PEP: 8 Title: Style Guide for Python Code Version: $Revision$ Author: guido@python.org (Guido van Rossum), barry@python.org (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], with some additions from Barry's style guide[5]. Where there's conflict, Guido's style rules for the purposes of this PEP. This PEP may still be incomplete (in fact, it may never be finished ). 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-only are strongly recommended over tabs. Most editors have features that make this easy to do. (In Emacs, make sure indent-tabs-mode is nil). 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. Encodings (PEP 263) Code in the core Python distribution should aways use the ASCII or Latin-1 encoding (a.k.a. ISO-8859-1). Files using ASCII should not have a coding cookie. Latin-1 should only be used when a comment or docstring needs to mention an author name that requires Latin-1; otherwise, using \x escapes is the preferred way to include non-ASCII data in string literals. An exception is made for those files that are part of the test suite for the code implementing PEP 263. Imports - Imports should usually be on separate lines, e.g.: No: import sys, os Yes: import sys import os it's okay to say this though: from types import StringType, ListType - Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants. Imports should be grouped, with the order being 1. standard library imports 2. related major package imports (i.e. all email package imports next) 3. application specific imports You should put a blank line between each group of imports. - Relative imports for intra-package imports are highly discouraged. Always use the absolute package path for all imports. - When importing a class from a class-containing module, it's usually okay to spell this from MyClass import MyClass from foo.bar.YourClass import YourClass If this spelling causes local name clashes, then spell them import MyClass import foo.bar.YourClass and use "MyClass.MyClass" and "foo.bar.YourClass.YourClass" 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) - Compound statements (multiple statements on the same line) are generally discouraged. No: if foo == 'blah': do_blah_thing() Yes: if foo == 'blah': do_blah_thing() No: do_one(); do_two(); do_three() Yes: do_one() do_two() do_three() 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! Comments should be complete sentences. 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 should use two spaces after a sentence-ending period, since it makes Emacs wrapping and filling work consistenty. 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 PEP 257 [3]. - Write docstrings for all public modules, functions, classes, and methods. Docstrings are not necessary for non-public methods, but you should have a comment that describes what the method does. This comment should appear after the "def" line. - PEP 257 describes good docstring conventions. Note that most importantly, the """ that ends a multiline docstring should be on a line by itself, e.g.: """Return a foobang Optional plotz says to frobnicate the bizbaz first. """ - For one liner docstrings, it's okay to keep the closing """ on the same line. 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 the currently recommended naming standards. New modules and packages (including 3rd party frameworks) should be written to these standards, but where an existing library has a different style, internal consistency is preferred. 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: - b (single lowercase letter) - B (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]). This is also sometimes known as StudlyCaps. - 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 Names to Avoid Never use the characters `l' (lowercase letter el), `O' (uppercase letter oh), or `I' (uppercase letter eye) as single character variable names. In some fonts, these characters are indistinguisable from the numerals one and zero. When tempted to use `l' use `L' instead. Module Names Modules should have short, lowercase names, without underscores. 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 -- this won't be a problem on Unix, but it may be a problem when the code is transported to Mac or Windows. 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 C/C++ module has a leading underscore (e.g. _socket). Python packages should have short, all-lowercase names, without underscores. 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). The trend seems to be toward CapWords exception names. 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 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. Function Names Function names should be lowercase, possibly with words separated by underscores to improve readability. mixedCase is allowed only in contexts where that's already the prevailing style (e.g. threading.py), to retain backwards compatibility. Method Names and Instance Variables The story is largely the same as with functions: in general, use lowercase with words separated by underscores as necessary to improve readability. Use one leading underscore only for internal methods and instance variables which are not intended to be part of the class's public interface. Python does not enforce this; it is up to programmers to respect the convention. Use two leading underscores to denote class-private names. Python "mangles" these names with the class name: if class Foo has an attribute named __a, it cannot be accessed by Foo.__a. (An insistent user could still gain access by calling Foo._Foo__a.) Generally, double leading underscores should be used only to avoid name conflicts with attributes in classes designed to be subclassed. Designing for inheritance Always decide whether a class's methods and instance variables should be public or non-public. In general, never make data variables public unless you're implementing essentially a record. It's almost always preferrable to give a functional interface to your class instead (and some Python 2.2 developments will make this much nicer). Also decide whether your attributes should be private or not. The difference between private and non-public is that the former will never be useful for a derived class, while the latter might be. Yes, you should design your classes with inheritence in mind! Private attributes should have two leading underscores, no trailing underscores. Non-public attributes should have a single leading underscore, no trailing underscores. Public attributes should have no leading or trailing underscores, unless they conflict with reserved words, in which case, a single trailing underscore is preferrable to a leading one, or a corrupted spelling, e.g. class_ rather than klass. (This last point is a bit controversial; if you prefer klass over class_ then just be consistent. :). Programming Recommendations - Code should be written in a way that does not disadvantage other implementations of Python (PyPy, Jython, IronPython, Pyrex, Psyco, and such). For example, do not rely on CPython's efficient implementation of in-place string concatenation for statements in the form a+=b or a=a+b. Those statements run more slowly in Jython. In performance sensitive parts of the library, the ''.join() form should be used instead. This will assure that concatenation occurs in linear time across various implementations. - Comparisons to singletons like None should always be done with 'is' or 'is not'. Also, beware of writing "if x" when you really mean "if x is not None" -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might be a value that's false in a Boolean context! - Class-based exceptions are always preferred over string-based exceptions. Modules or packages should define their own domain-specific base exception class, which should be subclassed from the built-in Exception class. Always include a class docstring. E.g.: class MessageError(Exception): """Base class for errors in the email package.""" - Use string methods instead of the string module unless backward-compatibility with versions earlier than Python 2.0 is important. String methods are always much faster and share the same API with unicode strings. - Avoid slicing strings when checking for prefixes or suffixes. Use startswith() and endswith() instead, since they are cleaner and less error prone. For example: No: if foo[:3] == 'bar': Yes: if foo.startswith('bar'): The exception is if your code must work with Python 1.5.2 (but let's hope not!). - Object type comparisons should always use isinstance() instead of comparing types directly. E.g. No: if type(obj) is type(1): Yes: if isinstance(obj, int): When checking if an object is a string, keep in mind that it might be a unicode string too! In Python 2.3, str and unicode have a common base class, basestring, so you can do: if isinstance(obj, basestring): In Python 2.2, the types module has the StringTypes type defined for that purpose, e.g.: from types import StringTypes if isinstance(obj, StringTypes): In Python 2.0 and 2.1, you should do: from types import StringType, UnicodeType if isinstance(obj, StringType) or \ isinstance(obj, UnicodeType) : - For sequences, (strings, lists, tuples), use the fact that empty sequences are false, so "if not seq" or "if seq" is preferable to "if len(seq)" or "if not len(seq)". - Don't write string literals that rely on significant trailing whitespace. Such trailing whitespace is visually indistinguishable and some editors (or more recently, reindent.py) will trim them. - Don't compare boolean values to True or False using == (bool types are new in Python 2.3): No: if greeting == True: Yes: if greeting: 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 [5] Barry's GNU Mailman style guide http://barry.warsaw.us/software/STYLEGUIDE.txt Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil End: