Boolean logic

Every if and while statement relies on a condition that is either True or False. Understanding how Python builds and evaluates those conditions gives you much finer control over your programs' behaviour.

Comparison operators

Six operators compare two values and always return a boolean:

x = 7

x == 7    # True   — equal to
x != 5    # True   — not equal to
x > 3     # True   — greater than
x < 10    # True   — less than
x >= 7    # True   — greater than or equal to
x <= 6    # False  — less than or equal to

A common mistake is writing = (assignment) where == (comparison) was meant. Python will raise a SyntaxError in most of those cases, but it's still worth internalising the distinction early.

Logical operators

Three keywords combine boolean values:

age = 20
has_ticket = True

age >= 18 and has_ticket    # True  — both conditions met
age < 18 or has_ticket      # True  — one is enough
not has_ticket              # False — flipped

Truthy and falsy values

Python's if statement doesn't require a literal True or False — it accepts any expression and evaluates it as "truthful" or "falsy". These values are always falsy:

• False and None
• Zero: 0, 0.0
• Empty containers: "", [], {}, set()

Everything else is truthy. This lets you write natural-sounding guards:

name = input_value   # could be an empty string

if name:
    print(f"Hello, {name}")
else:
    print("No name provided")

That if name: is more idiomatic than if name != "": and says the same thing.

Short-circuit evaluation

Python evaluates and and or lazily — this is short-circuit evaluation: it stops as soon as the result is determined:

• a and b: if a is falsy, Python never looks at b (the result is already False).
• a or b: if a is truthy, Python never looks at b (the result is already True).

This is useful as a guard pattern:

text = ""
length = text and len(text)   # text is falsy → short-circuits to ""
                               # avoids calling len() on None, etc.

And for providing defaults:

display_name = user_name or "Anonymous"

Chained comparisons

Python lets you chain comparisons the way mathematicians write them, which reads far more naturally than the equivalent and:

x = 7

1 < x < 10          # True  — x is between 1 and 10 (exclusive)
0 <= x <= 100       # True  — x is in the 0–100 range

# Equivalent but more verbose:
1 < x and x < 10

Each chained comparison evaluates each value once and short-circuits, so 1 < x < 10 is both readable and efficient.

Where to go next

You can now write precise conditions for any branch or loop. Next: reading and writing files — applying conditions when processing real data from disk.