3 Python Style Rules toto
3.1 Semicolons toto
Do not terminate your lines with semicolons, and do not use semicolons to put two statements on the same line.
3.2 Line length toto
Maximum line length is 80 characters.
Explicit exceptions to the 80 character limit:
- Long import statements.
- URLs, pathnames, or long flags in comments.
- Long string module level constants not containing whitespace that would be inconvenient to split across lines such as URLs or pathnames.
- Pylint disable comments. (e.g.:
# pylint: disable=invalid-name
)toto
Do not use backslash line continuation except for with
statements requiring three or more context managers.
Make use of Python’s implicit line joining inside parentheses, brackets and braces.toto
If necessary, you can add an extra pair of parentheses around an expression.
Yes: foo_bar(self, width, height, color='black', design=None, x='foo',
emphasis=None, highlight=0)
if (width == 0 and height == 0 and
color == 'red' and emphasis == 'strong'):
When a literal string won’t fit on a single line, use parentheses for implicit line joining.
x = ('This will build a very long long '
'long long long long long long string')
Within comments, put long URLs on their own line if necessary.
Yes:# See details attoto
# http://www.example.com/us/developer/documentation/api/content/v2.0/csv_file_name_extension_full_specification.htmltoto
No:# See details attoto
# http://www.example.com/us/developer/documentation/api/content/\toto
# v2.0/csv_file_name_extension_full_specification.htmltoto
It is permissible to use backslash continuation when defining a with
statement whose expressions span three or more lines. For two lines of expressions, use a nested with
statement:
Yes: with very_long_first_expression_function() as spam, \
very_long_second_expression_function() as beans, \
third_thing() as eggs:
place_order(eggs, beans, spam, beans)
No: with VeryLongFirstExpressionFunction() as spam, \
VeryLongSecondExpressionFunction() as beans:
PlaceOrder(eggs, beans, spam, beans)
Yes: with very_long_first_expression_function() as spam:
with very_long_second_expression_function() as beans:
place_order(beans, spam)
Make note of the indentation of the elements in the line continuation examples above; see the indentation section for explanation.toto
In all other cases where a line exceeds 80 characters, and the yapf auto-formatter does not help bring the line below the limit, the line is allowed to exceed this maximum.
3.3 Parentheses toto
Use parentheses sparingly.
It is fine, though not required, to use parentheses around tuples. Do not use them in return statements or conditional statements unless using parentheses for implied line continuation or to indicate a tuple.
Yes: if foo:
bar()
while x:
x = bar()
if x and y:
bar()
if not x:
bar()
# For a 1 item tuple the ()s are more visually obvious than the comma.toto
onesie = (foo,)
return foo
return spam, beans
return (spam, beans)
for (x, y) in dict.items(): ...
No: if (x):
bar()
if not(x):
bar()
return (foo)
3.4 Indentation toto
Indent your code blocks with 4 spaces.
Never use tabs or mix tabs and spaces. In cases of implied line continuation, you should align wrapped elements either vertically, as per the examples in the line length section; or using a hanging indent of 4 spaces,toto
in which case there should be nothing after the open parenthesis or bracket on the first line.
Yes:# Aligned with opening delimitertoto
foo = long_function_name(var_one, var_two,
var_three, var_four)
meal = (spam,
beans)
# Aligned with opening delimiter in a dictionarytoto
foo = {
long_dictionary_key: value1 +
value2,
...
}
# 4-space hanging indent; nothing on first linetoto
foo = long_function_name(
var_one, var_two, var_three,
var_four)
meal = (
spam,
beans)
# 4-space hanging indent in a dictionarytoto
foo = {
long_dictionary_key:
long_dictionary_value,
...
}
No:# Stuff on first line forbiddentoto
foo = long_function_name(var_one, var_two,
var_three, var_four)
meal = (spam,
beans)
# 2-space hanging indent forbiddentoto
foo = long_function_name(
var_one, var_two, var_three,
var_four)
# No hanging indent in a dictionarytoto
foo = {
long_dictionary_key:
long_dictionary_value,
...
}
3.4.1 Trailing commas in sequences of items? toto
Trailing commas in sequences of items are recommended only when the closing container token ]
, )
, or }
does not appear on the same line as the final element. The presence of a trailing comma is also used as a hint to our Python code auto-formatter YAPF to direct it to auto-format the container of items to one item per line when the ,
after the final element is present.
