This page covers how to read a value typed by the user, the basic categories of values Python works with, and how to convert between them.

Keyboard Input

The built-in input() function pauses a script and waits for the user to type something and press Enter. Whatever text they typed is returned as the value of the input() call. An optional argument sets the prompt message shown before the user types:

name = input("Enter your name: ")
print("Hello, " + name + "!")

Enter your name: Kip
Hello, Kip!

input() always returns text, even if the user types a number. Trying to do arithmetic directly on the result, such as age = input("Enter your age: ") followed by age + 1, raises an error - Python will not silently guess that you meant to treat age as a number. This is a common first surprise, and the Type Conversion section below explains how to handle it.

Data Types

Every value in Python has a data type, which determines what operations make sense for it. The type() function reports a value’s type:

age = 21
gravity = 9.807
craft_name = "Voyager"
is_crewed = False

print(type(age))
print(type(gravity))
print(type(craft_name))
print(type(is_crewed))


<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>

The four types used above are the most common ones you’ll encounter early on:

Type Description Example
int Whole number, positive or negative 21
float Decimal (floating point) number 9.807
str Text, enclosed in quotes "Voyager"
bool True or False False

Unlike C++, you never write these type names when creating a variable - Python infers the type from the value itself, as covered on the Variables & Output page.

Lists and Tuples

Two more common types hold more than one value at a time: lists and tuples. Both are ordered sequences of values, indexed the same way as a string, but they differ in whether that sequence can change after it’s created. This section covers just the basics. See Lists & Tuples for indexing, slicing, and the full set of list operations.

A list is created with square brackets and can be changed after creation. Values can be added, removed, or reassigned:

altitudes = [400, 550, 800, 1200]

print(altitudes)
print(altitudes[0])
print(len(altitudes))


[400, 550, 800, 1200]
400
4

A tuple is created with parentheses and, unlike a list, cannot be changed once created. Tuples are often used for a fixed group of related values, such as a coordinate pair:

coordinates = (37.9401, -75.4664)  # lat, lon of NASA Wallops Flight Facility
lat, lon = coordinates

print(f"Launch site: {lat}, {lon}")


Launch site: 37.9401, -75.4664

Writing lat, lon = coordinates is called tuple unpacking. It assigns each value in the tuple to its own variable in a single line.

Type Conversion

Because input() always returns a str, you often need to explicitly convert it to a number before using it in a calculation. Python provides a built-in function for each type that attempts to convert its argument:

Function Converts to
int(x) Integer
float(x) Floating point number
str(x) Text 
fuel_lb_text = "500"  # imagine this string came from input()
fuel_lb = float(fuel_lb_text)
fuel_kg = fuel_lb * 0.4536

print(type(fuel_lb_text))
print(type(fuel_lb))
print(f"{fuel_kg:.1f} kg")


<class 'str'>
<class 'float'>
226.8 kg

If the text cannot be interpreted as a number - for example, int("abc") - Python raises an error rather than guessing. Always convert user input to the type you actually need before doing arithmetic with it.

Example: Fuel Mass Conversion

Question

Prompt the user to enter a fuel mass in pounds, then print the equivalent mass in kilograms (1 lb = 0.4536 kg), rounded to 1 decimal place.

Solution

fuel_lb_text = input("Enter fuel mass in lb: ")
fuel_lb = float(fuel_lb_text)
fuel_kg = fuel_lb * 0.4536

print(f"{fuel_kg:.1f} kg")

Enter fuel mass in lb: 500
226.8 kg

The value returned by input() is converted to a float before it is used in the multiplication. If the float(...) conversion were left out, fuel_lb * 0.4536 would raise an error, since you cannot multiply text by a number.

Reading Questions

  1. What data type does input() always return, regardless of what the user types?
  2. What would go wrong if you tried to run input("Enter your age: ") + 1 directly?
  3. Which function converts a str to a float?
  4. What happens if you call int() on text that isn’t a valid whole number, like int("hello")?
  5. What is the data type of the value False?
  6. Write a line of code that reads a user’s height (in text) and converts it to a float named height_m.
  7. What is the main difference between a list and a tuple?
  8. What does tuple unpacking mean?

Practice Problem: Altitude Unit Conversion

Your Task

Write a script named altitude_conversion.py that:

  1. Prompts the user with input() for an altitude in meters, and converts the result to a float named altitude_m
  2. Computes altitude_ft, the equivalent altitude in feet (1 m = 3.281 ft)
  3. Prints the result with an f-string, rounded to 1 decimal place

Your variable name for the converted value must match exactly (altitude_ft) so that the checker below can find it.

Checking Your Work

Download check_altitude_conversion.py and save it in the same folder as your altitude_conversion.py script. Open a terminal in that folder and run:

python check_altitude_conversion.py

The checker supplies its own test value for the input() prompt, so it won’t wait for you to type anything - it just runs your script and reports whether altitude_ft is correct. This is practice, not a graded assignment. If something doesn’t pass, use the feedback to find and fix the issue, then run the checker again.