4.7.7. Function Annotations

Function annotations are completely optional, arbitrary metadata information about user-defined functions. Neither Python itself nor the standard library use function annotations in any way; this section just shows the syntax. Third-party projects are free to use function annotations for documentation, type checking, and other uses.

Annotations are stored in the __annotations__ attribute of the function as a dictionary and have no effect on any other part of the function. Parameter annotations are defined by a colon after the parameter name, followed by an expression evaluating to the value of the annotation. Return annotations are defined by a literal ->, followed by an expression, between the parameter list and the colon denoting the end of the def statement. The following example has a positional argument, a keyword argument, and the return value annotated with nonsense:

>>> def f(ham: 42, eggs: int = 'spam') -> "Nothing to see here":

...     print("Annotations:", f.__annotations__)

...     print("Arguments:", ham, eggs)

...

>>> f('wonderful')

Annotations: {'eggs': <class 'int'>, 'return': 'Nothing to see here', 'ham': 42}

Arguments: wonderful spam

 def f(ham: 42, eggs: int = 'spam') -> 'Nothing to see here':

     print('Annotations:', f.__annotations__)

     print('Arguments:', ham, eggs)

 f('wonderful')

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32

Type "copyright", "credits" or "license()" for more information.

>>> ================================ RESTART ================================

>>>

Annotations: {'eggs': <class 'int'>, 'ham': 42, 'return': 'Nothing to see here'}

Arguments: wonderful spam

>>> 

ONE : 这里给ham赋一个初始值'Hongten'

 #这里给ham赋一个初始值'Hongten'

 def f(ham: 42 = 'Hongten', eggs: int = 'spam') -> 'Nothing to see here':

     print('Annotations:', f.__annotations__)

     print('Arguments:', ham, eggs)

 f()

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32

Type "copyright", "credits" or "license()" for more information.

>>> ================================ RESTART ================================

>>>

Annotations: {'eggs': <class 'int'>, 'ham': 42, 'return': 'Nothing to see here'}

Arguments: Hongten spam

>>> 

//TWO :  这里把42变为：'这里是ham的注释'

 #这里把42变为：'这里是ham的注释'

 def f(ham: '这里是ham的注释' = 'Hongten', eggs: int = 'spam') -> 'Nothing to see here':

     print('Annotations:', f.__annotations__)

     print('Arguments:', ham, eggs)

 f()

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32

Type "copyright", "credits" or "license()" for more information.

>>> ================================ RESTART ================================

>>>

Annotations: {'eggs': <class 'int'>, 'return': 'Nothing to see here', 'ham': '这里是ham的注释'}

Arguments: Hongten spam

>>> 

//THREE :  这里把int变为str

 #这里把int变为str

 def f(ham: '这里是ham的注释' = 'Hongten', eggs: str = 'spam') -> 'Nothing to see here':

     print('Annotations:', f.__annotations__)

     print('Arguments:', ham, eggs)

 f()

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32

Type "copyright", "credits" or "license()" for more information.

>>> ================================ RESTART ================================

>>>

Annotations: {'eggs': <class 'str'>, 'ham': '这里是ham的注释', 'return': 'Nothing to see here'}

Arguments: Hongten spam

>>> 

//FOUR :  看看一段java函数代码

  /**

     * 判断一个字符串是否全为字母,此方法比上面的isAllChar方法效率要高，但是需要的是str中包含非字母字符在靠前面

     * 如："a2bcd",对于这个字符串，到字符"2"的时候就会终止判断

     *

     * @param str

     *            所需判断的字符串

     * @return str中是否全为字母字符

     */

    public static boolean isAllChars(String str) {

        if (str == null || str.equals("")) {

            return false;

        }

        boolean flag = true;

        for (int i = 0; i < str.length(); i++) {

            if ((str.charAt(i) < 'a' || str.charAt(i) > 'z')

                    && (str.charAt(i) < 'A' || str.charAt(i) > 'Z')) {

                flag = false;

                break;

            }

        }

        return flag;

    }

def f(ham: 42, eggs: int = 'spam') -> "Nothing to see here":

    print("Annotations:", f.__annotations__)

    print("Arguments:", ham, eggs)

#def关键字定义了函数f，在函数f中有两个参数：ham，eggs。

#其中ham没有默认值，而eggs是由默认值的，其默认值为'spam'.

#参数ham的注释部分为：42；参数eggs的注释部分为：int

# "Nothing to see here"是返回值的注释，这个必须用 '->'连接

#看了java代码后，你会有更直观的认识，注释嘛，你可以根据你自己的想法，想怎么写就怎么写，如42，int；不过不好的注释有时候会给别人阅读代码带来很大的麻烦

#如果上面的代码是这样写：

def f(ham: int = 42, eggs: str = 'spam') -> 'Nothing to see here':

    print("Annotations:", f.__annotations__)

    print("Arguments:", ham, eggs)

#我想很多人阅读代码的时候就很容易理解啦

#上面只是个人观点，如果不正确的地方，还请大家指正 #同时也希望大家相互学习：hongtenzone@foxmail.com