Python variables, data types and input()

What is a variable?

A variable is a name that stores a value, like a labelled box. You create one with =, which in Python means *store*, not *equals*.

name = "Asha"
marks = 87
print(name, marks)      # Asha 87

You can change what a variable holds at any time. The old value is replaced.

score = 10
score = score + 5       # take the old value, add 5, store it back
print(score)            # 15

Rules for naming variables

Good names say what the value means: fee_per_month is better than x. Python programmers write variable names in lower case with underscores (this is called *snake_case*).

The four basic data types

Use type() to check:

print(type(16))        # <class 'int'>
print(type(16.0))      # <class 'float'>
print(type("16"))      # <class 'str'>
print(type(True))      # <class 'bool'>

Note that "16" in quotes is a string, not a number. "16" + "4" gives "164", not 20.

input(): let the user type a value

city = input("Which city do you live in? ")
print("You live in", city)

The program pauses, shows the message, and waits for the user to type and press Enter.

input() always gives a string

This is the most common beginner bug:

age = input("Your age: ")     # the user types 15
print(age + 1)                # TypeError: can only concatenate str (not "int") to str

Convert the text to a number first:

age = int(input("Your age: "))
print("Next year you will be", age + 1)

Converting between types

int(9.8) cuts off the decimal part; it does not round. Use round(9.8) to round.

Converting text that is not a number gives a ValueError. You will learn to handle that safely in the errors and exceptions topic.

Multiple assignment and swapping

x, y, z = 1, 2, 3           # three variables in one line
a = b = 0                    # both start at 0

p, q = 5, 9
p, q = q, p                  # swap: p is now 9, q is now 5

In most other languages swapping needs a third, temporary variable. In Python one line does it.

Example: a tea stall bill

tea = int(input("Cups of tea: "))
samosa = int(input("Samosas: "))
total = tea * 10 + samosa * 15
print("Total bill: Rs", total)

Common mistakes

• Forgetting to convert input() before doing maths.
• Putting a number in quotes by accident: marks = "90" is text.
• Using a variable before giving it a value: NameError: name 'total' is not defined.
• Spaces or hyphens in names: first name = "Riya" is a SyntaxError.

Key points

• = stores a value in a variable; the newest value replaces the old one.
• The four basic types are int, float, str and bool; type() shows which one you have.
• input() always returns a string, so wrap it in int() or float() for numbers.
• a, b = b, a swaps two values.

Practice questions

c = float(input("Celsius: "))
f = c * 9 / 5 + 32
print("Fahrenheit:", f)