08.03.2018       Выпуск 220 (05.03.2018 - 11.03.2018)       Статьи

Синтаксис: *args and **kwargs

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Экспериментальная функция:

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In programming, we define a function to make a reusable code that performs similar operation. To perform that operation, we call a function with the specific value, this value is called a function argument in Python.

We would recommend you to read Python Function and Python Function Arguments.

Suppose, we define a function for addition of 3 numbers.

Example 1: Function to add 3 numbers

def adder(x,y,z):
    print("sum:",x+y+z)

adder(10,12,13)

When we run the above program, the output will be

sum: 35

In above program we have adder() function with three arguments x, y and z. When we pass three values while calling adder() function, we get sum of the 3 numbers as the output.


Lets see what happens when we pass more than 3 arguments in the adder() function.

def adder(x,y,z):
    print("sum:",x+y+z)

adder(5,10,15,20,25)

When we run the above program, the output will be

TypeError: adder() takes 3 positional arguments but 5 were given

In the above program, we passed 5 arguments to the adder() function instead of 3 arguments due to which we got TypeError.


Introduction to *args and **kwargs in Python

In Python, we can pass a variable number of arguments to a function using special symbols. There are two special symbols:

  1. *args (Non Keyword Arguments)
  2. **kwargs (Keyword Arguments)

We use *args and **kwargs as an argument when we are unsure about the number of arguments to pass in the functions.


Python *args

As in the above example we are not sure about the number of arguments that can be passed to a function. Python has *args which allow us to pass the variable number of non keyword arguments to function.

In the function, we should use an asterisk * before the parameter name to pass variable length arguments.The arguments are passed as a tuple and these passed arguments make tuple inside the function with same name as the parameter excluding asterisk *.

Example 2: Using *args to pass the variable length arguments to the function

def adder(*num):
    sum = 0
    
    for n in num:
        sum = sum + n

    print("Sum:",sum)

adder(3,5)
adder(4,5,6,7)
adder(1,2,3,5,6)

When we run the above program, the output will be

Sum: 8
Sum: 22
Sum: 17

In the above program, we used *num as a parameter which allows us to pass variable length argument list to the adder() function. Inside the function, we have a loop which adds the passed argument and prints the result. We passed 3 different tuples with variable length as an argument to the function.


Python **kwargs

Python passes variable length non keyword argument to function using *args but we cannot use this to pass keyword argument. For this problem Python has got a solution called **kwargs, it allows us to pass the variable length of keyword arguments to the function.

In the function, we use the double asterisk ** before the parameter name to denote this type of argument. The arguments are passed as a dictionary and these arguments make a dictionary inside function with name same as the parameter excluding double asterisk **.

Example 3: Using **kwargs to pass the variable keyword arguments to the function 

def intro(**data):
    print("\nData type of argument:",type(data))

    for key, value in data.items():
        print("{} is {}".format(key,value))

intro(Firstname="Sita", Lastname="Sharma", Age=22, Phone=1234567890)
intro(Firstname="John", Lastname="Wood", Email="johnwood@nomail.com", Country="Wakanda", Age=25, Phone=9876543210)

When we run the above program, the output will be

Data type of argument: <class 'dict'>
Firstname is Sita
Lastname is Sharma
Age is 22
Phone is 1234567890

Data type of argument: <class 'dict'>
Firstname is John
Lastname is Wood
Email is johnwood@nomail.com
Country is Wakanda
Age is 25
Phone is 9876543210

In the above program, we have a function intro() with **data as a parameter. We passed two dictionaries with variable argument length to the intro() function. We have for loop inside intro() function which works on the data of passed dictionary and prints the value of the dictionary.


Things to Remember:

  • *args and *kwargs are special keyword which allows function to take variable length argument.
  • *args passes variable number of non-keyworded arguments list and on which operation of the list can be performed.
  • **kwargs passes variable number of keyword arguments dictionary to function on which operation of a dictionary can be performed.
  • *args and **kwargs make the function flexible.


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