Python Coding Standards


  • Unless otherwise specified, follow PEP 8; this means using snake_case

  • Use black to format Python code with the configuration in the pyproject.toml in the root of the repository

  • Always use four spaces for indentation

  • For docstrings please follow Docstring Conventions PEP 257 and document code to aid sphinx

Formatting with black

To ensure that formatting matches across all developers please use the latest version of black. This is installed by the mantid-developer conda environment files and will eventually be enforced by a pre-commit hook. black automatically finds the configuration in the pyproject.toml file when executed inside the Mantid source directory. To format a given file you may use python -m black path/to/

To automatically format any Python files you have changed prior to commiting you may use the command: git diff --name-only --cached | grep '\.py' | xargs python -m black. This will in place format any files with the .py extension that you have staged. Adding configuration of black into pre-commit is planned.

The editor integration from black’s docuementation provide details.

None checks

Prefer if obj is not None: over if obj:. The latter invokes object.__nonzero__ whereas the former simply compares that obj references the same object as None.


Include the following licence agreement at the top of each file, replacing YEAR with the appropriate year:

# Mantid Repository :
# Copyright © YEAR ISIS Rutherford Appleton Laboratory UKRI,
#     NScD Oak Ridge National Laboratory, European Spallation Source
#     & Institut Laue - Langevin
# SPDX - License - Identifier: GPL - 3.0 +


Imports should be grouped in the following order:

  1. stdlib

  2. third party libraries

  3. local modules

Each group should be alphabetically sorted and separate by a newline, e.g.

import sys

from qtpy.QtWidgets import QMainWindow

from mypackage.subpkg import MyClass


Use the qtpy module to hide the differences between PyQt versions. When working with signals, use the new style. For naming new custom signals, use the sig_ prefix:

from qtpy.QtCore import Signal

class MyWidget(...):
    """Funky new widget"""

    # Signals
    sig_run_a_thing_happened = Signal(str, str, str, bool, bool)