True, False and comparisons

Up to now, every program you’ve written did the same thing every time: line 1, line 2, line 3, done. Real programs react. A game checks whether you’ve run out of lives; a shop checks whether you’ve got enough money; a thermostat checks whether the room is too cold. Every one of those checks is a yes-or-no question, and in Python the answer is a bool: True or False. You met bools briefly in the lesson on values and types. Now they take centre stage.

Asking questions with comparisons

A comparison compares two values and gives back True or False. Python has six:

Predict each line, then run:

print(10 == 10)
print(10 != 3)
print(4 > 9)
print(4 <= 4)
age = 15
print(age >= 13)

The most common mistake: = versus ==

One equals sign and two equals signs mean completely different things:

• x = 5 is assignment: “x now refers to 5”. It’s an instruction.
• x == 5 is a comparison: “is x equal to 5?”. It’s a question, and its answer is a bool.

x = 5
print(x == 5)
print(x == 6)

If you accidentally write = where a question belongs, Python usually stops with a SyntaxError and often suggests “Maybe you meant ’==’ instead of ’=’?”. Read the hint; it’s nearly always right.

Comparisons can be stored

A comparison is an expression like any other, so you can store its answer in a variable. Names that read like yes-or-no questions work well:

temperature = 31
is_hot = temperature > 30
print(is_hot)
print(type(is_hot))

Comparing text

== works on strings too. Two strings are equal only if every character matches exactly, including capital letters and spaces:

password = "Banana"
print(password == "Banana")
print(password == "banana")
print("cat" != "dog")

< and > also work on strings: they compare in dictionary order, roughly, so "apple" < "banana" is True. But careful: all capital letters count as “smaller” than all lowercase letters, so "Zebra" < "apple" is also True. Comparing a string with a number using < is a TypeError. And == between a string and a number is simply False: "7" == 7 is False, because text is never equal to a number.

Combining questions: and, or, not

Real decisions often depend on more than one thing. To ride a roller coaster you might need to be tall enough and old enough. Python has three words for combining bools:

• a and b is True only if both are True.
• a or b is True if at least one is True.
• not a flips it: True becomes False and False becomes True.

height = 135
age = 9
tall_enough = height >= 120
old_enough = age >= 10
print(tall_enough and old_enough)
print(tall_enough or old_enough)
print(not old_enough)

Change age to 10 and run again. Which answers change?

Here’s the whole of and and or in one table:

In everyday English “or” sometimes means “one or the other but not both”. In Python, or is True when both are True too.

Ranges in one go

Is a number between two others? You could write x >= 1 and x <= 10. Python also lets you chain comparisons the way you would in maths:

score = 7
print(1 <= score <= 10)
print(0 < score < 5)

Precedence, and when to use brackets

Comparisons are worked out before not, not before and, and and before or. That means a or b and c is read as a or (b and c). Rather than memorising this, use brackets whenever you mix and and or; they make your meaning clear to Python and to people.

is_weekend = True
is_holiday = False
raining = True
go_to_park = (is_weekend or is_holiday) and not raining
print(go_to_park)

Your turn

height = 125
age = 7
with_adult = True

tall_enough = True
old_enough = True
can_ride = True
print(tall_enough, old_enough, can_ride)

Check yourself