#3 Python 类型提示（Type Hints）

Python Python3 2021-01-28

Type Hint, 英文直译应该是输入提示。

动态类型语言有一个优点，同时也是缺点：不好做静态类型检查，IDE 或者其他开发工具很难根据代码去准确判断一个变量的类型。
Python 3.0 开始引入并逐渐完善类型注解（Type Annotation）则给 Python 静态类型检查提供了可能性。
PS: Python 运行时会忽略类型注解，不会给任何提示或警告。
PS: 之前的一些工具可以通过注释来做类型检查 (Type Comment)，起到相同的作用。

PEP

SF 3107 [2006-12-02] (3.0 ) Function Annotations  开始引入函数注解
SF 3141 [2007-04-23] (    ) A Type Hierarchy for Numbers
SF  424 [2012-07-14] (3.4 ) A method for exposing a length hint
SF  451 [2013-08-08] (3.4 ) A ModuleSpec Type for the Import System
SP  484 [2014-09-29] (3.5 ) Type Hints            类型提示
IF  483 [2014-12-19] (    ) The Theory of Type Hints
IF  482 [2015-01-08] (    ) Literature Overview for Type Hints
SF  526 [2016-08-09] (3.6 ) Syntax for Variable Annotations
SA  544 [2017-03-05] (3.8 ) Protocols: Structural subtyping (static duck typing)
SA  560 [2017-09-03] (3.7 ) Core support for typing module and generic types
SA  563 [2017-09-08] (3.7 ) Postponed Evaluation of Annotations
SA  561 [2017-09-09] (3.7 ) Distributing and Packaging Type Information
SA  585 [2019-03-03] (3.9 ) Type Hinting Generics In Standard Collections
SA  586 [2019-03-14] (3.8 ) Literal Types
SA  591 [2019-03-15] (3.8 ) Adding a final qualifier to typing
SA  589 [2019-03-20] (3.8 ) TypedDict: Type Hints for Dictionaries with a Fixed Set of Keys
SA  593 [2019-04-26] (3.9 ) Flexible function and variable annotations
SA  604 [2019-08-28] (3.10) Allow writing union types as ``X | Y``
SA  613 [2020-01-21] (    ) Explicit Type Aliases     引入类型别名
SA  647 [2020-10-07] (3.10) User-Defined Type Guards  引入 TypeGuard 类型，缩小类型检查时的范围
S   649 [2021-01-11] (    ) Deferred Evaluation Of Annotations Using Descriptors
S   655 [2021-01-30] (3.10) Marking individual TypedDict items as required or potentially-missing

基础用法

如果担心类型检查会，可以使用 @no_type_check 装饰器。

def greeting(name: str) -> str:
    return 'Hello ' + name

Vector = list[float]

def scale(scalar: float, vector: Vector) -> Vector:
    return [scalar * num for num in vector]

Union

from typing import NoReturn

Address = tuple[str, int]

def connect(Union[Address, str]) -> NoReturn:
    pass

Optional[T] 是 Union[T, None] 的简写。

def get_argument(name:str, default:Optional[str]=None) -> Union[str, None]:
    pass

其他常用类型

  • Any
  • Callable
  • ClassVar
  • NewType

stub 文件

https://github.com/python/typeshed
Collection of library stubs for Python, with static types

类型检查工具

IDE，比如 PyCharm，可以配置类型检查。
VSCode 或者 Atom 之类的编辑器也可以通过插件支持类型检查。

  1. mypy
  2. pyright
  3. pytype
  4. pyre

参考：

mypy

参考资料与拓展阅读

#2 Python f-string

Python Python3 2021-01-25

以前的字符串格式换方法

1. format 方法格式化

这种方法用的不多（对应 string.Formatter）。

Format String Syntax

replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"

field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*
arg_name          ::=  [identifier | digit+]
attribute_name    ::=  identifier
element_index     ::=  digit+ | index_string
index_string      ::=  <any source character except "]"> +

conversion        ::=  "r" | "s" | "a"

format_spec       ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill              ::=  <any character>
align             ::=  "<" | ">" | "=" | "^"
sign              ::=  "+" | "-" | " "
width             ::=  digit+
grouping_option   ::=  "_" | ","
precision         ::=  digit+
type              ::=  "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

print('{} {}'.format('Hello', 'World'))
print('There are three people in my family: {0}, {1}, and I, and I love my {0} a litte more.'.format('father', 'mother'))

2. 模板字符串格式化

这种方法我只在文档中看到，从没真的用过。
PS: 被废弃的 PEP 215 曾建议采用 $'a = $a, b = $b' 这种语法。

Template strings

from string import Template
Template('$who likes $what').substitute(who='tim', what='kung pao')
Template('$who likes $what').safe_substitute({'who': 'time'})

3. 百分号格式化

这应该是现在的最主流的字符串格式化方式。

printf-style String Formatting

print('Hello %s' % 'World')
print('action %s cost %.3f seconds' % ('download', 0.123456789))
print('%(language)s has %(number)03d quote types.' % {'language': "Python", "number": 2})

Python 3.6 新加入 f-string

Formatted string literals

f_string          ::=  (literal_char | "{{" | "}}" | replacement_field)*
replacement_field ::=  "{" f_expression ["="] ["!" conversion] [":" format_spec] "}"
f_expression      ::=  (conditional_expression | "*" or_expr)
                         ("," conditional_expression | "," "*" or_expr)* [","]
                       | yield_expression
conversion        ::=  "s" | "r" | "a"
format_spec       ::=  (literal_char | NULL | replacement_field)*
literal_char      ::=  <any code point except "{", "}" or NULL>

从 format 方法格式化语法中复用了很多 (格式化和 conversion 部分), 不过变得更强大了。
主要是里面支持条件语句，表达式 (包括 yield)。

a = 3.1415926
f'{a}'
f'{a:.2f}'

f"{1 + 1}", f"{{1 + 1}}", f"{{{1 + 1}}}"
# ('2', '{1 + 1}', '{2}')

注意：f-sting 里面不能使用反斜杠转义！

f'{John\'s}'
# SyntaxError: f-string expression part cannot include a backslash

r, s, a

  • !r -> repr()
  • !s -> str()
  • !a -> ascii()

PS: ascii 方法是 Python 3 引入，和 repr 相似，但是 ascii 方法仅使用 ASCII 字符。
例如：a = '中国'; print(f'{a!a} {ascii(a)}') 输出 '\u4e2d\u56fd' '\u4e2d\u56fd'。

print(f'{a!r}')

=

有人提议加入 !d 表示输出表达式本身，然后加上等于号，加上计算值，例如 f'1 + 1!d' => 1 + 1=2。
后来实现成了这样：

a = 3.14
b = 1
print(f'a + b=')   # a + b=4.140000000000001
print(f'a + b = ') # a + b = 4.140000000000001

很有趣！

参考资料与拓展阅读