Translate Styleguide

The Translate styleguide is the styleguide for all Translate projects, including Translate Toolkit, Pootle, Virtaal and others. Patches are required to follow these guidelines.

This Styleguide follows PEP 8 with some clarifications. It is based almost verbatim on the Flask Styleguide.

pre-commit hooks

The Translate styleguide can be checked by pre-commit. The Translate toolkit repository repository contains configuration for it to verify the committed files are sane. After installing it (it is already included in the requirements/dev.txt) turn it on by running pre-commit install in Translate toolkit checkout. This way all your changes will be automatically checked.

You can also trigger check manually, to check all files run:

pre-commit run --all


These are the Translate conventions for Python coding style.



4 real spaces, no tabs. Exceptions: modules that have been copied into the source that don’t follow this guideline.

Maximum line length

79 characters with a soft limit for 84 if absolutely necessary. Try to avoid too nested code by cleverly placing break, continue and return statements.

Continuing long statements

To continue a statement you can use backslashes (preceded by a space) in which case you should align the next line with the last dot or equal sign, or indent four spaces:

MyModel.query.filter(MyModel.scalar > 120) \
             .order_by( \

my_long_assignment = MyModel.query.filter(MyModel.scalar > 120) \
                     .order_by( \

this_is_a_very_long(function_call, 'with many parameters') \

If you break in a statement with parentheses or braces, align to the braces:

this_is_a_very_long(function_call, 'with many parameters',
                    23, 42, 'and even more')

If you need to break long strings, on function calls or when assigning to variables, try to use implicit string continuation:

this_holds_a_very_long_string("Very long string with a lot of characters "
                              "and words on it, so many that it is "
                              "necessary to break it in several lines to "
                              "improve readability.")
long_string_var = ("Very long string with a lot of characters and words on "
                   "it, so many that it is necessary to break it in "
                   "several lines to improve readability.")

For lists or tuples with many items, break immediately after the opening brace:

items = [
    'this is the first', 'set of items', 'with more items',
    'to come in this line', 'like this'

Blank lines

Top level functions and classes are separated by two lines, everything else by one. Do not use too many blank lines to separate logical segments in code. Example:

def hello(name):
    print('Hello %s!' % name)

def goodbye(name):
    print('See you %s.' % name)

class MyClass:
    """This is a simple docstring"""

    def __init__(self, name): = name

    def annoying_name(self):
        return + '!!!!111'


  • Double quotes are suggested over single quotes, but always try to respect the surrounding coding style. This is overruled by escaping which you should always try to avoid.

    # Good.
    str1 = "Sauron's eye"
    str2 = 'Its name is "Virtaal".'
    # Bad.
    str3 = 'Sauron\'s eye'
    str4 = "Its name is \"Virtaal\"."

String formatting

While str.format() is more powerful than %-formatting, the latter has been the canonical way of formatting strings in Python for a long time and the Python core team has shown no desire to settle on one syntax over the other. For simple, serial positional cases (non-translatable strings), the old “%s” way of formatting is preferred. For anything more complex, including translatable strings, str.format is preferred as it is significantly more powerful and often cleaner.

# Good
print("Hello, {thing}".format(thing="world"))
print("Hello, {}".format("world"))
print("%s=%r" % ("hello", "world"))  # non-translatable strings

# Bad
print("%s, %s" % ("Hello", "world"))  # Translatable string.
print("Hello, %(thing)s" % {"thing": "world"})  # Use {thing}.


Like in PEP 8, but:

  • Imports should be grouped in the following order:

    1. __future__ library imports

    2. Python standard library imports

    3. Third party libraries imports

    4. Translate Toolkit imports

    5. Current package imports, using explicit relative imports (See PEP 328)

  • A blank line must be present between each group of imports (like in PEP8).

  • Imports on each group must be arranged alphabetically by module name:

    • Shortest module names must be before longer ones: from django.db import ... before from django.db.models import ....

  • import ... calls must precede from ... import ones on each group:

    • On each of these subgroups the entries should be alphabetically arranged.

    • No blank lines between subgroups.

  • On from ... import

    • Use a CONSTANT, Class, function order, where the constants, classes and functions are in alphabetical order inside of its respective groups.

