Intro to Python · Lesson 5 of 12 · 12 min

Making decisions with if

Comparisons give True or False, and if, elif and else use them to choose which lines run. Learn why indentation matters and why the order of your tests changes the answer.

COMPARISONS

A comparison gives True or False

Comparison operators ask a question about two values and answer with a bool: True or False. == means equal, != not equal, then <, >, <= and >=. Watch the difference between = and ==: one = stores a value, two compare. Text comparisons are exact, capital letters included.

5 > 3 is True, 5 == 3 is False, and "Yes" == "yes" is False because Y and y are different characters.

print(5 > 3)
print(5 == 3)
print(5 != 3)
print("Yes" == "yes")

Output

True
False
True
False

Each line prints the answer to a yes-or-no question. True and False are real values in Python, with capital letters and no quotes, and they're what if uses to decide.

Check yourself

Which operator checks whether two values are equal?

Show the answer

Right. == compares and gives True or False. A single = stores a value, so if x = 5: is a SyntaxError.

IF, ELIF, ELSE

Indented lines run only when the test is True

if temp > 30: is followed by a colon and then lines indented by 4 spaces; those lines run only when the test is True. elif (else if) adds another test, and else: catches everything that no test matched. Python checks the tests from top to bottom and runs only the first branch that is True, then skips the rest. The first line that isn't indented runs every time.

The indentation isn't decoration: it's how Python knows which lines belong to which branch.

temp = 32
if temp > 30:
    print("hot")
elif temp > 15:
    print("mild")
else:
    print("cold")
print("done")

Output

hot
done

temp is 32, so the first test is True: hot is printed and the elif and else are skipped. The last line isn't indented, so done prints no matter which branch ran. With 18 you'd get mild, and with 5, cold.

Check yourself

temp = 18
if temp > 30:
    print("hot")
elif temp > 15:
    print("mild")
else:
    print("cold")

```
mild
```
```
hot
```
```
cold
```
```
mild
cold
```

Show the answer

mild

Step through it

Two tests and an else, checked from the top
The flow chart starts at a temp box, then two test diamonds, temp > 30 and temp > 15, and an else box at the bottom. Each has a side exit to what it prints: "hot", "mild" or "cold". Nothing is lit yet: the program checks the tests from top to bottom.
temp = 32: the first test is True
With temp = 32, the first diamond lights up and its exit, marked True, runs straight across to "hot". The second test and else stay dim: once one branch runs, Python skips the others.
temp = 18: False, then True
With temp = 18, the first test is False, so the path drops down to the second diamond. temp > 15 is True, and the path turns right to "mild". A False test simply sends Python on to the next one.
temp = 5: both False, so else runs
With temp = 5, both diamonds say False and the path falls all the way down to else, which lights up "cold". else has no test of its own: it catches every value that no test above it matched.

Check yourself

If both the if test and an elif test are True, both of their blocks run.

Show the answer

False

False. Python runs only the first branch whose test is True and skips everything else in that if/elif/else chain. With temp = 32, both temp > 30 and temp > 15 are True, but only hot is printed.

ORDER MATTERS

Put the strictest test first

Because only the first True branch runs, the order of your tests changes the answer. A score of 95 is also at least 50, so if score >= 50 comes first, 95 gets "pass" and never reaches the "excellent" test. Put the narrowest test at the top and work down to the widest. To combine conditions, use and (both must be True), or (at least one) and not (flips True and False), as in age >= 18 and has_ticket.

Testing score >= 90 first, then score >= 50, gives 95 "excellent" and 60 "pass".

Check yourself

score = 95
if score >= 50:
print("pass")
elif score >= 90:
print("excellent")

Show the answer

The bug is on line 2.

Found it. The program runs without an error but prints pass, because 95 >= 50 is True and the first True branch wins. Swap the tests so score >= 90 is checked first.

age = 20
has_ticket = True
if age >= 18 and has_ticket:
    print("come in")
answer = "Yes"
if answer.lower() == "yes":
    print("ok")

Output

come in
ok

The first if needs both conditions, and both are True. The second shows a habit worth having: text comparisons are exact, so lower() turns Yes into yes before comparing, and the test works whether the user typed Yes, YES or yes.

Check yourself

Omid writes age = input("Age? "), types 20, and the next line is if age > 17:. What happens?

The if runs, because 20 is more than 17
The if is skipped, because "20" is text
The program stops with a TypeError
Python converts "20" to a number by itself

Show the answer

The program stops with a TypeError

Right. age holds the text "20", and Python won't compare text with a number using >. Writing age = int(input("Age? ")) fixes it.

Lesson recap

Comparisons (==, !=, <, >, <=, >=) give True or False; = stores, == compares.
if test: runs its indented lines (4 spaces) only when the test is True; the colon is required.
Python checks if, then each elif, top to bottom, runs only the first True branch, and else catches the rest.
Order matters: put the strictest test first, or 95 gets "pass" before it can reach "excellent".
Combine tests with and, or and not; compare text exactly (use lower()), and convert input() before comparing it with a number.

Making decisions with if

A comparison gives True or False

Check yourself

Indented lines run only when the test is True

Check yourself

Step through it

Check yourself

Put the strictest test first

Check yourself

Check yourself

Lesson recap

Keep it, don't just read it

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