Python Tutorial » User Input

Reading User Input
Taking input from user is a vital part of any application as it makes the application more interactive and user friendly. Further, a real world applications takes different actions based on user input.
Python provides a built-in function called input() which is used for reading user entered values from the keyboard.

This method takes a string as an argument which represents the message to be displayed to the user when asking him for entering some values.
input() function returns the value read from the keyboard always in string format. This means that if you are expecting anything other than string in your program, you need to convert it manually to that type.  Example,

# read user entered data
name = input("Enter your name")       
# print input value
print("Hello !!" + name)

Output will be as below. Text in green is the user entered data.

Note that input() function blocks program execution till the user enters some values and hits enter key. Till then the program keeps waiting for user input.

Type Conversion
Suppose you want to accept two numbers from user, add them and show their sum to the user. If you are familiar with input method, you would write the code as

# read a value
first_number = input("Enter first number")   
# read another value
second_number = input("Enter second number") 
# add values
sum = first_number + second_number           
# display sum
print("Sum is " + sum)

When the above code is executed, it behaves as below

Notice the output, it should have been 43 but it is instead 2023 which was unexpected. What is the reason?

The reason for this output is that input() function returns values in string format as mentioned earlier. Thus, while performing addition it was actually concatenating strings as ‘20‘ + ‘23‘ which results in ‘2023‘.

Therefore, in order for your program to work correctly, you should convert the return value of input() function as per your requirement.
Return value of input() can be converted to int, float using methods int and float respectively. Both these methods take a value as an argument and return the corresponding representation. Example,

int('54')    # will return 54
int(2.8)     # will return 2
float('5.4') # will return 5.4
float(3)     # will return 3.0

Remember that value to both these methods should be convertible to int or float or in other words, it should be a number. Trying to pass an invalid value will result in an error. Example,

# ValueError: invalid literal for int() with base 10: 'two'
print(int("two"))   

# ValueError: could not convert string to float: '2we'
print(float("2we"))

Now, the corrected version of the program to add two numbers will look like

# read a value
first_number = input("Enter first number")   
# convert string to int
first_number = int(first_number)             
# read another value
second_number = input("Enter second number") 
# convert string to int
second_number = int(second_number)           
# add values
sum = first_number + second_number           
# display sum
print("Sum is " + sum)

It is also possible to read input and convert it in the same line as

first_number = int(input("Enter first number"))

Return value of input() function is passed as argument to int method and its return value is assigned to a variable.
This is called method chaining where the output of one method becomes the input of another. Method chaining works from innermost method to outer methods.

Converting to string
Just like a string can be converted to an int or float, there is a method to do the opposite, that is, to convert an int or float to string using str() function.
This method takes a value as an argument and returns the same value converted to a string.
Example of usage follows

 # convert a float to string
value_string = str(2.3)    
# prints 2.3
print(value_string)         
# prints <class 'str'>
print(type(value_string))   
# convert an int to string
value_string = str(23)      
# prints 23
print(value_string)         
# prints <class 'str'>
print(type(value_string))