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Google Python team authored and gpshead committed May 8, 2021
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Expand Up @@ -15,100 +15,25 @@ See README.md for details.
- [1 Background](#s1-background)
- [2 Python Language Rules](#s2-python-language-rules)
* [2.1 Lint](#s2.1-lint)
+ [2.1.1 Definition](#s2.1.1-definition)
+ [2.1.2 Pros](#s2.1.2-pros)
+ [2.1.3 Cons](#s2.1.3-cons)
+ [2.1.4 Decision](#s2.1.4-decision)
* [2.2 Imports](#s2.2-imports)
+ [2.2.1 Definition](#s2.2.1-definition)
+ [2.2.2 Pros](#s2.2.2-pros)
+ [2.2.3 Cons](#s2.2.3-cons)
+ [2.2.4 Decision](#s2.2.4-decision)
* [2.3 Packages](#s2.3-packages)
+ [2.3.1 Pros](#s2.3.1-pros)
+ [2.3.2 Cons](#s2.3.2-cons)
+ [2.3.3 Decision](#s2.3.3-decision)
* [2.4 Exceptions](#s2.4-exceptions)
+ [2.4.1 Definition](#s2.4.1-definition)
+ [2.4.2 Pros](#s2.4.2-pros)
+ [2.4.3 Cons](#s2.4.3-cons)
+ [2.4.4 Decision](#s2.4.4-decision)
* [2.5 Global variables](#s2.5-global-variables)
+ [2.5.1 Definition](#s2.5.1-definition)
+ [2.5.2 Pros](#s2.5.2-pros)
+ [2.5.3 Cons](#s2.5.3-cons)
+ [2.5.4 Decision](#s2.5.4-decision)
* [2.6 Nested/Local/Inner Classes and Functions](#s2.6-nested)
+ [2.6.1 Definition](#s2.6.1-definition)
+ [2.6.2 Pros](#s2.6.2-pros)
+ [2.6.3 Cons](#s2.6.3-cons)
+ [2.6.4 Decision](#s2.6.4-decision)
* [2.7 Comprehensions & Generator Expressions](#s2.7-comprehensions)
+ [2.7.1 Definition](#s2.7.1-definition)
+ [2.7.2 Pros](#s2.7.2-pros)
+ [2.7.3 Cons](#s2.7.3-cons)
+ [2.7.4 Decision](#s2.7.4-decision)
* [2.8 Default Iterators and Operators](#s2.8-default-iterators-and-operators)
+ [2.8.1 Definition](#s2.8.1-definition)
+ [2.8.2 Pros](#s2.8.2-pros)
+ [2.8.3 Cons](#s2.8.3-cons)
+ [2.8.4 Decision](#s2.8.4-decision)
* [2.9 Generators](#s2.9-generators)
+ [2.9.1 Definition](#s2.9.1-definition)
+ [2.9.2 Pros](#s2.9.2-pros)
+ [2.9.3 Cons](#s2.9.3-cons)
+ [2.9.4 Decision](#s2.9.4-decision)
* [2.10 Lambda Functions](#s2.10-lambda-functions)
+ [2.10.1 Definition](#s2.10.1-definition)
+ [2.10.2 Pros](#s2.10.2-pros)
+ [2.10.3 Cons](#s2.10.3-cons)
+ [2.10.4 Decision](#s2.10.4-decision)
* [2.11 Conditional Expressions](#s2.11-conditional-expressions)
+ [2.11.1 Definition](#s2.11.1-definition)
+ [2.11.2 Pros](#s2.11.2-pros)
+ [2.11.3 Cons](#s2.11.3-cons)
+ [2.11.4 Decision](#s2.11.4-decision)
* [2.12 Default Argument Values](#s2.12-default-argument-values)
+ [2.12.1 Definition](#s2.12.1-definition)
+ [2.12.2 Pros](#s2.12.2-pros)
+ [2.12.3 Cons](#s2.12.3-cons)
+ [2.12.4 Decision](#s2.12.4-decision)
* [2.13 Properties](#s2.13-properties)
+ [2.13.1 Definition](#s2.13.1-definition)
+ [2.13.2 Pros](#s2.13.2-pros)
+ [2.13.3 Cons](#s2.13.3-cons)
+ [2.13.4 Decision](#s2.13.4-decision)
* [2.14 True/False Evaluations](#s2.14-truefalse-evaluations)
+ [2.14.1 Definition](#s2.14.1-definition)
+ [2.14.2 Pros](#s2.14.2-pros)
+ [2.14.3 Cons](#s2.14.3-cons)
+ [2.14.4 Decision](#s2.14.4-decision)
* [2.16 Lexical Scoping](#s2.16-lexical-scoping)
+ [2.16.1 Definition](#s2.16.1-definition)
+ [2.16.2 Pros](#s2.16.2-pros)
+ [2.16.3 Cons](#s2.16.3-cons)
+ [2.16.4 Decision](#s2.16.4-decision)
* [2.17 Function and Method Decorators](#s2.17-function-and-method-decorators)
+ [2.17.1 Definition](#s2.17.1-definition)
+ [2.17.2 Pros](#s2.17.2-pros)
+ [2.17.3 Cons](#s2.17.3-cons)
+ [2.17.4 Decision](#s2.17.4-decision)
* [2.18 Threading](#s2.18-threading)
* [2.19 Power Features](#s2.19-power-features)
+ [2.19.1 Definition](#s2.19.1-definition)
+ [2.19.2 Pros](#s2.19.2-pros)
+ [2.19.3 Cons](#s2.19.3-cons)
+ [2.19.4 Decision](#s2.19.4-decision)
* [2.20 Modern Python: Python 3 and from \_\_future\_\_ imports](#s2.20-modern-python)
+ [2.20.1 Definition](#s2.20.1-definition)
+ [2.20.2 Pros](#s2.20.2-pros)
+ [2.20.3 Cons](#s2.20.3-cons)
+ [2.20.4 Decision](#s2.20.4-decision)
* [2.21 Type Annotated Code](#s2.21-type-annotated-code)
+ [2.21.1 Definition](#s2.21.1-definition)
+ [2.21.2 Pros](#s2.21.2-pros)
+ [2.21.3 Cons](#s2.21.3-cons)
+ [2.21.4 Decision](#s2.21.4-decision)
- [3 Python Style Rules](#s3-python-style-rules)
* [3.1 Semicolons](#s3.1-semicolons)
* [3.2 Line length](#s3.2-line-length)
Expand Down Expand Up @@ -288,8 +213,11 @@ that the arguments are actually unused.
### 2.2 Imports

