Input and Output

A program often needs to interact with its user — to collect some input data from the end user, process it, and present the desired output. Python provides two built-in functions for this: input() for reading data from the user, and print() for displaying data on the screen.

Taking input: the input() function

The input() function prompts the user to enter data and reads what they type. Its syntax is:

input([Prompt])

• Prompt is an optional string that is displayed on the screen before the input is taken — typically a message telling the user what to enter. When a prompt is given, it is shown first, and then the user types their data.
• Typing is terminated by pressing the Enter key.
• input() takes exactly what was typed at the keyboard, converts it into a string, and assigns it to the variable on the left-hand side of the assignment operator =.

The crucial point: input() treats everything as a string. Even if the user types a number, the function returns it as string data.

>>> fname = input("Enter your first name: ")
Enter your first name: Arnab
>>> age = input("Enter your age: ")
Enter your age: 19
>>> type(age)
<class 'str'>

Here fname receives the string 'Arnab' — as expected. But age receives the string '19', not the number 19, which is why type(age) reports <class 'str'>.

Converting the input to a number

If we need to do arithmetic with the input, we must typecast it — change the data type of the accepted string to an appropriate numeric type. Wrapping the input() call in int() converts the string to an integer:

>>> age = int(input("Enter your age: "))
Enter your age: 19
>>> type(age)
<class 'int'>

Caution: if the user enters a non-numeric value (say nineteen), the conversion fails and an error is generated.

Producing output: the print() function

Python uses the print() function to send data to the standard output device — the screen. print() first evaluates the expression given to it and then displays the result. It outputs a complete line and then moves to the next line for subsequent output. Its syntax is:

print(value [, ..., sep = ' ', end = '\n'])

Two optional parameters control the formatting:

• sep — the separator placed between the output values when several are printed. It can be any character, integer or string; the default separator is a space.
• end — a string appended after the last value. The default is a newline ('\n'), which is why each print() normally starts a fresh line.

print("Hello")
print(10 * 2.5)
print("I" + "love" + "my" + "country")
print("I'm", 16, "years old")

Output:

Hello
25.0
Ilovemycountry
I'm 16 years old

Joining with + versus separating with ,

The third and fourth statements above look similar but behave differently:

• The third uses + to concatenate strings. Concatenation joins the pieces with no space added — hence Ilovemycountry. …