Yes: golomb3 = [0, 1, 3]
Yes: golomb4 = [
0,
1,
4,
6,
]
No: golomb4 = [
0,
1,
4,
6
]
3.5 Blank Lines toto
Two blank lines between top-level definitions, be they function or class definitions. One blank line between method definitions and between the class
line and the first method. No blank line following a def
line. Use single blank lines as you judge appropriate within functions or methods.
3.6 Whitespace toto
Follow standard typographic rules for the use of spaces around punctuation.
No whitespace inside parentheses, brackets or braces.
Yes: spam(ham[1], {eggs: 2}, [])
No: spam( ham[ 1 ], { eggs: 2 }, [ ] )
No whitespace before a comma, semicolon, or colon. Do use whitespace after a comma, semicolon, or colon, except at the end of the line.
Yes: if x == 4:
print(x, y)
x, y = y, x
No: if x == 4 :
print(x , y)
x , y = y , x
No whitespace before the open paren/bracket that starts an argument list, indexing or slicing.
Yes: dict['key'] = list[index]
No: dict ['key'] = list [index]
No trailing whitespace.
Surround binary operators with a single space on either side for assignment (=
), comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not
), and Booleans (and, or, not
). Use your better judgment for the insertion of spaces around arithmetic operators (+
, -
, *
, /
, //
, %
, **
, @
).
Never use spaces around =
when passing keyword arguments or defining a default parameter value, with one exception: when a type annotation is present, do use spaces around the =
for the defaulttoto
parameter value.
Yes: def complex(real, imag=0.0): return Magic(r=real, i=imag)
Yes: def complex(real, imag: float = 0.0): return Magic(r=real, i=imag)
No: def complex(real, imag = 0.0): return Magic(r = real, i = imag)
No: def complex(real, imag: float=0.0): return Magic(r = real, i = imag)
Don’t use spaces to vertically align tokens on consecutive lines, since it becomes a maintenance burden (applies to :
, #
, =
, etc.):toto
Yes:
foo = 1000# commenttoto
long_name = 2# comment that should not be alignedtoto
dictionary = {
'foo': 1,
'long_name': 2,
}
No:
foo = 1000# commenttoto
long_name = 2# comment that should not be alignedtoto
dictionary = {
'foo' : 1,
'long_name': 2,
}
3.7 Shebang Line toto
Most .py
files do not need to start with a #!
line. Start the main file of atoto
program with #!/usr/bin/python
with an optional single digit 2
or 3
suffix pertoto
PEP-394.
This line is used by the kernel to find the Python interpreter, but is ignored by Python when importing modules. It is only necessary on a file that will be executed directly.
3.8 Comments and Docstrings toto
Be sure to use the right style for module, function, method docstrings and inline comments.
3.8.1 Docstrings toto
Python uses docstrings to document code. A docstring is a string that is the first statement in a package, module, class or function. These strings can be extracted automatically through the __doc__
member of the object and are used by pydoc
. (Try running pydoc
on your module to see how it looks.) Always use the three double-quote """
format for docstrings (per PEP 257). A docstring should be organized as a summary line (one physical line) terminated by a period, question mark, or exclamation point, followed by a blank line, followed by the rest of the docstring starting at the same cursor position as the first quote of the first line. There are more formatting guidelines for docstrings below.
3.8.2 Modules toto
Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL)
Files should start with a docstring describing the contents and usage of the module.
"""A one line summary of the module or program, terminated by a period.
Leave one blank line. The rest of this docstring should contain an
overall description of the module or program. Optionally, it may also
contain a brief description of exported classes and functions and/or usage
examples.
Typical usage example:
foo = ClassFoo()
bar = foo.FunctionBar()
"""
3.8.3 Functions and Methods toto
In this section, “function” means a method, function, or generator.
A function must have a docstring, unless it meets all of the following criteria:
- not externally visible
- very short
- obvious
A docstring should give enough information to write a call to the function without reading the function’s code. The docstring should be descriptive-style ("""Fetches rows from a Bigtable."""
) rather than imperative-style ("""Fetch rows from a Bigtable."""
), except for @property
data descriptors, which should describe the function’s calling syntax and its semantics, not its implementation. For tricky code, comments alongside the code are more appropriate than using docstrings.