Use `import` statements for packages and modules only, not for individual
classes or functions. Note that there is an explicit exemption for imports from
the [typing module](#typing-imports).
classes or functions. Imports from the [typing module](#typing-imports),
[typing_extensions module](https://github.com/python/typing/tree/master/typing_extensions),
and the
[six.moves module](https://six.readthedocs.io/#module-six.moves)
are exempt from this rule.

<a id="s2.2.1-definition"></a>
<a id="221-definition"></a>
Expand Down Expand Up @@ -343,10 +271,6 @@ Do not use relative names in imports. Even if the module is in the same package,
use the full package name. This helps prevent unintentionally importing a
package twice.

Imports from the [typing module](#typing-imports) and the
[six.moves module](https://six.readthedocs.io/#module-six.moves)
are exempt from this rule.

<a id="s2.3-packages"></a>
<a id="23-packages"></a>

Expand Down Expand Up @@ -584,7 +508,7 @@ assignments to global variables are done when the module is first imported.
Avoid global variables.

While they are technically variables, module-level constants are permitted and
encouraged. For example: `MAX_HOLY_HANDGRENADE_COUNT = 3`. Constants must be
encouraged. For example: `_MAX_HOLY_HANDGRENADE_COUNT = 3`. Constants must be
named using all caps with underscores. See [Naming](#s3.16-naming) below.

