Python The parameter type of the function

• 1. Positional arguments
• 2. Default parameter
• 3. The key parameters
• 4. Variable length parameter
• • 4.1) *parameter
  • 4.2) **parameter

Python When you define a function You don't have to specify The type of parameter , Argument types passed entirely by the caller and Python The interpreter's understanding and inference determine .
Next , This article will introduce the parameter types of the following four functions ：

• Positional arguments 、 Default parameter 、 Key parameters and variable length parameters .

1. Positional arguments

Positional arguments ： Actual and formal parameters when calling a function The sequence must be strictly consistent , And arguments and formal parameters The quantity must be the same .

The following format ：

def showArgs(a, b, c):
print(a, b, c)
show(1, 2, 3) # 1 2 3
show(1, 2) # TypeError: showArgs() missing 1 required positional argument: 'c'
show(1, 2, 3, 4) # TypeError: showArgs() takes from 1 to 3 positional arguments but 4 were given

2. Default parameter

Default parameter ： When calling a parameter with a default value , You can not assign a value to a parameter that has a default value （ If there is , will Use the passed value ）, That is, different from the above position parameters , Parameter passing can be missing , But you can't pass on more .

How to use it ： namely , stay When defining a function ：def demo(a=1, b=2) A default value is given when filling in parameters .

Let's take an example ：

def showArgs(a, b=3, c=2):
print(a, b, c)
showArgs(1, 2, 3) # 1 2 3
showArgs(1, 2) # 1 2 2
showArgs(1, 2, 3, 4) # TypeError: showArgs() takes from 1 to 3 positional arguments but 4 were given

matters needing attention ：

• The default value parameter must appear in the function parameter list Rightmost end , The right side of any default value parameter cannot have non default value parameters
• Assignment of default value parameter It will only be interpreted once when the function is defined
  （ Therefore, the default value parameter should preferably be of immutable type ）

3. The key parameters

Use , The key parameters , The order of arguments and formal parameters can be different , However, if the parameter has no default value , The number of parameters should also be consistent .

How to use it ： stay When you call a function ： demo(a=1, b=2) To specify the value of the parameter .

Let's take an example ：

def showArgs(a, b, c):
print(a, b, c)
showArgs(1, 2, c=3) # 1 2 3
showArgs(a=1, c=22, b=33) # 1 33 22

matters needing attention ：

• Key parameters cannot be followed by position parameters

Wrong situation ：

showArgs(1, 22, b=33)

It will report this error ：TypeError: showArgs() got multiple values for argument 'b', Because though b Key parameters are used , But the first two are positional parameters , Through position parameters ,b It has been assigned

showArgs(a=11, c=222, 333)

It will report this error ：SyntaxError: positional argument follows keyword argument, Key parameters cannot be followed by positional parameters

4. Variable length parameter

Variable length parameter ： That is, the parameter length is not fixed
It has the following two forms ：

• *parameter: Receive multiple Location parameter And put it in Tuples in
• **parameter: Receive multiple The key parameters And put it in Dictionaries in

Among them parameter You can customize the name you want

4.1) *parameter

Must receive Positional arguments ！！！
Its usage is as follows :

def showArgs(*args):
print(args)
showArgs(1, 2, 3) # (1, 2, 3)
showArgs(7, 8) # (7, 8)

4.2) **parameter

Must receive The key parameters ！！！
Its usage is as follows ：

def showArgs(**args):
print(args)
showArgs(a=1, b=2, c=3) # {'a': 1, 'b': 2, 'c': 3}
showArgs(a=7, b=8) # {'a': 7, 'b': 8}