Conditional Logic & Control Flow

▶Today's videos

Watch each video, then work the matching sections below. Watching alone will not do it.

1The if statement

One new idea, one new piece of punctuation, and one rule about whitespace that Python takes more seriously than any other language you will meet.

temperature = 31

if temperature > 30:
    print("It is hot.")
    print("Drink water.")

print("This always runs.")

Three things to notice:

1. The condition is any expression that produces True or False.
2. The line ends with a colon. Forgetting it is the single most common syntax error of the week.
3. The indented lines are the body. They run only if the condition is true. The unindented line after them is outside the if and always runs.

if / else

age = 15

if age >= 18:
    print("You may vote.")
else:
    print("Not yet.")

if / elif / else

Python checks each condition in order and runs the body of the first one that is true. Then it skips the rest entirely.

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(grade)

elif versus a stack of separate ifs

n = 15

# elif: at most ONE branch runs
if n > 10:
    print("more than 10")
elif n > 5:
    print("more than 5")

print("---")

# separate ifs: EVERY true condition runs
if n > 10:
    print("more than 10")
if n > 5:
    print("more than 5")

Use elif when the cases are alternatives. Use separate ifs when the checks are independent and several can apply at once.

2From English to boolean

This is the real skill of the day. Syntax takes twenty minutes to learn. Saying exactly what you mean takes practice.

The method: write the rule as a sentence. Underline every condition. Decide whether the connectors are "and" (all must hold) or "or" (any will do). Then translate word by word.

Decision tables

When a rule has three or more inputs, stop guessing at nested ifs and draw the table first.

Now the code writes itself, and you can point at the table when someone asks whether you handled a case.

De Morgan's laws

Two identities that let you push a not inward and usually make a condition readable:

not (a and b)   ==   (not a) or  (not b)
not (a or  b)   ==   (not a) and (not b)

Worked example. "Reject the order if it is not the case that the customer is verified and the address is valid":

# straight translation - hard to read aloud
if not (is_verified and address_valid):
    reject()

# De Morgan - reads exactly like the business rule
if not is_verified or not address_valid:
    reject()

3Nesting, flattening and guard clauses

Nested conditionals are sometimes necessary and usually avoidable. Knowing which is which is a craft skill.

Here is the leap year rule, written the obvious nested way:

year = 2024

if year % 4 == 0:
    if year % 100 == 0:
        if year % 400 == 0:
            is_leap = True
        else:
            is_leap = False
    else:
        is_leap = True
else:
    is_leap = False

print(is_leap)

Correct, and genuinely hard to check. The same rule, flattened:

year = 2024
is_leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
print(is_leap)

Read it aloud: "divisible by four, and either not a century year or divisible by four hundred." That is the actual rule. The flat version is the specification.

Guard clauses

When you are validating input, deep nesting appears almost immediately:

# the arrow of doom
if username:
    if len(username) >= 3:
        if username.isalnum():
            print("Welcome,", username)
        else:
            print("Letters and digits only")
    else:
        print("Too short")
else:
    print("Username required")

Handle each failure immediately and get it out of the way. The happy path ends up unindented at the bottom, where it is easy to find:

# guard clauses - each problem dealt with and dismissed
if not username:
    print("Username required")
elif len(username) < 3:
    print("Too short")
elif not username.isalnum():
    print("Letters and digits only")
else:
    print("Welcome,", username)

The conditional expression

# instead of four lines
if score >= 50:
    status = "pass"
else:
    status = "fail"

# one line
status = "pass" if score >= 50 else "fail"

Use it when the whole thing fits comfortably on one line and reads like English. Do not nest them.

4The seven classic traps

Each of these has cost every programmer alive at least one afternoon.

A complete worked program

# shipping_cost.py
BASE_RATE = 8.00
OVERSIZE_SURCHARGE = 15.00
FREE_THRESHOLD = 50.00

order_total = 62.50
is_member = False
is_oversized = True
destination = "domestic"

# Decide the surcharge first - it applies either way
surcharge = OVERSIZE_SURCHARGE if is_oversized else 0.00

# Then decide the base shipping
if is_member:
    shipping = 0.00
    reason = "member benefit"
elif order_total >= FREE_THRESHOLD and not is_oversized:
    shipping = 0.00
    reason = "free over $50"
elif destination == "international":
    shipping = BASE_RATE * 3
    reason = "international rate"
else:
    shipping = BASE_RATE
    reason = "standard rate"

total = order_total + shipping + surcharge

print(f"Order:      ${order_total:>7.2f}")
print(f"Shipping:   ${shipping:>7.2f}   ({reason})")
print(f"Surcharge:  ${surcharge:>7.2f}")
print(f"Total:      ${total:>7.2f}")

Notice the structure: one decision that is genuinely independent is computed separately with a ternary; the mutually exclusive cases go in one ordered elif chain, narrowest first; and every branch sets both the number and a reason, so the output can explain itself.

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