Python Style Guide

Python Style Guide

Author: Guido van Rossum

This style guide has been converted to several PEPs (Python Enhancement Proposals): PEP 8 for the main text, PEP 257 for docstring conventions. See the PEP index.

A Foolish Consistency is the Hobgoblin of Little Minds

But most importantly: know when to be inconsistent -- sometimes the style guide just doesn't apply. When in doubt, use your best judgement. Look at other examples and decide what looks best. And don't hesitate to ask!

Table of Contents

• Lay-out -- how to use tabs, spaces, and newlines.
• Comments -- on proper use of comments (and documentation strings).
• Names -- various naming conventions.

Lay-out

Indentation

Tabs or Spaces?

Maximum Line Length

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. Make sure to indent the continued line appropriately. Emacs Python-mode does this right. Some examples:

class Rectangle(Blob):

    def __init__(self, width, height,
                 color='black', emphasis=None, highlight=0):
        if width == 0 and height == 0 and \
           color == 'red' and emphasis == 'strong' or \
           highlight > 100:
            raise ValueError, "sorry, you lose"
        if width == 0 and height == 0 and (color == 'red' or
                                           emphasis is None):
            raise ValueError, "I don't think so"
        Blob.__init__(self, widt, height,
                      color, emphasis, highlight)

Blank Lines

When blank lines are used to separate method definitions, there is also a blank line between the `class' line and the first method definition.

Use blank lines in functions, sparingly, to indicate logical sections.

Whitespace in Expressions and Statements

Pet Peeves

• Immediately inside parentheses, brackets or braces, as in: spam( ham[ 1 ], { eggs: 2 } ). Always write this as spam(ham[1], {eggs: 2}).

• Immediately before a comma, semicolon, or colon, as in: if x == 4 : print x , y ; x , y = y , x. Always write this as if x == 4: print x, y; x, y = y, x.

• Immediately before the open parenthesis that starts the argument list of a function call, as in spam (1). Always write this as spam(1).

• Immediately before the open parenthesis that starts an indexing or slicing, as in: dict ['key'] = list [index]. Always write this as dict['key'] = list[index].

• More than one space around an assignment (or other) operator to align it with another, as in:

  x             = 1
  y             = 2
  long_variable = 3
  

  Always write this as

  x = 1
  y = 2
  long_variable = 3
  

Other Recommendations

• Always surround these binary operators with a single space on either side: assignment (=), comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not), Booleans (and, or, not).

• Use your better judgement for the insertion of spaces around arithmetic operators. Always be consistent about whitespace on either side of a binary operator. Some examples:

  i = i+1
  submitted = submitted + 1
  x = x*2 - 1
  hypot2 = x*x + y*y
  c = (a+b) * (a-b)
  c = (a + b) * (a - b)
  

• Don't use spaces around the '=' sign when used to indicate a keyword argument or a default parameter value. For instance:

  def complex(real, imag=0.0):
      return magic(r=real, i=imag)
  

Comments

If a comment is a phrase or sentence, its first word should be capitalized, unless it is an identifier that begins with a lower case letter (never alter the case of identifiers!).

If a comment is short, the period at the end is best omitted. Block comments generally consist of one or more paragraphs built out of complete sentences, and each sentence should end in a period.

You can use two spaces after a sentence-ending period.

As always when writing English, Strunk and White apply.

Python coders from non-English speaking countries: please write your comments in English, unless you are 120% sure that the code will never be read by people who don't speak your language.

Block Comments

Inline Comments

Inline comments are unnecessary and in fact distracting if they state the obvious. Don't do this:

x = x+1                 # Increment x

x = x+1                 # Compensate for border

Documentation Strings

The doc string of a script (a stand-alone program) should be usable as its "usage" message, printed when the script is invoked with incorrect or missing arguments (or perhaps with a "-h" option, for "help"). Such a doc string should document the script's function and command line syntax, environment variables, and files. Usage messages can be fairly elaborate (several screenfuls) and should be sufficient for a new user to use the command properly, as well as a complete quick reference to all options and arguments for the sophisticated user.

For consistency, always use """triple double quotes""" around doc strings.

There are two forms of doc strings: one-liners and multi-line doc strings.

One-line Doc Strings

def kos_root():
    """Return the pathname of the KOS root directory."""
    global _kos_root
    if _kos_root: return _kos_root
    ...

• Triple quotes are used even though the string fits on one line. This makes it easy to later expand it.
• The closing quotes are on the same line as the opening quotes. This looks better for one-liners.
• There's no blank line either before or after the doc string.
• The doc string is a phrase ending in a period. It prescribes the function's effect as a command ("Do this", "Return that"), not as a description: e.g. don't write "Returns the pathname ..."

