PEP: 8 Title: Style Guide for Python Code Version: $Revision$ Author: guido@python.org (Guido van Rossum), barry@zope.com (Barry Warsaw) Status: Active Type: Informational Created: 05-Jul-2001 Post-History: 05-Jul-2001 Introduction This document gives coding conventions for the Python code comprising the standard library for the main Python distribution. Please see the companion informational PEP describing style guidelines for the C code in the C implementation of Python[1]. This document was adapted from Guido's original Python Style Guide essay[2]. This PEP inherits that essay's incompleteness. A Foolish Consistency is the Hobgoblin of Little Minds A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one module or function is most important. 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! Two good reasons to break a particular rule: (1) When applying the rule would make the code less readable, even for someone who is used to reading code that follows the rules. (2) To be consistent with surrounding code that also breaks it (maybe for historic reasons) -- although this is also an opportunity to clean up someone else's mess (in true XP style). Code lay-out Indentation Use the default of Emacs' Python-mode: 4 spaces for one indentation level. For really old code that you don't want to mess up, you can continue to use 8-space tabs. Emacs Python-mode auto-detects the prevailing indentation level used in a file and sets its indentation parameters accordingly. Tabs or Spaces? Never mix tabs and spaces. The most popular way of indenting Python is with spaces only. The second-most popular way is with tabs only. Code indented with a mixture of tabs and spaces should be converted to using spaces exclusively. (In Emacs, select the whole buffer and hit ESC-x untabify.) When invoking the python command line interpreter with the -t option, it issues warnings about code that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are highly recommended! For new projects, spaces are strongly recommended over tabs. Most editors have features that make this easy to do. Maximum Line Length There are still many devices around that are limited to 80 character lines; plus, limiting windows to 80 characters makes it possible to have several windows side-by-side. The default wrapping on such devices looks ugly. Therefore, please limit all lines to a maximum of 79 characters (Emacs wraps lines that are exactly 80 characters long.) For flowing long blocks of text (docstrings or comments), limiting the length to 72 characters is recommended. 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, width, height, color, emphasis, highlight) Blank Lines Separate top-level function and class definitions with two blank lines. Method definitions inside a class are separated by a single blank line. Extra blank lines may be used (sparingly) to separate groups of related functions. Blank lines may be omitted between a bunch of related one-liners (e.g. a set of dummy implementations). 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. Python accepts the control-L (i.e. ^L) form feed character as whitespace; Emacs (and some printing tools) treat these characters as page separators, so you may use them to separate pages of related sections of your file. Whitespace in Expressions and Statements Pet Peeves Guido hates whitespace in the following places: - 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 (Don't bother to argue with him on any of the above -- Guido's grown accustomed to this style over 20 years.) 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 judgment 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 Comments that contradict the code are worse than no comments. Always make a priority of keeping the comments up-to-date when the code changes! 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. 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 Block comments generally apply to some (or all) code that follows them, and are indented to the same level as that code. Each line of a block comment starts with a # and a single space (unless it is indented text inside the comment). Paragraphs inside a block comment are separated by a line containing a single #. Block comments are best surrounded by a blank line above and below them (or two lines above and a single line below for a block comment at the start of a a new section of function definitions). Inline Comments An inline comment is a comment on the same line as a statement. Inline comments should be used sparingly. Inline comments should be separated by at least two spaces from the statement. They should start with a # and a single space. Inline comments are unnecessary and in fact distracting if they state the obvious. Don't do this: x = x+1 # Increment x But sometimes, this is useful: x = x+1 # Compensate for border Documentation Strings Conventions for writing good documentation strings (a.k.a. "docstrings") are immortalized in their own PEP[3]. Version Bookkeeping If you have to have RCS or CVS crud in your source file, do it as follows. __version__ = "$Revision$" # $Source$ These lines should be included after the module's docstring, before any other code, separated by a blank line above and below. Naming Conventions The naming conventions of Python's library are a bit of a mess, so we'll never get this completely consistent -- nevertheless, here are some guidelines. Descriptive: Naming Styles There are a lot of different naming styles. It helps to be able to recognize what naming style is being used, independently from what they are used for. The following naming styles are commonly distinguished: - x (single lowercase letter) - X (single uppercase letter) - lowercase - lower_case_with_underscores - UPPERCASE - UPPER_CASE_WITH_UNDERSCORES - CapitalizedWords (or CapWords, or CamelCase -- so named because of the bumpy look of its letters[4]) - mixedCase (differs from CapitalizedWords by initial lowercase character!) - Capitalized_Words_With_Underscores (ugly!) There's also the style of using a short unique prefix to group related names together. This is not used much in Python, but it is mentioned for completeness. For example, the os.stat() function returns a tuple whose items traditionally have names like st_mode, st_size, st_mtime and so on. The X11 library uses a leading X for all its public functions. (In Python, this style is generally deemed unnecessary because attribute and method names are prefixed with an object, and function names are prefixed with a module name.)< In addition, the following special forms using leading or trailing underscores are recognized (these can generally 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 as of 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 Module names can be either CapWords or lowercase. There is no unambiguous convention to decide which to use. Modules that export a single class (or a number of closely related classes, plus some additional support) are often named in CapWords, with the module name being the same as the class name (e.g. the standard StringIO module). Modules that export a bunch of functions are usually named in all lowercase. 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 may be a problem 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. _socket). Python packages generally have a short all lowercase name. Class Names Almost without exception, class names use the CapWords convention. Classes for internal use have a leading underscore in addition. Exception Names If a module defines a single exception raised for all sorts of conditions, it is generally called "error" or "Error". It seems that built-in (extension) modules use "error" (e.g. os.error), while Python modules generally use "Error" (e.g. xdrlib.Error). Function Names Plain functions exported by a module can either use the CapWords style or lowercase (or lower_case_with_underscores). There is no strong preference, but it seems that the CapWords style is used for functions that provide major functionality (e.g. nstools.WorldOpen()), while lowercase is used more for "utility" functions (e.g. pathhack.kos_root()). Global Variable Names (Let's hope that these variables are meant for use inside one module only.) The conventions are about the same as those for exported functions. Modules that are designed for use via "from M import *" should prefix their globals (and internal functions and classes) with an underscore to prevent exporting them. Method Names The story is largely the same as for functions. Use lowercase for methods accessed by other classes or functions that are part of the implementation of an object type. Use one leading underscore for "internal" methods and instance variables when there is no chance of a conflict with subclass or superclass attributes or when a subclass might actually need access to them. Use two leading underscores (class-private names, enforced by Python 1.4) in those cases where it is important that only the current class accesses an attribute. (But realize that Python contains enough loopholes so that an insistent user could gain access nevertheless, e.g. via the __dict__ attribute.) References [1] PEP 7, Style Guide for C Code, van Rossum [2] http://www.python.org/doc/essays/styleguide.html [3] PEP 257, Docstring Conventions, Goodger, van Rossum [4] http://www.wikipedia.com/wiki/CamelCase Copyright This document has been placed in the public domain. 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