python函数

函数定义

def my_abs(x):
    return x if x >=0 else -x


if __name__ == '__main__':
    print(my_abs(5))
    print(my_abs(-5))

输出：

5
5

函数的返回

def return_item(res):
    return res

print("return:", return_item(1))
print("type:", type(return_item(1)))

print("return:", return_item("A"))
print("type:", type(return_item("A")))

输出：

return: 1
type: <class 'int'>
return: A
type: <class 'str'>

函数的多值返回

def return_items():
    return 1, 2, 3

a, b, c = return_items()
print("abc:", a, b, c)
print("return:", return_items())
print("type:", type(return_items()))

输出：

abc: 1 2 3
return: (1, 2, 3)
type: <class 'tuple'>

参数类型检查

def max(num1, num2):
    if not isinstance(num1, int) or not isinstance(num2, int):
        raise TypeError("must be int!")
    return num1 if num1 > num2 else num2


if __name__ == '__main__':
    print(max(1, 2))
    print(max("a", 2))

输出：

Traceback (most recent call last):
  File "C:/Users/focks/OneDrive/CrapCode/Python/TestAll/function.py", line 16, in <module>
    print(max("a", 2))
  File "C:/Users/focks/OneDrive/CrapCode/Python/TestAll/function.py", line 10, in max
    raise TypeError("must be int!")
TypeError: must be int!
2

函数的默认参数

def hello(name="world"):
    print("hello,", name, "!")


if __name__ == '__main__':
    hello()
    hello("god fish")

输出：

hello, world !
hello, god fish !

可变参数

可变参数传入的是一个tuple或list。

def calc(*numbers):
    sum = 0
    for n in numbers:
        sum = sum + n * n
    return sum


if __name__ == '__main__':
    print(calc(1, 2, 3, 4))
    num = [1, 2, 3, 4]
    print(calc(*num))

输出：

30
30

关键字参数

关键字参数传入的是一个字典。

def person(name, age, **kw):
    print('name:', name, 'age:', age, 'other:', kw)


if __name__ == '__main__':
    person("godfish", 10, school="hqu", color="green")
    godfish = {'school':"hqu", 'color':"green"}
    person("godfish", 10, **godfish)

输出：

name: godfish age: 10 other: {'school': 'hqu', 'color': 'green'}
name: godfish age: 10 other: {'school': 'hqu', 'color': 'green'}

命名关键字参数

命名关键字参数传入的是一个字典，字典的键必须在规定范围内，且规定范围内的所有键都必须要传入。

def person(name, age, *, city, job):
    print(name, age, city, job)


if __name__ == '__main__':
    person("godfish", 10, city="green city", job="wang")
    godfish = {'city': "green city", 'job': "wang"}
    person("godfish", 10, **godfish)
    
    # person("godfish", 10, like="hhh")
    # ->TypeError: person() got an unexpected keyword argument 'like'
    
    # person("godfish", 10, city="green city")
    # ->TypeError: person() missing 1 required keyword-only argument: 'job'

输出：

godfish 10 green city wang
godfish 10 green city wang

参数组合

节选自函数的参数。

在Python中定义函数，可以用必选参数、默认参数、可变参数、关键字参数和命名关键字参数，这5种参数都可以组合使用。但是请注意，参数定义的顺序必须是：必选参数、默认参数、可变参数、命名关键字参数和关键字参数。

比如定义一个函数，包含上述若干种参数：

def f1(a, b, c=0, *args, **kw):
    print('a =', a, 'b =', b, 'c =', c, 'args =', args, 'kw =', kw)

def f2(a, b, c=0, *, d, **kw):
    print('a =', a, 'b =', b, 'c =', c, 'd =', d, 'kw =', kw)

在函数调用的时候，Python解释器自动按照参数位置和参数名把对应的参数传进去。

>>> f1(1, 2)
a = 1 b = 2 c = 0 args = () kw = {}
>>> f1(1, 2, c=3)
a = 1 b = 2 c = 3 args = () kw = {}
>>> f1(1, 2, 3, 'a', 'b')
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {}
>>> f1(1, 2, 3, 'a', 'b', x=99)
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {'x': 99}
>>> f2(1, 2, d=99, ext=None)
a = 1 b = 2 c = 0 d = 99 kw = {'ext': None}

最神奇的是通过一个tuple和dict，你也可以调用上述函数：

>>> args = (1, 2, 3, 4)
>>> kw = {'d': 99, 'x': '#'}
>>> f1(*args, **kw)
a = 1 b = 2 c = 3 args = (4,) kw = {'d': 99, 'x': '#'}
>>> args = (1, 2, 3)
>>> kw = {'d': 88, 'x': '#'}
>>> f2(*args, **kw)
a = 1 b = 2 c = 3 d = 88 kw = {'x': '#'}

所以，对于任意函数，都可以通过类似func(*args, **kw)的形式调用它，无论它的参数是如何定义的。