Python User Input & Output

input("prompt? ") reads a line from the user (always as a string). int(input(...)) for numbers. print with sep= and end= and f"..." for formatted output.

Interactive scripts need to read input and produce output. Python's input() and print() cover both.

input(): reading from the user

name = input("What is your name? ")
print(f"Hello, {name}!")

input(prompt) displays prompt, waits for the user to type a line, returns the line as a string. Newline is stripped.

Run it:

What is your name? Alice
Hello, Alice!

input() always returns a string

age = input("How old are you? ")
# age is a string, even if user types digits
next_age = age + 1   # ERROR — can't add int to str

To do arithmetic, convert:

age = int(input("How old are you? "))
next_age = age + 1
print(f"Next year you will be {next_age}")

int(...) converts the string to int. float(...) for decimals.

If the user types non-numeric input, int(...) raises ValueError. Always handle bad input:

try:
    age = int(input("How old? "))
except ValueError:
    print("That's not a number!")

We dive into exceptions in lesson 16.

A temperature converter

print("Temperature Converter")
print("1. Celsius to Fahrenheit")
print("2. Fahrenheit to Celsius")

choice = input("Enter choice (1 or 2): ")
temp = float(input("Enter temperature: "))

if choice == "1":
  result = temp * 9/5 + 32
  print(f"{temp}°C = {result:.1f}°F")
else:
  result = (temp - 32) * 5/9
  print(f"{temp}°F = {result:.1f}°C")

Three input calls (technically two — choice and temp), one if/else, formatted output. The whole "interactive program" pattern in 12 lines.

{result:.1f} formats to 1 decimal place — episode 3's f-string syntax.

print(): customizing the output

print("Python", "is", "awesome")
# Python is awesome (default: space-separated)

print(*args) — accepts any number of arguments. Default separator is space; default end is newline.

sep= keyword

print("2025", "01", "15", sep="-")
# 2025-01-15

print("Alice", "Bob", "Charlie", sep=" | ")
# Alice | Bob | Charlie

sep=delimiter controls what goes between args. Useful for CSV-style output, formatted tables.

end= keyword

print("Loading", end="")
print("...", end="")
print(" Done!")
# Loading... Done!

end=text controls what goes after. Default is "\n" (newline). Empty string "" keeps the cursor on the same line. Useful for progress indicators.

For real progress bars, use tqdm — much nicer.

Formatted columns

print(f"{'Name':<10} {'Score':>5}")
print(f"{'Alice':<10} {95:>5}")
print(f"{'Bob':<10} {87:>5}")
print(f"{'Charlie':<10} {92:>5}")

Output:

Name       Score
Alice         95
Bob           87
Charlie       92

{value:<10} left-aligns, width 10. {value:>10} right-aligns. {value:^10} centers.

For a :-format spec on a string: < left, > right, ^ center, then width.

For numbers, you can combine alignment with formatting:

print(f"{3.14159:>10.2f}")    # '      3.14' — width 10, 2 decimals, right-aligned

Multiple inputs in one line

parts = input("Enter name and age (comma-separated): ").split(",")
name = parts[0].strip()
age = int(parts[1].strip())
print(f"{name} is {age}")

Or with tuple unpacking:

name, age = input().split(",")
name = name.strip()
age = int(age.strip())

For separated values, split + strip + convert is the pattern.

Reading until EOF

import sys

for line in sys.stdin:
    print(line.strip())

Reads from stdin until EOF (Ctrl-D on Linux/Mac, Ctrl-Z on Windows). Handles piped input:

cat names.txt | python script.py

For interactive use, input() per call is cleaner.

Validating input

def get_int(prompt, min_val=None, max_val=None):
    while True:
        raw = input(prompt)
        try:
            n = int(raw)
        except ValueError:
            print("Not a number. Try again.")
            continue
        if min_val is not None and n < min_val:
            print(f"Must be >= {min_val}")
            continue
        if max_val is not None and n > max_val:
            print(f"Must be <= {max_val}")
            continue
        return n

age = get_int("Age: ", min_val=0, max_val=120)

Loop until valid. Helper functions like this are useful for any interactive script.

getpass for hidden input

from getpass import getpass

password = getpass("Password: ")   # not echoed to terminal

For passwords or sensitive input. Same as input() but the typed characters don't appear.

Standard streams

import sys

print("hello", file=sys.stdout)   # default
print("error", file=sys.stderr)   # to stderr

stdout and stderr are separate streams. Errors should go to stderr (so users can pipe stdout to a file without losing them).

For command-line tools:

./script.py 2>errors.log >output.txt

2> redirects stderr; > redirects stdout.

Common stumbles

Forgetting int() for arithmetic. input() returns str; int(input()) for numbers.

input without prompt. Works, but the user doesn't know what to type. Always supply a prompt.

Crashing on bad input. Wrap int(input()) in try/except.

Forgetting f prefix. "value: {x}" is just text. f"value: {x}" interpolates.

Mixing print formats. Pick f-strings; the older % and .format() styles still work but are noisier.

Hardcoded \n. print("line\n") produces two newlines (the \n in the string + print's default). Either use print("line") (auto newline) or print("line", end="").

What's next

Lesson 6: conditionals. if, elif, else, plus and, or, not. Branching logic.

Recap

input(prompt) reads a line as a string. Convert with int() or float() for numbers — wrap in try/except for bad input. print() accepts multiple args; sep= controls separator, end= controls trailing output (default newline). Format with f-strings: alignment (< > ^), width, decimals ({x:.2f}). For passwords, getpass. Standard streams sys.stdout, sys.stderr for redirection.

Next lesson: conditionals.