Multi-line Doc Strings

The entire doc string is indented the same as the quotes at its first line (see example below). Doc string processing tools will strip an amount of indentation from the second and further lines of the doc string equal to the indentation of the first non-blank line after the first line of the doc string. Relative indentation of later lines in the doc string is retained.

I recommend inserting a blank line between the last paragraph in a multi-line doc string and its closing quotes, placing the closing quotes on a line by themselves. This way, Emacs' fill-paragraph command can be used on it.

I also recommend inserting a blank line before and after all doc strings (one-line or multi-line) that document a class -- generally speaking, the class' methods are separated from each other by a single blank line, and the doc string needs to be offset from the first method by a blank line; for symmetry, I prefer having a blank line between the class header and the doc string. Doc strings documenting function generally don't have this requirement, unless the function's body is written as a number of blank-line separated sections -- in this case, treat the doc string as another section, and precede it with a blank line.

The doc string for a module should generally list the classes, exceptions and functions (and any other objects) that are exported by the module, with a one-line summary of each. (These summaries generally give less detail than the summary line in the object's doc string.)

The doc string for a function or method should summarize its behavior and document its arguments, return value(s), side effects, exceptions raised, and restrictions on when it can be called (all if applicable). Optional arguments should be indicated. It should be documented whether keyword arguments are part of the interface.

The doc string for a class should summarize its behavior and list the public methods and instance variables. If the class is intended to be subclassed, and has an additional interface for subclasses, this interface should be listed separately (in the doc string). The class constructor should be documented in the doc string for its __init__ method. Individual methods should be documented by their own doc string.

If a class subclasses another class and its behavior is mostly inherited from that class, its doc string should mention this and summarize the differences. Use the verb "override" to indicate that a subclass method replaces a superclass method and does not call the superclass method; use the verb "extend" to indicate that a subclass method calls the superclass method (in addition to its own behavior).

Do not use the Emacs convention of mentioning the arguments of functions or methods in upper case in running text. Python is case sensitive and the argument names can be used for keyword arguments, so the doc string should document the correct argument names. It is best to list each argument on a separate line, with two dashes separating the name from the description, like this:

def complex(real=0.0, imag=0.0):
    """Form a complex number.

    Keyword arguments:
    real -- the real part (default 0.0)
    imag -- the imaginary part (default 0.0)

    """
    if imag == 0.0 and real == 0.0: return complex_zero
    ...

Version Bookkeeping

__version__ = "$Revision: 6104 $"
# $Source$

Naming Conventions

Descriptive: Naming Styles

• x (single lowercase letter)
• X (single uppercase letter)
• lowercase
• lower_case_with_underscores
• UPPERCASE
• UPPER_CASE_WITH_UNDERSCORES
• CapitalizedWords (or CapWords)
• mixedCase (differs from CapitalizedWords by initial lowercase character!)
• Capitalized_Words_With_Underscores (ugly!)

In addition, the following special forms using leading or trailing underscores are recognized (these can gerally be combined with any case convention):

• _single_leading_underscore: weak "internal use" indicator (e.g. "from M import *" does not import objects whose name starts with an underscore).
• single_trailing_underscore_: used by convention to avoid conflicts with Python keyword, e.g. Tkinter.Toplevel(master, class_="ClassName").
• __double_leading_underscore: class-private names in Python 1.4.
• __double_leading_and_trailing_underscore__: "magic" objects or attributes that live in user-controlled namespaces, e.g. __init__, __import__ or __file__. Sometimes these are defined by the user to trigger certain magic behavior (e.g. operator overloading); sometimes these are inserted by the infrastructure for its own use or for debugging purposes. Since the infrastructure (loosely defined as the Python interpreter and the standard library) may decide to grow its list of magic attributes in future versions, user code should generally refrain from using this convention for its own use. User code that aspires to become part of the infrastructure could combine this with a short prefix inside the underscores, e.g. __bobo_magic_attr__.

Prescriptive: Naming Conventions

Module Names

Since module names are mapped to file names, and some file systems are case insensitive and truncate long names, it is important that module names be chosen to be fairly short and not in conflict with other module names that only differ in the case -- this won't be a problem on Unix, but it will be when the code is transported to Mac or Windows.

There is an emerging convention that when an extension module written in C or C++ has an accompanying Python module that provides a higher level (e.g. more object oriented) interface, the Python module's name CapWords, while the C/C++ module is named in all lowercase and has a leading underscore (e.g. Tkinter/_tkinter).

"Packages" (groups of modules, supported by the "ni" module) generally have a short all lowercase name.

Class Names

Exception Names

Function Names

Global Variable Names

Method Names