Python Variables

A variable is a name that refers to a value, so you can use and change that value later. Python makes variables very simple: there’s no special keyword, you just assign.

Creating variables#

name = "Asha"
age = 21
height_m = 1.62
is_student = True

print(name, age, height_m, is_student)

= is the assignment operator: it stores the value on the right under the name on the left. A variable is created the first time you assign to it. No let, var or type declaration is needed.

Changing variables#

score = 10
print(score)   # 10

score = 15
print(score)   # 15

score = score + 5
print(score)   # 20

score = score + 5 means: take the current value of score, add 5, store the result back in score.

Shortcut operators#

points = 100
points += 20   # points = points + 20 → 120
points -= 5    # 115
points *= 2    # 230
points //= 3   # 76 (whole-number division)
print(points)

Python has no ++ or -- operators. Use count += 1.

Variables can change type#

value = 42
print(type(value))   # <class 'int'>

value = "forty-two"
print(type(value))   # <class 'str'>

Python is dynamically typed: a variable can refer to any type of value, and type() tells you what it currently is. It’s flexible, but changing a variable’s type midway usually makes code confusing, so avoid it.

Names refer to objects#

In Python, a variable is a label attached to a value (an object), not a box containing it. Two names can point to the same object:

a = [1, 2, 3]
b = a          # b refers to the SAME list
b.append(4)
print(a)       # [1, 2, 3, 4]: a changed too!

For numbers and strings, this never surprises you, because they can’t be changed in place (they’re immutable). For lists and dictionaries (which are mutable), remember that assignment doesn’t copy. Use b = a.copy() when you need an independent copy. You’ll see this again in the Lists lesson.

Several assignments at once#

x, y, z = 1, 2, 3
print(x, y, z)

first, last = "Meera Shah".split()
print(first)
print(last)

a = b = 0       # both names refer to 0
print(a, b)

The number of names must match the number of values, or you get a ValueError.

Swapping without a temporary variable#

left = "tea"
right = "coffee"

left, right = right, left
print(left, right)   # coffee tea

Constants#

Python has no true constants, but by convention names in UPPER_CASE shouldn’t be changed:

GST_RATE = 0.18
MAX_LOGIN_ATTEMPTS = 5

price = 1000
print(price * (1 + GST_RATE))

Putting such values in named constants at the top of a file makes the code self-explaining and easy to update.

Naming variables well#

# Hard to understand
x = 1499
y = 3
z = x * y

# Clear
unit_price = 1499
quantity = 3
order_total = unit_price * quantity
print(order_total)

Rules and conventions:

• Use snake_case: lower-case words joined with underscores.
• Be descriptive: user_count, not uc.
• Booleans read well as questions: is_active, has_paid.
• Don’t use names of built-in functions, like list, str, sum, max or input: you’d hide the real function.

sum = 10          # bad: hides the built-in sum()
# sum([1, 2, 3])  # TypeError: 'int' object is not callable
del sum           # removes our variable, restoring the built-in
print(sum([1, 2, 3]))

Printing variables with f-strings#

The easiest way to combine text and variables is an f-string: put f before the quotes and variables inside { }:

name = "Rohan"
lessons_done = 7
total_lessons = 20

print(f"{name} has finished {lessons_done} of {total_lessons} lessons.")
print(f"Progress: {lessons_done / total_lessons:.0%}")
print(f"Next lesson: {lessons_done + 1}")

Output:

Rohan has finished 7 of 20 lessons.
Progress: 35%
Next lesson: 8

• Any expression can go inside { }.
• After a colon you can add a format: :.2f for 2 decimals, :.0% for a percentage, :, for thousands separators.

amount = 1234567.891
print(f"{amount:,.2f}")   # 1,234,567.89

A handy trick for debugging: print(f"{order_total=}") prints order_total=4497.

Deleting variables#

del name removes a name. You’ll rarely need it.

Common mistakes#

• Using a variable before assigning it → NameError.
• Typos in names: totl instead of total creates a new variable or a NameError.
• Shadowing built-ins like list, str or input.
• **Forgetting the f** in f-strings, which prints the braces literally.
• **Thinking b = a copies a list.**

Exercise#

1. Create variables for a product’s name, price and quantity, and print a line like “3 × Notebook = ₹360” with an f-string.
2. Start with balance = 1000, apply -= 250 and += 75, then print it.
3. Swap two variables in one line.
4. Create a constant DISCOUNT = 0.1 and print a price after the discount with 2 decimals.
5. Show that b = a for a list doesn’t copy it, then fix it with .copy().

Summary#

• name = value creates or updates a variable; no keyword or type is needed.
• Names refer to objects; assigning a list to another name doesn’t copy it.
• Use snake_case names, UPPER_CASE for constants, and never shadow built-ins.
• f-strings (f"{value}") are the easiest way to show variables in text.

Next, you’ll learn Python’s built-in data types.