SHOULD be used to iterate over large data sets efficiently

Use iterators, generators (classes that act like iterators) and generator expressions (expressions that act like iterators) to iterate over large data sets efficiently.

if False: and if True: SHOULD NOT be used

Code must not be placed inside if False: or if True: blocks, nor left commented out. Instead, debugging code and alternative implementations must be placed inside a “named” if statement. Such blocks should have a comment describing why they are disabled. They may have a comment describing the conditions under which said code can be removed (like the completion of a ticket or a particular date). For example, for code that will likely be removed in the future, once testing is completed:

# Delete old_thing() and the below "if" statement once all unittests are finished (DM-123456).
use_old_method = False
if use_old_method:

It is often beneficial to lift such debugging flags into the method’s keyword arguments to allow users to decide which branch to run. For example:

def foo(x, debug_plots=False):
    if debug_plots:

or, using lsstDebug, which can be controlled as part of a command line task:

import lsstDebug
def foo(x):
    if lsstDebug.Info(__name__).debug_plots:

The deprecated decorator SHOULD be used to deprecate code

For more on deprecating code, see Deprecating Interfaces.

Properties SHOULD be used when they behave like regular instance attributes

Properties SHOULD be added to Python objects to provide syntactic sugar for a getter (and possibly setter) when all of the following conditions are true:

  • The getter method must return the same type the setter method accepts, or the types must have very similar interfaces (e.g. because they are part of the same class hierarchy, or they share an important common interface, such as a Python Sequence).

  • Either the returned object must be immutable or modifying it must modify the object on which the property is defined in the expected way. Note that it may be useful to have a getter return an immutable object (e.g. tuple instead of list) to meet this criterion. This prevents confusing behavior in which a.b.c = v could be a silent no-op.

  • The getter (and setter, if it exists) must be computationally trivial; either the direct return of an internal object or an extremely simple calculation (e.g. the width of a bounding box from its starting and ending x coordinates). In general, getter methods that begin with something other than “get” should not have associated properties.

Some examples:

  • Image.getBBox() SHOULD NOT have an associated property, because the returned object (Box2I) is mutable, but modifying it does not modify the bounding box of the Image.

  • Psf.computeShape() SHOULD NOT have an associated property, because the getter is not computationally trivial - as suggested by the method name.

  • Image.getArray() SHOULD have an associated property, because the returned object is a view that can be modified to modify the original image.

  • Exposure.getWcs() SHOULD have an associated property, because the returned object is a data member of the Exposure that is returned via shared_ptr in C++, which allows modifications to the Wcs to automatically affect the Exposure.

Note that C++ getters that return STL container types cannot have properties in Python unless the usual pybind11 conversion (which typically yields list, dict, or set objects) is augmented with a conversion to an immutable type (such as tuple or frozenset), because these conversions otherwise always yield mutable objects that do not modify the parent.

The existing getters and setters MUST NOT be removed when defining a property.

Python exceptions SHOULD be raised and checked in Python code

When raising an exception in Python code, consideration should be given to defining a module-specific exception for increased precision. Such an exception SHOULD inherit from an appropriate standard Python exception, unless it also needs to be thrown from C++ code, in which case it MUST be defined using the LSST-specific pex_exceptions library. If a module-specific exception is not used, then the appropriate standard Python exception SHOULD be raised.

When writing an except clause, the exception type caught SHOULD be, in order of preference, a module-specific exception (either Python or C++), a standard Python exception, or a generic pex_exceptions exception for which there is no corresponding Python exception. In particular, most generic pex_exceptions exceptions should be caught as their standard Python counterparts. For example, catch lsst.pex.exceptions.OverflowError as the Python OverflowError, but catch lsst.pex.exceptions.LengthError as such.