A method that overrides a method from a base class may have a simple docstring sending the reader to its overridden method’s docstring, such as """See base class."""
. The rationale is that there is no need to repeat in many places documentation that is already present in the base method’s docstring. However, if the overriding method’s behavior is substantially different from the overridden method, or details need to be provided (e.g., documenting additional side effects), a docstring with at least those differences is required on the overriding method.
Certain aspects of a function should be documented in special sections, listed below. Each section begins with a heading line, which ends with a colon. All sections other than the heading should maintain a hanging indent of two or four spaces (be consistent within a file). These sections can be omitted in cases where the function’s name and signature are informative enough that it can be aptly described using a one-line docstring.
- Args:toto
List each parameter by name. A description should follow the name, and be separated by a colon and a space. If the description is too long to fit on a single 80-character line, use a hanging indent of 2 or 4 spaces (be consistent with the rest of the file).
The description should include required type(s) if the code does not contain a corresponding type annotation. If a function accepts *foo
(variable length argument lists) and/or **bar
(arbitrary keyword arguments), they should be listed as *foo
and **bar
.
- Returns: (or Yields: for generators)toto
Describe the type and semantics of the return value. If the function only returns None, this section is not required. It may also be omitted if the docstring starts with Returns or Yields (e.g. """Returns row from Bigtable as a tuple of strings."""
) and the opening sentence is sufficient to describe return value.
- Raises:toto
List all exceptions that are relevant to the interface. You should not document exceptions that get raised if the API specified in the docstring is violated (because this would paradoxically make behavior under violation of the API part of the API).
def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
"""Fetches rows from a Bigtable.
Retrieves rows pertaining to the given keys from the Table instance
represented by big_table. Silly things may happen if
other_silly_variable is not None.
Args:
big_table: An open Bigtable Table instance.
keys: A sequence of strings representing the key of each table row
to fetch.
other_silly_variable: Another optional variable, that has a much
longer name than the other args, and which does nothing.
Returns:
A dict mapping keys to the corresponding table row data
fetched. Each row is represented as a tuple of strings. For
example:
{'Serak': ('Rigel VII', 'Preparer'),
'Zim': ('Irk', 'Invader'),
'Lrrr': ('Omicron Persei 8', 'Emperor')}
If a key from the keys argument is missing from the dictionary,
then that row was not found in the table.
Raises:
IOError: An error occurred accessing the bigtable.Table object.
"""
3.8.4 Classes toto
Classes should have a docstring below the class definition describing the class. If your class has public attributes, they should be documented here in an Attributes
section and follow the same formatting as a function’s Args
section.toto
class SampleClass(object):
"""Summary of class here.
Longer class information....
Longer class information....
Attributes:
likes_spam: A boolean indicating if we like SPAM or not.
eggs: An integer count of the eggs we have laid.
"""
def __init__(self, likes_spam=False):
"""Inits SampleClass with blah."""
self.likes_spam = likes_spam
self.eggs = 0
def public_method(self):
"""Performs operation blah."""
3.8.6 Punctuation, Spelling and Grammar toto
Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.
Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.
Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.
3.9 Classes toto
If a class inherits from no other base classes, explicitly inherit from object
. This also applies to nested classes.
Yes: class SampleClass(object):
pass
class OuterClass(object):
class InnerClass(object):
pass
class ChildClass(ParentClass):
"""Explicitly inherits from another class already."""
No: class SampleClass:
pass
class OuterClass:
class InnerClass:
pass
Inheriting from object
is needed to make properties work properly in Python 2 and can protect your code from potential incompatibility with Python 3. It also defines special methods that implement the default semantics of objects including __new__
, __init__
, __delattr__
, __getattribute__
, __setattr__
, __hash__
, __repr__
, and __str__
.
3.10 Strings toto
Use the format
method or the %
operator for formatting strings, even when the parameters are all strings. Use your best judgment to decide between +
and %
(or format
) though.
