【python】入门学习（四）

函数：

定义函数

#area.py

from math import pi
def area(radius):
    """Return the area of a circle with the given radius."""
    return pi * radius ** 2

>>> ================================ RESTART ================================
>>> 
>>> area(5.5)
95.03317777109125
>>> print(area.__doc__)
Return the area of a circle with the given radius.
>>> 

doctest #可用于自动运行文档字符串中的python示例代码

全局变量访问时一定要加上global

#error
name = 'Jack'
def say_hello():
    print('Hello ' + name + '!')
def change_name(new_name):
    name = new_name

>>> say_hello()
Hello Jack!
>>> change_name('Mary')
>>> say_hello()
Hello Jack!

#correct
name = 'Jack'
def say_hello():
    print('Hello ' + name + '!')
def change_name(new_name):
    global name
    name = new_name

>>> say_hello()
Hello Jack!
>>> change_name('Mary')
>>> say_hello()
Hello Mary!

main():被认为是程序的起点，可选不一定要。运行时必须输入main()

python中参数的传递都是按引用传参，python不支持按值传参

在引用传参中，无法修改参数的值。下面的函数不起作用：

#reference.py
def set1(x):
    x = 1

>>> ================================ RESTART ================================
>>> 
>>> y = 5
>>> set1(y)
>>> y
5

函数参数默认值：

注意：包含默认参数的形参一定要放在无默认参数的形参后面

        只有第一次调用函数时给默认参数赋值！ #还不理解，先记下来

#greetings.py
def greet(name, greeting = 'Hello'):
    print(greeting, name + '!')

>>> greet('bob')
Hello bob!
>>> greet('bob', 'Good morning')
Good morning bob!

使用关键字传参，即在使用时也指明形参，可以不理会顺序，很好用：

#greetings.py
def greet(name = 'Bob', greeting = 'Hello'):
    print(greeting, name + '!')

>>> greet(greeting = 'Good evening', name = 'Mary')
Good evening Mary!
>>> greet(greeting = 'Good evening')
Good evening Bob!

也可以用模块化的方式来调用，模块中不包括main函数

>>> import greetings
>>> greetings.greet()
Hello Bob!
>>> dir(greetings)
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'greet']