Detailed explanation of parameters of functions in Python

1. Types of function parameters in Python

• Required parameters: parameters that must be passed in when calling the function, only the parameter name is defined when the function is defined
• Keyword parameters: When passed in, it is passed in as a parameter name value pair of the function
• Default parameter: When the function is defined, a value is set by default for the parameter. When the function is called, the parameter is not transmitted, that is, the default value is used.
• Variable length parameters: parameters modified with * or **; * modified parameters are a tuple (tuple), ** modified parameters must be a dictionary (dict), usually written as *args or **args

2, Python's required parameters

• The number of parameters passed in must be the same as the number of formal parameters

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


def demo01(a,b):
    print(a, type(a))
    print(b, type(b))


# demo01(1) #TypeError missing 1 required positional argument: 'b' is missing an unknown parameter, demo01 must pass 2 parameters
# demo01(1, 2, 3) #TypeError demo01() takes 2 positional arguments but 3 were given, demo01 function has 2 positional arguments but 3 were received;
# Required parameters: parameters that must be passed in when calling the function, only the parameter name is defined when the function is defined
# The number of parameters passed in must be the same as the number of formal parameters
demo01(1, 2)      # yes
demo01(1, [1, 2])      # yes
demo01([2, 3], (1, 2))      # yes
demo01(1, {2, 3, 4})      # yes
demo01(2, {"code":'1001', "name":"zhang", "age":18})      # yes

3. Keyword Parameters

• The parameter is passed in the way of associating the parameter name and the parameter value, the mode of the key-value pair, and the parameter name is the key.

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


def demo01(a,b):
    print(a, type(a), end="__")
    print(b, type(b))

# The parameter before the parameter position of the first keyword entry and exit is selected by keyword parameter; in the following example, the variable passed by the first keyword is a, and a is in the second position of the function definition, then the first The parameters before the two positions must be passed in the form of keyword parameters.
# demo01(1, a=2) # demo01() got multiple values for argument 'a'


demo01(a=1, b=2)        # yes
demo01(1, b=2)          # yes
demo01(b=1, a=2)        # yes
demo01(b=1, a=[1, 2])           # yes
demo01(b=[2, 3], a=(1, 2))      # yes
demo01(b=1, a={2, 3, 4})        # yes
demo01(b=2, a={"code":'1001', "name":"zhang", "age":18})      # yes

4. Default parameters

• When calling the function, the default value of the parameter is not passed in, an d it is processed according to the passed parameter value when it is passed in.

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


## default parameters
def demo02(City = "LongNan"):
    print("City is the default parameter, the default value is: China,The current value is:", City)

# don't pass parameters
demo02()
# Incoming parameters
demo02("Beijing")

5. Variable length parameters

• The passed parameter will generate a tuple type variable for internal use of the function

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


## variable length parameter
def demo03(*args):
    print(args,type(args))

# The passed parameter will generate a tuple type variable for internal use of the function
demo03(1)
demo03("code")
demo03(1,"code")

• The number of parameters passed in cannot be less than the number of required parameters
• The formal parameters after *args must be passed as keyword arguments

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


# The formal parameters after *args must be passed as keyword parameters,
def demo04(a, b, *args,c):
    print("a The parameter values ​​passed in are:{},b The parameter values ​​passed in are:{}, args The parameters passed in are:{}, c The in and out parameters are:{}".format(a, b, args,c))
# The number of parameters passed in cannot be less than the number of mandatory parameters, a,b,c are mandatory parameters
# demo04(1, 2)      # TypeError
# demo04(1, 2, 3)   # TypeError
demo04(1, 2, c=3)

• Parameters before *args can only be passed by position, not by keyword
• When the number of incoming parameters is more than the required parameters, first assign the required parameters by position, and then generate a tuple for the remaining parameters and pass them to args

• **Modified parameters must be passed as keyword parameters, and the Python interpreter will generate a dictionary with the passed-in keyword and keyword value for internal use in the function

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


def demo05(**kwargs):
    print("kwargs The parameters passed in are:{}".format(kwargs),type(kwargs))


kwargs = {"code":'1002', "name":"zhang"}

#  demo05(kwargs)   # TypeError
# **Modified parameters must be passed as keyword parameters, and the Python interpreter will generate a dictionary with the passed-in keyword and keyword value for internal use in the function
demo05(**kwargs)                    # kwargs The parameter passed in is: {'code': '1002', 'name': 'zhang'} <class 'dict'>
demo05(code='1002',name="zhang")    # kwargs The parameter passed in is: {'code': '1002', 'name': 'zhang'} <class 'dict'>

• **The modified parameter must be the last one

# coding:utf-8
# Time:2022/6/27 20:04
# Author:YangXiaoPeng


# **The modified parameter must be the last one
"""
# SyntaxError: invalid syntax
def demo06(a, b, *args, c, **kwargs, d):
    pass
"""


def demo06(a, b, *args, c, **kwargs):
    print("a The parameter value passed in is:{},b The parameter value passed in is:{}, args The parameter passed in is:{}".format(a, b, args),end='')
    print(",c The parameter passed in is:{},kwargs The parameter passed in is:{}".format(c,kwargs))


demo06(1, 2, 3, 4, 5, c=3, code='1002', name="zhang", d=3)