Yes: x = a + b
x = '%s, %s!' % (imperative, expletive)
x = '{}, {}'.format(first, second)
x = 'name: %s; score: %d' % (name, n)
x = 'name: {}; score: {}'.format(name, n)
x = f'name: {name}; score: {n}'# Python 3.6+toto
No: x = '%s%s' % (a, b)# use + in this casetoto
x = '{}{}'.format(a, b)# use + in this casetoto
x = first + ', ' + second
x = 'name: ' + name + '; score: ' + str(n)
Avoid using the +
and +=
operators to accumulate a string within a loop. Since strings are immutable, this creates unnecessary temporary objects and results in quadratic rather than linear running time. Instead, add each substring to a list and ''.join
the list after the loop terminates (or, write each substring to a io.BytesIO
buffer).
Yes: items = ['<table>']
for last_name, first_name in employee_list:
items.append('<tr><td>%s, %s</td></tr>' % (last_name, first_name))
items.append('</table>')
employee_table = ''.join(items)
No: employee_table = '<table>'
for last_name, first_name in employee_list:
employee_table += '<tr><td>%s, %s</td></tr>' % (last_name, first_name)
employee_table += '</table>'
Be consistent with your choice of string quote character within a file. Pick '
or "
and stick with it. It is okay to use the other quote character on a string to avoid the need to \\
escape within the string.
Yes:
Python('Why are you hiding your eyes?')
Gollum("I'm scared of lint errors.")
Narrator('"Good!" thought a happy Python reviewer.')
No:
Python("Why are you hiding your eyes?")
Gollum('The lint. It burns. It burns us.')
Gollum("Always the great lint. Watching. Watching.")
Prefer """
for multi-line strings rather than '''
. Projects may choose to use '''
for all non-docstring multi-line strings if and only if they also use '
for regular strings. Docstrings must use """
regardless.
Multi-line strings do not flow with the indentation of the rest of the program. If you need to avoid embedding extra space in the string, use either concatenated single-line strings or a multi-line string with textwrap.dedent()
toto
to remove the initial space on each line:
No:
long_string = """This is pretty ugly.
Don't do this.
"""
Yes:
long_string = """This is fine if your use case can accept
extraneous leading spaces."""
Yes:
long_string = ("And this is fine if you can not accept\n" +
"extraneous leading spaces.")
Yes:
long_string = ("And this too is fine if you can not accept\n"
"extraneous leading spaces.")
Yes:
import textwrap
long_string = textwrap.dedent("""\
This is also fine, because textwrap.dedent()
will collapse common leading spaces in each line.""")
3.11 Files and Sockets toto
Explicitly close files and sockets when done with them.
Leaving files, sockets or other file-like objects open unnecessarily has many downsides:
- They may consume limited system resources, such as file descriptors. Code that deals with many such objects may exhaust those resources unnecessarily if they’re not returned to the system promptly after use.
- Holding files open may prevent other actions such as moving or deleting them.
- Files and sockets that are shared throughout a program may inadvertently be read from or written to after logically being closed. If they are actually closed, attempts to read or write from them will throw exceptions, making the problem known sooner.
Furthermore, while files and sockets are automatically closed when the file object is destructed, tying the lifetime of the file object to the state of the file is poor practice:
- There are no guarantees as to when the runtime will actually run the file’s destructor. Different Python implementations use different memory management techniques, such as delayed Garbage Collection, which may increase the object’s lifetime arbitrarily and indefinitely.
- Unexpected references to the file, e.g. in globals or exception tracebacks, may keep it around longer than intended.
The preferred way to manage files is using the “with” statement:toto
with open("hello.txt") as hello_file:
for line in hello_file:
print(line)
For file-like objects that do not support the “with” statement, use contextlib.closing()
:
import contextlib
with contextlib.closing(urllib.urlopen("http://www.python.org/")) as front_page:
for line in front_page:
print(line)
3.14 Statements toto
Generally only one statement per line.
However, you may put the result of a test on the same line as the test only if the entire statement fits on one line. In particular, you can never do so with try
/except
since the try
and except
can’t both fit on the same line, and you can only do so with an if
if there is no else
.
No:
if foo: bar(foo)
else: baz(foo)
try: bar(foo)
except ValueError: baz(foo)
try:
bar(foo)
except ValueError: baz(foo)
3.15 Accessors toto
If an accessor function would be trivial, you should use public variables instead of accessor functions to avoid the extra cost of function calls in Python. When more functionality is added you can use property
to keep the syntax consistent.