If needed, globals should be declared at the module level and made internal to
Expand Down Expand Up @@ -637,9 +561,9 @@ outer function longer and less readable.
#### 2.6.4 Decision

They are fine with some caveats. Avoid nested functions or classes except when
closing over a local value. Do not nest a function just to hide it from users of
a module. Instead, prefix its name with an \_ at the module level so that it can
still be accessed by tests.
closing over a local value other than `self` or `cls`. Do not nest a function
just to hide it from users of a module. Instead, prefix its name with an \_ at
the module level so that it can still be accessed by tests.

<a id="s2.7-comprehensions"></a>
<a id="s2.7-list_comprehensions"></a>
Expand Down Expand Up @@ -1705,7 +1629,7 @@ Yes: with very_long_first_expression_function() as spam, \
```python
No: with VeryLongFirstExpressionFunction() as spam, \
VeryLongSecondExpressionFunction() as beans:
PlaceOrder(eggs, beans, spam, beans)
PlaceOrder(beans, spam)
```
```python
Expand Down Expand Up @@ -2911,6 +2835,17 @@ containing `exec "$0.py" "$@"`.
<a id="s3.17-main"></a>
<a id="317-main"></a>
<a id="math-notation"></a>
#### 3.16.5 Mathematical Notation
For mathematically heavy code, short variable names that would otherwise violate
the style guide are preferred when they match established notation in a
reference paper or algorithm. When doing so, reference the source of all naming
conventions in a comment or docstring or, if the source is not accessible,
clearly document the naming conventions. Prefer PEP8-compliant
`descriptive_names` for public APIs, which are much more likely to be
encountered out of context.
<a id="main"></a>
### 3.17 Main
Expand Down Expand Up @@ -3150,22 +3085,22 @@ 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 interpreted as `a: Optional[Text] = None`,
but that is no longer the preferred behavior.
484 allowed `a: str = None` to be interpreted as `a: Optional[str] = None`, but
that is no longer the preferred behavior.
```python
Yes:
def func(a: Optional[Text], b: Optional[Text] = None) -> Text:
def func(a: Optional[str], b: Optional[str] = None) -> str:
...
def multiple_nullable_union(a: Union[None, Text, int]) -> Text
def multiple_nullable_union(a: Union[None, str, int]) -> str:
...
```
```python
No:
def nullable_union(a: Union[None, Text]) -> Text:
def nullable_union(a: Union[None, str]) -> str:
...
def implicit_optional(a: Text = None) -> Text:
def implicit_optional(a: str = None) -> str:
...
```
Expand All @@ -3185,7 +3120,7 @@ long:
```python
_ShortName = module_with_long_name.TypeWithLongName
ComplexMap = Mapping[Text, List[Tuple[int, int]]]
ComplexMap = Mapping[str, List[Tuple[int, int]]]
```
Other examples are complex nested types and multiple return variables from a
Expand Down Expand Up @@ -3248,7 +3183,7 @@ The latter is commonly used as the return type from a function.
```python
a = [1, 2, 3] # type: List[int]
b = (1, 2, 3) # type: Tuple[int, ...]
c = (1, "2", 3.5) # type: Tuple[int, Text, float]
c = (1, "2", 3.5) # type: Tuple[int, str, float]
```
<a id="s3.19.10-typevars"></a>
Expand Down Expand Up @@ -3276,7 +3211,7 @@ def next(l: List[T]) -> T:
A TypeVar can be constrained:
```python
AddableType = TypeVar("AddableType", int, float, Text)
AddableType = TypeVar("AddableType", int, float, str)
def add(a: AddableType, b: AddableType) -> AddableType:
return a + b
```
Expand Down Expand Up @@ -3469,13 +3404,13 @@ remember that in many cases [`TypeVar`](#typing-type-var) might be more
appropriate:
```python
def get_names(employee_ids: List[Any]) -> Dict[Any, Text]:
def get_names(employee_ids: List[Any]) -> Dict[Any, str]:
"""Returns a mapping from employee ID to employee name for given IDs."""
```
```python
T = TypeVar('T')
def get_names(employee_ids: List[T]) -> Dict[T, Text]:
def get_names(employee_ids: List[T]) -> Dict[T, str]:
"""Returns a mapping from employee ID to employee name for given IDs."""
```
Expand Down

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