This lesson shows a simple function to take user input for console applications. Console application are also known as command-line application. Python has a function input which can be used to take user input.
User input with built-in function
When interacting with user, we may need to take user responses for certain variables and based on user inputs, we may respond differently. For example, in an application which calculates student grades, we may have to ask for student marks for each subject and based on those, we can assign some grades or calculate average or GPA.
Python provides a function input which can be used to take user input. As per [official documentation for this function], this function takes an argument of prompt and waits for user input until user enters something with new line. Basic usage looks like this. Here, whatever user enters will be stored in s variable in the form of string data type.
1s = input("Enter something: ")
Let’s see a simple example asking for user’s name and then greeting them.
1name = input("Enter your name: ")
2print(f"Hello {name}")
Execute this code and the console will wait for your input after prompt Enter your name: .
python main.py
Enter your name: Johny
Hello Johny
On caveat with input function is that it always returns str as output data type. That is ther variable being assigned will have value as string. If we want to take input as numerical value, then we will have to cast to required data type.
Type Casting
Type casting is converting variable of one type into another. In Python, it’s relatively straight forward. We just use the function for the data type to which we want to convert current value. Below code shows demo of type casting.
1i = 10 # int
2decimal = 10.20 # float
3int_to_str = str(i) # convert to str
4float_to_int = int(decimal) # convert float to int
5float_to_str = str(decimal) # convert float to str
6int_to_float = float(i) # convert int to float
7print(type(int_to_str)) # str
8print(type(float_to_str)) # str
9print(type(int_to_float)) # float
10print(type(float_to_int)) # int
<class 'str'>
<class 'str'>
<class 'float'>
<class 'int'>
How to take integer input from user?
Let’s get back to input function and see what happens when we want to take int input.
1name = input("Enter your name: ")
2age = input("Enter your age: ")
3age = age + 1
4print(f"Hello {name}, next year you'll be {age} years old.")
python main.py
Enter your name: John
Enter your age: 21
When executed, we get below error.
TypeError: can only concatenate str (not "int") to str
Here, when we try to input age. It takes 21 as age variable. However, this value is of type str and on next line, when we try to add 1 to this variable. It fails to understand that we are trying to perform addition. Python interpreter thinks we are trying to perform string concatenation (joining two strings). It cannot join str with int and hence throws TypeError. To fix this, we have to first cast age to int type.
1name = input("Enter your name: ")
2age = input("Enter your age: ")
3age = int(age)
4age = age + 1
5print(f"Hello {name}, next year you'll be {age} years old.")
Hello John, next year you'll be 22 years old.
Conclusion
This lesson shows how you can take user input in command-line applications. We also saw how we have to explicitly cast from string data type to respective data type if we want input of type other than string.
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