workshop_3.md

Workshop 3 — Conditionals and Logical Operators

Prerequisites

• Workshop 1: variables, data types, type casting, f-strings, operators
• Workshop 2: input(), string methods, booleans, truthiness

1. Making decisions

Every program so far ran top to bottom, executing every single line. An if statement lets a program choose whether to run a block of code, based on a condition:

age = 20

if age >= 18:
    print("You may enter.")

The condition is age >= 18. As we saw in Workshop 2, a comparison produces a bool — True or False. If it's True, the indented block runs. If it's False, Python skips it entirely.

2. Indentation is the block

Most languages mark a block with { and }. Python uses indentation — the spaces at the start of a line:

age = 20

if age >= 18:
    print("You may enter.")      # inside the if
    print("Welcome!")            # also inside the if
print("Have a nice day.")        # outside — always runs

Two rules that Python enforces:

• The line with if ends with a colon :.
• Every line in the block is indented by the same amount. Use 4 spaces — that's the convention the whole Python world follows.

Get either wrong and you get an error before your program even starts:

if age >= 18
    print("hi")
# SyntaxError: expected ':'

if age >= 18:
print("hi")
# IndentationError: expected an indented block after 'if' statement

3. else

else gives you the other path — it runs only when the condition was False:

age = 15

if age >= 18:
    print("You may enter.")
else:
    print("You are too young.")

Exactly one of the two blocks runs. Never both, never neither.

4. elif

To test several conditions in order, use elif ("else if"):

score = 75

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

Python checks the conditions top to bottom and stops at the first True one. score = 75 is not >= 90, not >= 80, but it is >= 70 — so it prints Grade: C and never looks at the else.

This ordering matters. Written the other way around, every passing score gets a C:

score = 95

if score >= 70:
    print("Grade: C")      # this wins — 95 >= 70 is True
elif score >= 90:
    print("Grade: A")      # never reached

An if can have any number of elif branches, and the else is optional.

5. = is not ==

A single = assigns. A double == compares. Mixing them up is the most common beginner mistake in any language:

age = 20          # assignment: age now refers to 20
print(age == 20)  # comparison: True

Python protects you here — using = in a condition is a syntax error, not a silent bug:

if age = 20:
    print("hi")
# SyntaxError: invalid syntax. Maybe you meant '==' instead of '='?

6. Logical operators

To combine conditions, use and, or, and not:

Operator	Result is True when...
a and b	both a and b are True
a or b	at least one of a, b is True
not a	a is False

age = 25
has_ticket = True

if age >= 18 and has_ticket:
    print("Enjoy the film.")

if age < 18 or not has_ticket:
    print("Sorry, you can't come in.")

The full truth table:

a	b	a and b	a or b
True	True	True	True
True	False	False	True
False	True	False	True
False	False	False	False

Note these are the words and, or, not — not &&, ||, ! as in some other languages.

7. Each side needs a full condition

A very common mistake is writing a condition the way you'd say it out loud:

day = "saturday"

if day == "saturday" or "sunday":    # WRONG
    print("Weekend!")

This looks right and even seems to work — but it's always True, for every day. Python reads it as (day == "saturday") or ("sunday"), and "sunday" is a non-empty string, which is truthy (Workshop 2). So the or always succeeds.

Spell out both comparisons:

if day == "saturday" or day == "sunday":   # correct
    print("Weekend!")

8. Chaining comparisons

When you need a value to fall inside a range, Python lets you write the comparison the way it looks in mathematics:

temperature = 22

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

That means exactly the same as 18 <= temperature and temperature <= 25, just shorter.

9. Conditions don't have to be comparisons

An if accepts any value and applies the truthiness rules from Workshop 2 — empty or zero is False, everything else is True:

name = input("What is your name?\n").strip()

if name:
    print(f"Hello, {name}!")
else:
    print("You didn't type a name.")

If the user just pressed Enter, name is "", which is falsy, so the else runs. Writing if name: is the idiomatic Python way to say "if this isn't empty".

10. in — is this text inside that text?

For strings, in checks whether one piece of text appears inside another. It produces a bool, so it works anywhere a condition does:

email = input("Email: ").strip().lower()

if "@" in email:
    print("Looks like an email address.")
else:
    print("That's missing an @.")

not in is its opposite:

if "@" not in email:
    print("That's missing an @.")

11. Nested conditions

An if block can contain another if. Indent the inner one one level further:

age = 25
has_ticket = False

if age >= 18:
    if has_ticket:
        print("Enjoy the film.")
    else:
        print("Please buy a ticket first.")
else:
    print("This film is 18+.")

Nesting gets hard to read quickly. When the inner condition is just "and also this", prefer and:

if age >= 18 and has_ticket:
    print("Enjoy the film.")

12. One-line conditional expressions

When you only need to choose a value, a conditional expression (sometimes called a ternary) is more compact than four lines of if/else:

age = 20
status = "adult" if age >= 18 else "minor"
print(status)          # adult

Read it left to right: the value is "adult" if age >= 18, otherwise "minor". Use it for picking a value, not for running several statements — an ordinary if/else is clearer for those.

What's next

See workshop_3/ for runnable examples, and your homework in python-127-homework-3.