    • If the import line exceeds the 80 chars, then split it using parentheses to continue the import on the next line (aligning the imported items with the opening parenthesis).

from __future__ import absolute_import

import re
import sys.path as sys_path
import time
from datetime import timedelta
from os import path

from lxml.html import fromstring

from translate.filters import checks
from import base
from import (EOF, WHITESPACE, AndroidFile,
                                         AndroidUnit, android_decode,

from . import php2po


  • Never use lambda functions:

    # Good.
    def stores(self):
      return self.child.stores
    # Bad.
    stores = property(lambda self: self.child.stores)
  • Try to use @property instead of get_* or is_* methods that don’t require passing any parameter:

    # Good.
    def terminology(self):
    def is_monolingual(self):
    # Also good.
    def get_stores_for_language(self, language):
    # Bad.
    def get_terminology(self):
    def is_monolingual(self):
  • Always use @property instead of property(...), even for properties that also have a setter or a deleter:

    # Good.
    def units(self):
    # Also good.
    def x(self):
      """I'm the 'x' property."""
      return self._x
    def x(self, value):  # Note: Method must be named 'x' too.
      self._x = value
    def x(self):  # Note: Method must be named 'x' too.
      del self._x
    # Bad.
    def _get_units(self):
    units = property(_get_units)
    # Also bad.
    def getx(self):
      return self._x
    def setx(self, value):
      self._x = value
    def delx(self):
      del self._x
    x = property(getx, setx, delx, "I'm the 'x' property.")

Expressions and Statements

General whitespace rules

  • No whitespace for unary operators that are not words (e.g.: -, ~ etc.) as well on the inner side of parentheses.

  • Whitespace is placed between binary operators.

# Good.
exp = -1.05
value = (item_value / item_count) * offset / exp
value = my_list[index]
value = my_dict['key']

# Bad.
exp = - 1.05
value = ( item_value / item_count ) * offset / exp
value = (item_value/item_count)*offset/exp
value=( item_value/item_count ) * offset/exp
value = my_list[ index ]
value = my_dict ['key']

Slice notation

While PEP 8 calls for spaces around operators a = b + c this results in flags when you use a[b+1:c-1] but would allow the rather unreadable a[b + 1:c - 1] to pass. PEP 8 is rather quiet on slice notation.

  • Don’t use spaces with simple variables or numbers

  • Use brackets for expressions with spaces between binary operators

    # Good.
    a[(start - 1):(end + var + 2)]  # Brackets help group things and don't hide the slice
    a[-1:(end + 1)]
    # Bad.
    a[start: end]  # No spaces around :
    a[start-1:end+var+2]  # Insanely hard to read, especially when your expressions are more complex
    a[start - 1:end + 2]  # You lose sight of the fact that it is a slice
    a[- 1:end]  # -1 is unary, no space


String slice formatting is still under discussion.


  • Against arbitrary types: == and !=

  • Against singletons with is and is not (e.g.: foo is not None)

  • Never compare something with True or False (for example never do foo == False, do not foo instead)

Negated containment checks

  • Use foo not in bar instead of not foo in bar

Instance checks

  • isinstance(a, C) instead of type(A) is C, but try to avoid instance checks in general. Check for features.

If statements

  • Use () brackets around complex if statements to allow easy wrapping, don’t use backslash to wrap an if statement.

  • Wrap between and, or, etc.

  • Keep not with the expression

  • Use () alignment between expressions

  • Use extra () to eliminate ambiguity, don’t rely on an understanding of Python operator precedence rules.

    # Good.
    if length >= (upper + 2):
    if (length >= 25 and
        string != "Something" and
        not careful):
    # Bad.
    if length >= upper + 2:
    if (length...
        and string !=...

Naming Conventions


This has not been implemented or discussed. The Translate code is not at all consistent with these conventions.

  • Class names: CamelCase, with acronyms kept uppercase (HTTPWriter and not HttpWriter)

  • Variable names: lowercase_with_underscores

  • Method and function names: lowercase_with_underscores


  • precompiled regular expressions: name_re

Protected members are prefixed with a single underscore. Double underscores are reserved for mixin classes.

To prevent name clashes with keywords, one trailing underscore may be appended. Clashes with builtins are allowed and must not be resolved by appending an underline to the name. If your code needs to access a shadowed builtin, rebind the builtin to a different name instead. Consider using a different name to avoid having to deal with either type of name clash, but don’t complicate names with prefixes or suffixes.