On the other hand, if access is more complex, or the cost of accessing the variable is significant, you should use function calls (following the Naming guidelines) such as get_foo()
andtoto
set_foo()
. If the past behavior allowed access through a property, do not bind the new accessor functions to the property. Any code still attempting to access the variable by the old method should break visibly so they are made aware of the change in complexity.
3.16 Naming toto
module_name
, package_name
, ClassName
, method_name
, ExceptionName
, function_name
, GLOBAL_CONSTANT_NAME
, global_var_name
, instance_var_name
, function_parameter_name
, local_var_name
.
Function names, variable names, and filenames should be descriptive; eschew abbreviation. In particular, do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word.
Always use a .py
filename extension. Never use dashes.
3.16.1 Names to Avoid toto
- single character names except for counters or iterators. You may use “e” as an exception identifier in try/except statements.
- dashes (
-
) in any package/module name
__double_leading_and_trailing_underscore__
names (reserved by Python)
3.16.2 Naming Conventions toto
“Internal” means internal to a module, or protected or private within a class.
Prepending a single underscore (_
) has some support for protecting module variables and functions (not included with from module import *
). While prepending a double underscore (__
aka “dunder”) to an instance variable or method effectively makes the variable or method private to its class (using name mangling) we discourage its use as it impacts readability and testability and isn’t really private.
Place related classes and top-level functions together in a module. Unlike Java, there is no need to limit yourself to one class per module.
Use CapWords for class names, but lower_with_under.py for module names. Although there are some old modules named CapWords.py, this is now discouraged because it’s confusing when the module happens to be named after a class. (“wait – did I write import StringIO
or from StringIO import StringIO
?”)
Underscores may appear in unittest method names starting with test
to separate logical components of the name, even if those components use CapWords. One possible pattern is test<MethodUnderTest>_<state>
; for example testPop_EmptyStack
is okay. There is no One Correct Way to name test methods.
3.16.3 File Naming toto
Python filenames must have a .py
extension and must not contain dashes (-
). This allows them to be imported and unittested. If you want an executable to be accessible without the extension, use a symbolic link or a simple bash wrapper containing exec "$0.py" "$@"
.
3.16.4 Guidelines derived from Guido’s Recommendations toto
Type
|
Public
|
Internal
|
Packages
|
lower_with_under
|
|
Modules
|
lower_with_under
|
_lower_with_under
|
Classes
|
CapWords
|
_CapWords
|
Exceptions
|
CapWords
|
|
Functions
|
lower_with_under()
|
_lower_with_under()
|
Global/Class Constants
|
CAPS_WITH_UNDER
|
_CAPS_WITH_UNDER
|
Global/Class Variables
|
lower_with_under
|
_lower_with_under
|
Instance Variables
|
lower_with_under
|
_lower_with_under (protected)
|
Method Names
|
lower_with_under()
|
_lower_with_under() (protected)
|
Function/Method Parameters
|
lower_with_under
|
|
Local Variables
|
lower_with_under
|
|
While Python supports making things private by using a leading double underscore __
(aka. “dunder”) prefix on a name, this is discouraged. Prefer the use of a single underscore. They are easier to type, read, and to access from small unittests. Lint warnings take care of invalid access to protected members.
3.17 Main toto
Even a file meant to be used as an executable should be importable and a mere import should not have the side effect of executing the program’s main functionality. The main functionality should be in a main()
function.
In Python, pydoc
as well as unit tests require modules to be importable. Your code should always check if __name__ == '__main__'
before executing your main program so that the main program is not executed when the module is imported.
def main():
...
if __name__ == '__main__':
main()
All code at the top level will be executed when the module is imported. Be careful not to call functions, create objects, or perform other operations that should not be executed when the file is being pydoc
ed.
3.18 Function length toto
Prefer small and focused functions.
We recognize that long functions are sometimes appropriate, so no hard limit is placed on function length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.
Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.
You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces.
3.19 Type Annotations toto
3.19.1 General Rules toto
- Familiarize yourself with PEP-484.
- In methods, only annotate
self
, or cls
if it is necessary for proper type information. e.g., @classmethod def create(cls: Type[T]) -> T: return cls()
- If any other variable or a returned type should not be expressed, use
Any
.
- You are not required to annotate all the functions in a module.
- At least annotate your public APIs.
- Use judgment to get to a good balance between safety and clarity on the one hand, and flexibility on the other.
