Python静态类型

Mypy是Python的可选静态类型检查器。您可以在Python程序中添加类型提示（PEP 484），并使用mypy进行静态类型检查。查找程序中的错误，甚至不运行它们！

安装Mypy

Mypy需要运行Python 3.5或更高版本。下面代码是用的Python 3.7 使用pip安装mypy：

$ python3 -m pip install mypy

看几个简单例子：
平时我们代码大概都是下面的样子：

# 参数name的类型是未知的
# 返回的类型也是未知的
def greeting(name):
    return 'Hello ' + name

稍微修改一些

# 参数name后面跟了:str 代表name期望是str类型
# 参数括号后面跟了->str代表返回类型为str类型
def greeting(name: str) -> str:
    return 'Hello ' + name
x: str = 'xxx' # 声明一个变量为str类型
greeting(x) # Hello xxx
greeting(123) # TypeError: can only concatenate str (not "int") to str

到目前为止仅有str、float等基本类型，使用list、tuple复杂类型需要引用typing中的类

LIST

数组类型

from typing import List

# 参数names为list类型并且元素都是str类型
# 返回为None
def greet_all(names: List[str]) -> None:
    for name in names:
        print('Hello ' + name)

names = ["Alice", "Bob", "Charlie"]
ages = [10, 20, 30]

greet_all(names)   # Ok!
greet_all(ages)    #出错了 Error due to incompatible types 

Dict

字典类型

from typing import Dict

# Dict[int, str] int代表key类型， str代表val类型
def test(t: Dict[int, str]) -> Dict:
    return t

test({1: '234'}) # {1: '234'}

Iterable

可以迭代类型包括 List、Set、Tuple、Str、Dict

def greeting(names: Iterable[str]) -> None:
    for name in names:
        print(name)

greeting(['aaa', 'bbb']) # list aaa bbb
greeting(('ccc', 'ddd')) # tuple ccc ddd
greeting({'eee', 'fff'}) # set eee fff
greeting({'ggg': 'hhh'}) # dict ggg
greeting('123') # str 1 2 3 
greeting(678) # error: Argument 1 to "greeting" has incompatible type "int"; expected "Iterable[str]"

Union

接受多个指定类型，但不接受除此外的类型

from typing import Union
# user_id 只能为int或者str
def normalize_id(user_id: Union[int, str]) -> str:
    if isinstance(user_id, int):
        return 'user-{}'.format(100000 + user_id)
    else:
        return user_id
normalize_id(1) # user-100001
normalize_id('2') # 2

Optional

可选类型，给参数设置默认值

from typing import Optional

# Optional[str]只是Union [str，None]的简写或别名。它主要是为了方便帮助功能签名看起来更清洁
# Optional[str, int] 只能包含一个类型, 这样是不正确的
def greeting(name: Optional[str] = None) -> str:
    if name is None:
        name = 'stranger'
    return 'Hello, ' + name
greeting() # Hello, stranger
greeting('123') # Hello, 123

Any

有时候我们不确定是什么类型的时候可以用到Any

from typing import Any

def greeting(name: Any = None) -> str:
    if name is None:
        name = 'stranger'

    return 'Hello, ' + str(name)

greeting() # Hello, stranger
greeting('123') # Hello, 123
greeting(234) # Hello, 234
greeting([345]) # Hello, [345]

TypeVar

自定义类型

from typing import TypeVar

T = TypeVar('T') # 任意类型
A = TypeVar('A', int, str) # A类型只能为int或str
def test(t: A) -> None:
    print(t)
test(1)

Type comments

No first-class syntax support for explicitly marking variables as being of a specific type is added by this PEP(pep-0484). To help with type inference in complex cases, a comment of the following format may be used:

x = []                # type: List[Employee]
x, y, z = [], [], []  # type: List[int], List[int], List[str]
x, y, z = [], [], []  # type: (List[int], List[int], List[str])
a, b, *c = range(5)   # type: float, float, List[float]
x = [1, 2]            # type: List[int]

Type comments should be put on the last line of the statement that contains the variable definition. They can also be placed on with statements and for statements, right after the colon.

Examples of type comments on with and for statements:

with frobnicate() as foo:  # type: int
    # Here foo is an int
    ...

for x, y in points:  # type: float, float
    # Here x and y are floats
    ...

In stubs it may be useful to declare the existence of a variable without giving it an initial value. This can be done using PEP 526 variable annotation syntax:

from typing import IO

stream: IO[str]    # Note: no initial value!

参考链接： 

https://www.jianshu.com/p/37f945cd91c3

官方链接：

https://docs.python.org/3.6/library/typing.html#distinct

https://www.python.org/dev/peps/pep-0484/

https://www.python.org/dev/peps/pep-0585/