Python: Input and Output

The print() Function

print() is the most commonly used built-in function in Python. It writes its arguments to standard output (your terminal) followed by a newline. You can pass it any number of values separated by commas, and it will display them all on one line with a space between each by default.

The input() Function

input() pauses the program and waits for the user to type something and press Enter. It optionally accepts a prompt string that is displayed before the cursor so the user knows what to type. A critical point that catches almost every beginner: input() always returns a string, regardless of what the user types. If you need a number, you must convert it explicitly.

print() with sep and end Parameters

The sep parameter controls what character or string is placed between each value you pass to print. The end parameter controls what is placed after the last value. Changing these two parameters gives you fine control over exactly how your output looks without building a formatted string first.

String Formatting with % Style

The % formatting style is the oldest way to embed values inside strings in Python, borrowed directly from C's printf. You write a template string with %s, %d, or %f as placeholders, then follow the string with a % operator and a tuple of values. While you will still encounter this style in older codebases, f-strings are preferred for new code.

str.format() Method

The str.format() method was introduced in Python 2.6 as a cleaner replacement for % formatting. You place curly brace placeholders {} inside a string and call .format() with the values to insert. Placeholders can be positional, named, or contain format specifications just like % style.

f-strings (Python 3.6+)

f-strings (formatted string literals) are the modern, recommended way to format strings in Python. Prefix a string with f or F and then place any valid Python expression inside curly braces directly in the string. Python evaluates the expression at runtime and inserts the result. f-strings are not just more readable than the older alternatives; they are also faster because the interpolation is done at the C level.

Formatting Numbers (Width and Precision)

Python's format mini-language applies to all three formatting styles and gives you fine control over how numbers are displayed: total field width, decimal precision, alignment, sign display, and thousands separators. These features are essential when building tables, reports, or any output that needs to look aligned and professional.

Reading Multiple Inputs

Sometimes you need to read several values at once, either on separate lines or all on one line separated by spaces. The standard pattern for a single line of space-separated values is to call input().split() which splits the string on whitespace and returns a list of strings, then pipe that through map() to convert each piece to the required type.

Command-line Arguments with sys.argv

When you run a Python script from the terminal you can pass extra information directly on the command line: python greet.py Alice 30. Python stores these arguments in a list called sys.argv, available after you import the sys module. The first element sys.argv[0] is always the script name itself; actual user-provided arguments start at index 1.

Note: sys.argv reads actual command-line values when you run a .py file in your terminal. The in-browser compiler above will show the script structure but the arguments list will vary in that environment.

Quiz - Test Your Knowledge

Eight questions covering every topic in this tutorial. Work through them carefully before checking your answers.