Combining conditions

Part of the free LogicWiz Python course, Module 2: Decisions and loops. The examples and practice run in the app.

Is 2100 a leap year? year % 4 == 0 says yes. The calendar says no. One condition is not enough; you need to combine them.

You'll learn to

• Join conditions with and, or, not
• Check membership with in
• Know which values count as True or False (truthiness)
• Turn a flowchart into code: largest of three, leap year

Learn

and, or, not

age = 19
has_id = True
print(age >= 18 and has_id)
print(age < 12 or age > 60)
print(not has_id)

and is True only if both sides are True. or is True if at least one is. not flips True and False.

and vs or

in: is it inside?

print("a" in "chai")
print("z" in "chai")
ch = "e"
print(ch in "aeiou")
print("dosa" in "idli dosa vada")

in checks whether something appears inside a string. ch in "aeiou" is the neat way to test for a vowel.

Truthiness

if does not need a real True or False. Python treats some values as falsy:

• 0 and 0.0
• "" (an empty string)
• None

Everything else is truthy, even "0", "False" and -1.

So if name: means "if name is not empty". Also: and / or give back one of their operands, not always True or False. name or "Guest" gives name if it is truthy, otherwise "Guest".

Truthiness in action

print(bool(0), bool(""), bool("0"), bool(-1))
name = ""
print(name or "Guest")
print(0 or 5)
print(3 and 0)
if name:
    print("Hello", name)
else:
    print("Please type your name")

Flowchart to code: largest of three

a, b, c = 12, 45, 30
if a >= b and a >= c:
    big = a
elif b >= c:
    big = b
else:
    big = c
print("Largest:", big)

Each diamond in the flowchart becomes a condition. If a beats both, it is the answer. If not, the answer is whichever of b and c is bigger.

The leap year rule

year = int(input())
if (year % 4 == 0 and year % 100 != 0) or year % 400 == 0:
    print(year, "is a leap year")
else:
    print(year, "is not a leap year")

Divisible by 4 and not by 100, or divisible by 400. So 2024 and 2000 are leap years; 2100 and 1900 are not. Try them.

if ch == "a" or "e": is always True. Python reads it as (ch == "a") or ("e"), and "e" is a non-empty string, so it is truthy. Write ch == "a" or ch == "e", or better, ch in "ae".

CBSE asks: leap year, largest of three numbers, vowel or consonant, and the order of logical operators: not first, then and, then or. So True or False and False is True or (False and False) = True.

In one line

and needs both, or needs one, not flips; 0, "" and None are falsy; write ch == "a" or ch == "e", never ch == "a" or "e".