- Annotate code that is prone to type-related errors (previous bugs or complexity).
- Annotate code that is hard to understand.
- Annotate code as it becomes stable from a types perspective. In many cases, you can annotate all the functions in mature code without losing too much flexibility.
3.19.2 Line Breaking toto
Try to follow the existing indentation rules.toto
After annotating, many function signatures will become “one parameter per line”.
def my_method(self,
first_var: int,
second_var: Foo,
third_var: Optional[Bar]) -> int:
...
Always prefer breaking between variables, and not for example between variable names and type annotations. However, if everything fits on the same line, go for it.
def my_method(self, first_var: int) -> int:
...
If the combination of the function name, the last parameter, and the return type is too long, indent by 4 in a new line.
def my_method(
self, first_var: int) -> Tuple[MyLongType1, MyLongType1]:
...
When the return type does not fit on the same line as the last parameter, the preferred way is to indent the parameters by 4 on a new line and align the closing parenthesis with the def.
Yes:
def my_method(
self, other_arg: Optional[MyLongType]
) -> Dict[OtherLongType, MyLongType]:
...
pylint
allows you to move the closing parenthesis to a new line and align with the opening one, but this is less readable.
No:
def my_method(self,
other_arg: Optional[MyLongType]
) -> Dict[OtherLongType, MyLongType]:
...
As in the examples above, prefer not to break types. However, sometimes they are too long to be on a single line (try to keep sub-types unbroken).
def my_method(
self,
first_var: Tuple[List[MyLongType1],
List[MyLongType2]],
second_var: List[Dict[
MyLongType3, MyLongType4]]) -> None:
...
If a single name and type is too long, consider using an alias for the type. The last resort is to break after thetoto
colon and indent by 4.
Yes:
def my_function(
long_variable_name:
long_module_name.LongTypeName,
) -> None:
...
No:
def my_function(
long_variable_name: long_module_name.
LongTypeName,
) -> None:
...
3.19.3 Forward Declarations toto
If you need to use a class name from the same module that is not yet defined – for example, if you need the class inside the class declaration, or if you use a class that is defined below – use a string for the class name.
class MyClass(object):
def __init__(self,
stack: List["MyClass"]) -> None:
3.19.4 Default Values toto
As per PEP-008, usetoto
spaces around the =
only for arguments that have both a type annotation and a default value.
Yes:
def func(a: int = 0) -> int:
...
No:
def func(a:int=0) -> int:
...
3.19.5 NoneType toto
In the Python type system, NoneType
is a “first class” type, and for typing purposes, None
is an alias for NoneType
. If an argument can be None
, it has to be declared! You can use Union
, but if there is only one other type, use Optional
.
Use explicit Optional
instead of implicit Optional
. Earlier versions of PEP 484 allowed a: Text = None
to be interpretted as a: Optional[Text] = None
, but that is no longer the preferred behavior.
Yes:
def func(a: Optional[Text], b: Optional[Text] = None) -> Text:
...
def multiple_nullable_union(a: Union[None, Text, int]) -> Text
...
No:
def nullable_union(a: Union[None, Text]) -> Text:
...
def implicit_optional(a: Text = None) -> Text:
...
3.19.6 Type Aliases toto
You can declare aliases of complex types. The name of an alias should be CapWorded. If the alias is used only in this module, it should be _Private.
For example, if the name of the module together with the name of the type is too long:
_ShortName = module_with_long_name.TypeWithLongName
ComplexMap = Mapping[Text, List[Tuple[int, int]]]
Other examples are complex nested types and multiple return variables from a function (as a tuple).
3.19.7 Ignoring Types toto
You can disable type checking on a line with the special comment # type: ignore
.toto
pytype
has a disable option for specific errors (similar to lint):
# pytype: disable=attribute-errortoto
3.19.8 Typing Variables toto
If an internal variable has a type that is hard or impossible to infer, you can specify its type in a couple ways.