Function and method arguments

  • Class methods: cls as first parameter

  • Instance methods: self as first parameter


We use Sphinx to generate our API and user documentation. Read the reStructuredText primer and Sphinx documentation as needed.

Special roles

We introduce a number of special roles for documentation:

  • :issue: – links to a toolkit issue Github.

  • :opt: – mark command options and command values.

    • :opt:`-P` gives -P

    • :opt:`--progress=dots` gives --progress=dots

    • :opt:`dots` gives dots

  • :man: – link to a Linux man page.

Code and command line highlighting

All code examples and format snippets should be highlighted to make them easier to read. By default Sphinx uses Python highlighting of code snippets (but it doesn’t always work). You will want to change that in these situations:

  • The examples are not Python e.g. talking about INI file parsing. In which case set the file level highlighting using:

    .. highlight:: ini
  • There are multiple different code examples in the document, then use:

    .. code-block:: ruby

    before each code block.

  • Python code highlighting isn’t working, then force Python highlighting using:

    .. code-block:: python


Generally we prefer explicit markup as this makes it easier for those following you to know what you intended. So use .. code-block:: python even though in some cases this is not required.

With command line examples, to improve readability use:

.. code-block:: console

Add $ command prompt markers and # comments as required, as shown in this example:

$ cd docs
$ make html  # Build all Sphinx documentation
$ make linkcheck  # Report broken links

User documentation

This is documentation found in docs/ and that is published on Read the Docs. The target is the end user so our primary objective is to make accessible, readable and beautiful documents for them.


Docstring conventions:

All docstrings are formatted with reStructuredText as understood by Sphinx. Depending on the number of lines in the docstring, they are laid out differently. If it’s just one line, the closing triple quote is on the same line as the opening, otherwise the text is on the same line as the opening quote and the triple quote that closes the string on its own line:

def foo():
    """This is a simple docstring."""

def bar():
    """This is a longer docstring with so much information in there
    that it spans three lines.  In this case the closing triple quote
    is on its own line.

Please read PEP 257 (Docstring Conventions) for a general overview, the important parts though are:

  • A docstring should have a brief one-line summary, ending with a period. Use Do this, Return that rather than Does ..., Returns ....

  • If there are more details there should be a blank line between the one-line summary and the rest of the text. Use paragraphs and formatting as needed.

  • Use reST field lists to describe the input parameters and/or return types as the last part of the docstring.

  • Use proper capitalisation and punctuation.

  • Don’t restate things that would appear in parameter descriptions.

def addunit(self, unit):
    """Append the given unit to the object's list of units.

    This method should always be used rather than trying to modify the
    list manually.

    :param Unit unit: Any object that inherits from :class:`Unit`.
Parameter documentation:

Document parameters using reST field lists as follows:

def foo(bar):
    """Simple docstring.

    :param SomeType bar: Something
    :return: Returns something
    :rtype: Return type
Cross referencing code:

When talking about other objects, methods, functions and variables it is good practice to cross-reference them with Sphinx’s Python cross-referencing.

Other directives:

Use paragraph-level markup when needed.


We still need to gather the useful ones that we want you to use and how to use them. E.g. how to talk about a parameter in the docstring. How to reference classes in the module. How to reference other modules, etc.

Module header:

The module header consists of a utf-8 encoding declaration, copyright attribution, license block and a standard docstring:


"""A brief description"""

Document the deprecation and version when deprecating features:

from translate.misc.deprecation import deprecated

@deprecated("Use util.run_fast() instead.")
def run_slow():
    """Run fast

    .. deprecated:: 1.5
       Use :func:`run_fast` instead.


  • The # symbol (pound or hash) is used to start comments.

  • A space must follow the # between any written text.

  • Line length must be observed.

  • Inline comments are preceded by two spaces.

  • Write sentences correctly: proper capitalisation and punctuation.

# Good comment with space before and full sentence.
statement  # Good comment with two spaces

#Bad comment no space before
statement # Bad comment, needs two spaces
Docstring comments:

Rules for comments are similar to docstrings. Both are formatted with reStructuredText. If a comment is used to document an attribute, put a colon after the opening pound sign (#):

class User:
    #: the name of the user as unicode string
    name = Column(String)
    #: the sha1 hash of the password + inline salt
    pw_hash = Column(String)