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Python Style Guide

Python code should adhere to PEP 8, with the following clarifications:

  • Only use four spaces for indentation, no tabs.
  • Use Unix-style end of line (LF, aka '\n' character).
  • Spaces around operators (except for keyword arguments).
  • Use CamelCase for classes and exception types. Use underscore_case for everything else.

Automated Formatting

Most Python code is automatically formatted using autopep8.

Use make format to format both C/C++ and Python code in the entire repository, or integrate it in your IDE.

Additions to PEP 8

Naming

  • Avoid shadowing names of Python built-in functions, constants, types, and exceptions. Instead of using object, use obj; as for other names, the Python built-ins are very generic, so often it is better to be more specific in your naming. For example, instead of naming a list list, name it objects_to_export. Not only does this avoid shadowing a built-in name, it also is more explicit and emphasizes the contents of the list. If you want to make the type of the variable explicit, use type annotations.
  • Avoid overly short names. Use mesh and curve instead of me and cu (or worse, m and c).

Unused Variables & Arguments

It's sometimes necessary to declare variables or arguments that aren't used, in this case it's a common convention to begin the variable with an underscore.

Tools such as pylint will skip them when reporting unused variables.

For example, a menu's draw method always takes a context argument, which you don't necessarily need.

class EXAMPLE_MT_menu(Menu):
    bl_label = ""

    def draw(self, _context):
        layout = self.layout
        layout.menu("EXAMPLE_MT_submenu_a")
        layout.menu("EXAMPLE_MT_submenu_b")

Or when iterating over a sequence some items may not be used.

for (vert, _edge, face) in elements:
    process(vert, face)

Exceptions to PEP 8

  • Line width:
    Maximum width for lines is 120 for all scripts.
  • Imports:
    We often put imports within functions body which is not pep8 compliant.
    This is done to speed up Blender's startup times to avoid loading in many files and libraries which the user may never access.

Conventions for Core Scripts

These scripts run as part of Blender at startup (scripts/startup and scripts in scripts/modules that run on startup). There are some additional conventions for these scripts.

  • Postpone importing Python modules, where imports may be in a function or a method body.

This is done for faster startup times and avoid unnecessary overhead when running tests or rendering in background mode.

  • No type annotations (besides those necessary for bpy.props use).

Currently we don't have a practical way to validate these, see this task for details.

  • Use percentage string formatting (no f-string of str.format(..) usage).

This is done because percentage formatting works well for string literals that need to be translated into other languages and it keeps the code-base simpler. While there are advantages to alternative string-formatting methods, core-scripts typically perform primitive operations, so the advantages to using other methods isn't significant, (see: Python String Formatting discussion).

  • Single quotes for enumerator literals, double quotes for everything else.

Use single quotes for enums in checks such as ob.type == 'MESH', double quotes for other strings such as layout.label(text="Label Text").