Python Variables Explained

A Python variable is a name pointing at a value. You create one by assigning to it — there is no declaration step and no type to write.

city = "Karachi"
population = 15000000
average_rating = 4.6
is_coastal = True

Assignment happens right to left#

total = 10 + 5    # the right side is worked out first, then stored
print(total)      # 15

This is why count = count + 1 makes sense in code even though it is nonsense in algebra. Python has a shorthand for it:

count = 0
count += 1     # same as count = count + 1
count -= 1
count *= 3

Naming rules and conventions#

Rules that Python enforces:

• Start with a letter or underscore, never a digit
• Only letters, digits and underscores — no spaces or dashes
• Case sensitive: Total and total are different
• Cannot be a keyword such as class, for, lambda or None

Conventions the community follows:

• snake_case for variables and functions: user_email
• CAPITALS for values treated as constants: TAX_RATE = 0.17
• A leading underscore for internal details: _cache

Assigning several at once#

x, y = 3, 4
a = b = c = 0

# swapping without a temporary variable
x, y = y, x

That swap line is a small piece of Python that people genuinely enjoy.

Deleting and checking#

score = 10
print(type(score))     # <class 'int'>
del score
print(score)           # NameError

You rarely need del. type() on the other hand is one of the most useful debugging tools you have — see Python data types explained.

The behaviour that surprises people#

For numbers and text, assigning makes an independent copy in every way that matters:

a = 5
b = a
b = 10
print(a)   # 5 — unaffected

For lists and dictionaries, both names point at the same object:

a = [1, 2, 3]
b = a
b.append(4)
print(a)   # [1, 2, 3, 4] — a changed too

To get a real copy:

b = a.copy()      # or list(a), or a[:]

Global and local#

A variable created inside a function belongs to that function.

counter = 0

def increment():
    counter = 5      # a new local variable, not the outer one

increment()
print(counter)       # still 0

Rather than reaching for the global keyword, return the value instead. Functions that only depend on their inputs are far easier to test and reason about. See Python functions explained.

Where to go next#

• Python data types explained
• Python lists explained
• Common Python errors