Functions in Python - GeeksforGeeks

A function is a set of statements that take inputs, do some specific computation and produces output. The idea is to put some commonly or repeatedly done task together and make a function, so that instead of writing the same code again and again for different inputs, we can call the function.
Python provides built-in functions like print(), etc. but we can also create your own functions. These functions are called user-defined functions.

# A simple Python function to check 
# whether x is even or odd 
def evenOdd( x ): 
    if (x % 2 == 0): 
        print "even"
    else: 
        print "odd"
  
# Driver code 
evenOdd(2) 
evenOdd(3) 

Output:

even
odd

Pass by Reference or pass by value?
One important thing to note is, in Python every variable name is a reference. When we pass a variable to a function, a new reference to the object is created. Parameter passing in Python is same as reference passing in Java.

# Here x is a new reference to same list lst 
def myFun(x): 
   x[0] = 20
  
# Driver Code (Note that lst is modified 
# after function call. 
lst = [10, 11, 12, 13, 14, 15]  
myFun(lst); 
print(lst)  

Output:

[20, 11, 12, 13, 14, 15]

When we pass a reference and change the received reference to something else, the connection between passed and received parameter is broken. For example, consider below program.

def myFun(x): 
  
   # After below line link of x with previous 
   # object gets broken. A new object is assigned 
   # to x. 
   x = [20, 30, 40] 
  
# Driver Code (Note that lst is not modified 
# after function call. 
lst = [10, 11, 12, 13, 14, 15]  
myFun(lst); 
print(lst)  

Output:

[10, 11, 12, 13, 14, 15]

Another example to demonstrate that reference link is broken if we assign a new value (inside the function).

def myFun(x): 
  
   # After below line link of x with previous 
   # object gets broken. A new object is assigned 
   # to x. 
   x = 20
  
# Driver Code (Note that lst is not modified 
# after function call. 
x = 10 
myFun(x); 
print(x)  

Output:

Exercise: Try to guess the output of following code.

def swap(x, y): 
    temp = x; 
    x = y; 
    y = temp; 
  
# Driver code 
x = 2
y = 3
swap(x, y) 
print(x) 
print(y) 

Output:

2
3

Default arguments:
A default argument is a parameter that assumes a default value if a value is not provided in the function call for that argument.The following example illustrates Default arguments.

# Python program to demonstrate 
# default arguments 
def myFun(x, y=50): 
    print("x: ", x) 
    print("y: ", y) 
  
# Driver code (We call myFun() with only 
# argument) 
myFun(10) 

Output:

('x: ', 10)
('y: ', 50)

Like C++ default arguments, any number of arguments in a function can have a default value. But once we have a default argument, all the arguments to its right must also have default values.

Keyword arguments:
The idea is to allow caller to specify argument name with values so that caller does not need to remember order of parameters.

# Python program to demonstrate Keyword Arguments 
def student(firstname, lastname):  
     print(firstname, lastname)  
    
    
# Keyword arguments                   
student(firstname ='Geeks', lastname ='Practice')     
student(lastname ='Practice', firstname ='Geeks')   

Output:

('Geeks', 'Practice')
('Geeks', 'Practice')

Variable length arguments:
We can have both normal and keyword variable number of arguments. Please see this for details.

# Python program to illustrate   
# *args for variable number of arguments 
def myFun(*argv):  
    for arg in argv:  
        print (arg) 
    
myFun('Hello', 'Welcome', 'to', 'GeeksforGeeks')  

Output:

Hello
Welcome
to
GeeksforGeeks

# Python program to illustrate   
# *kargs for variable number of keyword arguments 
  
def myFun(**kwargs):  
    for key, value in kwargs.items(): 
        print ("%s == %s" %(key, value)) 
  
# Driver code 
myFun(first ='Geeks', mid ='for', last='Geeks')     

Output:

last == Geeks
mid == for
first == Geeks

Anonymous functions: In Python, anonymous function means that a function is without a name. As we already know that def keyword is used to define the normal functions and the lambda keyword is used to create anonymous functions. Please see this for details.

# Python code to illustrate cube of a number   
# using labmda function  
    
cube = lambda x: x*x*x  
print(cube(7))  

Output:

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