- Type Comments:toto
Use a # type:
comment on the end of the linetoto
a = SomeUndecoratedFunction()# type: Foototo
- Annotated Assignmentstoto
Use a colon and type between the variable name and value, as with function arguments.
a: Foo = SomeUndecoratedFunction()
3.19.9 Tuples vs Lists toto
Unlike Lists, which can only have a single type, Tuples can have either a single repeated type or a set number of elements with different types. The latter is commonly used as return type from a function.
a = [1, 2, 3]# type: List[int]toto
b = (1, 2, 3)# type: Tuple[int, ...]toto
c = (1, "2", 3.5)# type: Tuple[int, Text, float]toto
3.19.10 TypeVars toto
The Python type system has generics. The factorytoto
function TypeVar
is a common way to use them.
Example:
from typing import List, TypeVar
T = TypeVar("T")
...
def next(l: List[T]) -> T:
return l.pop()
A TypeVar can be constrained:
AddableType = TypeVar("AddableType", int, float, Text)
def add(a: AddableType, b: AddableType) -> AddableType:
return a + b
A common predefined type variable in the typing
module is AnyStr
. Use it for multiple annotations that can be bytes
or unicode
and must all be the same type.
from typing import AnyStr
def check_length(x: AnyStr) -> AnyStr:
if len(x) <= 42:
return x
raise ValueError()
3.19.11 String types toto
The proper type for annotating strings depends on what versions of Python the code is intended for.
For Python 3 only code, prefer to use str
. Text
is also acceptable. Be consistent in using one or the other.
For Python 2 compatible code, use Text
. In some rare cases, str
may make sense; typically to aid compatibility when the return types aren’t the same between the two Python versions. Avoid using unicode
: it doesn’t exist in Python 3.
The reason this discrepancy exists is because str
means different things depending on the Python version.
No:
def py2_code(x: str) -> unicode:
...
For code that deals with binary data, use bytes
.
def deals_with_binary_data(x: bytes) -> bytes:
...
For Python 2 compatible code that processes text data (str
or unicode
in Python 2, str
in Python 3), use Text
. For Python 3 only code that process text data, prefer str
.
from typing import Text
...
def py2_compatible(x: Text) -> Text:
...
def py3_only(x: str) -> str:
...
If the type can be either bytes or text, use Union
, with the appropriate text type.
from typing import Text, Union
...
def py2_compatible(x: Union[bytes, Text]) -> Union[bytes, Text]:
...
def py3_only(x: Union[bytes, str]) -> Union[bytes, str]:
...
If all the string types of a function are always the same, for example if the return type is the same as the argument type in the code above, use AnyStr.toto
Writing it like this will simplify the process of porting the code to Python 3.
3.19.12 Imports For Typing toto
For classes from the typing
module, always import the class itself. You are explicitly allowed to import multiple specific classes on one line from the typing
module. Ex:
from typing import Any, Dict, Optional
Given that this way of importing from typing
adds items to the local namespace, any names in typing
should be treated similarly to keywords, and not be defined in your Python code, typed or not. If there is a collision between a type and an existing name in a module, import it using import x as y
.
from typing import Any as AnyType
3.19.13 Conditional Imports toto
Use conditional imports only in exceptional cases where the additional imports needed for type checking must be avoided at runtime. This pattern is discouraged; alternatives such as refactoring the code to allow top level imports should be preferred.
Imports that are needed only for type annotations can be placed within an if TYPE_CHECKING:
block.
- Conditionally imported types need to be referenced as strings, to be forward compatible with Python 3.6 where the annotation expressions are actually evaluated.
- Only entities that are used solely for typing should be defined here; this includes aliases. Otherwise it will be a runtime error, as the module will not be imported at runtime.
- The block should be right after all the normal imports.
- There should be no empty lines in the typing imports list.
- Sort this list as if it were a regular imports list.
import typing
if typing.TYPE_CHECKING:
import sketch
def f(x: "sketch.Sketch"): ...
3.19.14 Circular Dependencies toto
Circular dependencies that are caused by typing are code smells. Such code is a good candidate for refactoring. Although technically it is possible to keep circular dependencies, the build system will not let youtoto
do so because each module has to depend on the other.
Replace modules that create circular dependency imports with Any
. Set an alias with a meaningful name, and use the real type name fromtoto
this module (any attribute of Any is Any). Alias definitions should be separated from the last import by one line.
from typing import Any
some_mod = Any# some_mod.py imports this module.toto
...
def my_method(self, var: some_mod.SomeType) -> None:
...
3.19.15 Generics toto
When annotating, prefer to specify type parameters for generic types; otherwise, the generics’ parameters will be assumed to be Any
.toto
def get_names(employee_ids: List[int]) -> Dict[int, Any]:
...
