If Statement

An if statement lets your program make a decision. It checks a condition.

If the condition is True, Python runs a block of code.
If it’s False, Python can skip it or run a different block (with else).

is_raining = True

if is_raining:
    print("Take an umbrella.")
else:
    print("Enjoy the sunshine!")

Use elif (“else if”) to test another condition when the first one is False.

light = "yellow"

if light == "red":
    print("Stop")
elif light == "yellow":
    print("Slow down")
else:
    print("Go")

NOTE: == tests equality. = assigns a value. Don’t mix them up.

Boolean operators: and, or, not

You can combine conditions.

age = 16
with_parent = True

if age < 18 and with_parent:
    print("You can enter with a parent.")

role = "moderator"
if role == "admin" or role == "moderator":
    print("Access granted.")

is_weekend = False
if not is_weekend:
    print("It’s a weekday.")

A clearer pattern for many options use in. Instead of chaining many ors, use membership with in / not in.

role = "moderator"
elevated = {"admin", "moderator", "owner"}

if role in elevated:
    print("Access granted.")

in also works with lists, tuples, strings, and dicts (for dicts it checks keys):

code = "AM"
allowed = ["AM", "GE", "FR"]         # list
print(code in allowed)               # True

vowels = ("a", "e", "i", "o", "u")   # tuple
print("o" in vowels)                 # True

letters = {"a", "b", "c"}            # set
print("d" in letters)                # False

fruit_by_letter = {"a": "apple", "b": "banana"}  # dict
print("a" in fruit_by_letter)        # True  (checks keys, not values)

word = "notebook"                    # string
print("note" in word)                # True  (substring)

Truthy and falsy values

if doesn’t only use True/False. Many values are truthy or falsy.

Falsy values (treated as False):

False, None, 0, 0.0
empty string ""
empty list [], empty tuple ()
empty dict {}, empty set set()

Anything else is truthy.

items = []
if items:
    print("We have items.")
else:
    print("The list is empty.")   # runs

Chained comparisons

Python lets you chain comparisons for readability.

temperature = 23
if 18 <= temperature <= 25:
    print("Comfortable range.")

This is cleaner than: if temperature >= 18 and temperature <= 25:.

Common mistakes (and quick fixes)

Using = instead of == in conditions → if x == 5:
Forgetting the colon : → if condition:
Bad indentation → indent the block under if, elif, else.
Comparing to None with == → prefer is: if x is None:
Long chains of or → prefer in, e.g., if ext in {".py", ".js"}:

The walrus operator := (Python 3.8+)

The walrus operator assigns and returns a value in one expression. Use it when it makes code clearer, not just shorter.

limit = 100
msg = "OK" * 42  # length 84

if (left := limit - len(msg)) >= 0:
    print(f"Fits: {left} characters left")
else:
    print(f"Too long by {-left}")

Why the parentheses? (left := limit - len(msg)) >= 0 first assigns to left, then compares. Without parentheses, you might accidentally compare before assignment.