Coding style for PyObjC


This document describes the coding style for PyObjC. Please use this style for new code and try apply this style to existing code while working on it.

The management summary: 4-space indents in Python code, 1-TAB indents in C code.

Python code

The coding style for core Python is used (see PEP 8). For consistency with Cocoa we use mixed-case identifiers (like lookUpClass).

PyObjC extensions to Apple frameworks should be clearly marked as such, preferably by prefixing names with PyObjC or pyobjc. This should make it clear to users where they should look for documentation of an item: The Apple documentation or ours.

The for a framework wrapper should defer most work to, like so:

from pyobjc_setup import setup

   description = "Wrappers for the framework AddressBook on macOS",
   packages = [ "AddressBook" ],
   install_requires = [

The framework wrappers do not include a copy of pyobjc-api.h, but dynamically fetch that at build time.

C code

The coding style for core Python is used (see PEP 7). We use PyObjC instead of Py as the prefix for globally visible symbols.


Currently indentation is done using tabs instead of spaces, I’m slowly migrating code to use spaces for indentation.

All (Objective-)C files in Modules/objc/ should include "pyobjc.h" as their first include. The (Objective-)C files in the wrappers for frameworks should include "pyobjc-api.h" and should not use other headers files from pyobjc-core.

  • Use Py_CLEAR to set fields in data structure to NULL instead of using Py_DECREF

  • Use SET_FIELD or SET_FIELD_INCREF to set the value of a field in a data structure, instead of trying to manage the retain count manually.

    Both this and the previous item can avoid hard crashes when the DECREF of the current value causes a callback that accesses the same field before it is set to NULL.

  • Use C99 style struct initialization (at least for larger structs). This makes the code easier to read, and the compiler will warn when you use the wrong field names.


All items exported by the objc module and all PyObjC extensions to Apple frameworks (the AppKit and Foundation modules) should be documented using docstrings.

All documentation– both standalone documents and docstrings– should be marked up using reStructuredText [ReST].

ReST in DocStrings

reStructuredText can be used in doc strings. ReST in DocStrings works exactly like a standalone ReST document, but the ReST is broken up slightly differently.

To format the DocStrings to be ReST compatible, make the following changes/additions to the source. These examples were taken from source found in the DocUtils source tree.

  1. Add appropriate ReST style fields to the top of the document as comments:

    # Author: David Goodger
    # Contact:
    # Copyright: This module has been placed in the public domain.
  2. Add a module level variable that indicates that ReST is used to format the text contained in the docstrings:

    __docformat__ = 'reStructuredText'
  3. Format all other DocStrings as one normally would in Python. Use ReST style markup where appropriate. For example, bulleted lists and sections might commonly appear in the module or class docstrings. The docstrings for individual methods may use example blocks, hyperlinks, or any other ReST markup.