# These are both interpreted as get_names(employee_ids: List[Any]) -> Dict[Any, Any]toto
def get_names(employee_ids: list) -> Dict:
...
def get_names(employee_ids: List) -> Dict:
...
If the best type parameter for a generic is Any
, make it explicit, but remember that in many cases TypeVar
might be moretoto
appropriate:
def get_names(employee_ids: List[Any]) -> Dict[Any, Text]:
"""Returns a mapping from employee ID to employee name for given IDs."""
T = TypeVar('T')
def get_names(employee_ids: List[T]) -> Dict[T, Text]:
"""Returns a mapping from employee ID to employee name for given IDs."""
3.8 Comments and Docstrings toto
Be sure to use the right style for module, function, method docstrings and inline comments.
3.8.1 Docstrings toto
Python uses docstrings to document code. A docstring is a string that is the first statement in a package, module, class or function. These strings can be extracted automatically through the
__doc__
member of the object and are used bypydoc
. (Try runningpydoc
on your module to see how it looks.) Always use the three double-quote"""
format for docstrings (per PEP 257). A docstring should be organized as a summary line (one physical line) terminated by a period, question mark, or exclamation point, followed by a blank line, followed by the rest of the docstring starting at the same cursor position as the first quote of the first line. There are more formatting guidelines for docstrings below.3.8.2 Modules toto
Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL)
Files should start with a docstring describing the contents and usage of the module.
3.8.3 Functions and Methods toto
In this section, “function” means a method, function, or generator.
A function must have a docstring, unless it meets all of the following criteria:
A docstring should give enough information to write a call to the function without reading the function’s code. The docstring should be descriptive-style (
"""Fetches rows from a Bigtable."""
) rather than imperative-style ("""Fetch rows from a Bigtable."""
), except for@property
data descriptors, which should describe the function’s calling syntax and its semantics, not its implementation. For tricky code, comments alongside the code are more appropriate than using docstrings.A method that overrides a method from a base class may have a simple docstring sending the reader to its overridden method’s docstring, such as
"""See base class."""
. The rationale is that there is no need to repeat in many places documentation that is already present in the base method’s docstring. However, if the overriding method’s behavior is substantially different from the overridden method, or details need to be provided (e.g., documenting additional side effects), a docstring with at least those differences is required on the overriding method.Certain aspects of a function should be documented in special sections, listed below. Each section begins with a heading line, which ends with a colon. All sections other than the heading should maintain a hanging indent of two or four spaces (be consistent within a file). These sections can be omitted in cases where the function’s name and signature are informative enough that it can be aptly described using a one-line docstring.
List each parameter by name. A description should follow the name, and be separated by a colon and a space. If the description is too long to fit on a single 80-character line, use a hanging indent of 2 or 4 spaces (be consistent with the rest of the file).
The description should include required type(s) if the code does not contain a corresponding type annotation. If a function accepts
*foo
(variable length argument lists) and/or**bar
(arbitrary keyword arguments), they should be listed as*foo
and**bar
.Describe the type and semantics of the return value. If the function only returns None, this section is not required. It may also be omitted if the docstring starts with Returns or Yields (e.g.
"""Returns row from Bigtable as a tuple of strings."""
) and the opening sentence is sufficient to describe return value.List all exceptions that are relevant to the interface. You should not document exceptions that get raised if the API specified in the docstring is violated (because this would paradoxically make behavior under violation of the API part of the API).
3.8.4 Classes toto
Classes should have a docstring below the class definition describing the class. If your class has public attributes, they should be documented here in an
Attributes
section and follow the same formatting as a function’sArgs
section.toto3.8.5 Block and Inline Comments toto
The final place to have comments is in tricky parts of the code. If you’re going to have to explain it at the next code review, you should comment it now. Complicated operations get a few lines of comments before the operations commence. Non-obvious ones get comments at the end of the line.
To improve legibility, these comments should start at least 2 spaces away from the code with the comment character
#
, followed by at least one space beforetotothe text of the comment itself.
On the other hand, never describe the code. Assume the person reading the code knows Python (though not what you’re trying to do) better than you do.
3.8.6 Punctuation, Spelling and Grammar toto
Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.
Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.
Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.