Dev: Coding style

This document describes our coding style. Coding style refers to the following:

• How source code is formatted (indentation, spacing, etc.)
• How things are named (variables, functions, classes, modules, etc.)

General coding conventions

In general, we follow the standard Python style conventions as described in Python's PEP 8, the official Python Style Guide.

Other general comments:

• In a large file, top level classes and functions should be separated by 2 lines to make it easier to separate them visually.

• Use 4 spaces for indentation, never use hard tabs.

• Keep the ordering of methods the same in classes that have the same methods. This is particularly true for classes that implement similar interfaces and for interfaces that are similar.

Naming conventions

For naming conventions, we also follow the guidelines of PEP 8. Some of the existing code doesn't honor this perfectly, but for all new and refactored IPython code, we'll use:

• All lowercase module names. Long module names can have words separated by underscores (really_long_module_name.py), but this is not required. Try to use the convention of nearby files.

• CamelCase for class names.

• lowercase_with_underscores for methods, functions, variables and attributes.

• Implementation-specific private methods will use _single_underscore_prefix. Names with a leading double underscore will only be used in special cases, as they makes subclassing difficult (such names are not easily seen by child classes).

• Occasionally some run-in lowercase names are used, but mostly for very short names or where we are implementing methods very similar to existing ones in a base class (like runlines() where runsource() and runcode() had established precedent).

• The old IPython codebase has a big mix of classes and modules prefixed with an explicit IP of ip. This is not necessary and all new code should not use this prefix. The only case where this approach is justified is for classes or functions which are expected to be imported into external namespaces and a very generic name (like Shell) that is likely to clash with something else. However, if a prefix seems absolutely necessary the more specific IPY or ipy are preferred.

• All JavaScript code should follow these naming conventions as well.

Attribute declarations for objects

In general, objects should declare, in their class, all attributes the object is meant to hold throughout its life. While Python allows you to add an attribute to an instance at any point in time, this makes the code harder to read and requires methods to constantly use checks with hasattr() or try/except calls. By declaring all attributes of the object in the class header, there is a single place one can refer to for understanding the object's data interface, where comments can explain the role of each variable and when possible, sensible deafaults can be assigned.

If an attribute is meant to contain a mutable object, it should be set to None in the class and its mutable value should be set in the object's constructor. Since class attributes are shared by all instances, failure to do this can lead to difficult to track bugs. But you should still set it in the class declaration so the interface specification is complete and documented in one place.

A simple example:

class Foo(object):
    # X does..., sensible default given:
    x = 1
    # y does..., default will be set by constructor
    y = None
    # z starts as an empty list, must be set in constructor
    z = None
    
    def __init__(self, y):
        self.y = y
        self.z = []

New files

When starting a new Python file for IPython, you can use the following template as a starting point that has a few common things pre-written for you.