Every program you have written so far runs the same way every time. Python starts at the top, runs each line in turn, and reaches the bottom. Nothing is ever skipped, whatever the user types.

Real instructions are not like that. They contain decisions:

IF it is raining THEN
take your umbrella.

IF you haven’t done your homework THEN
you get grounded.

In both of those, the instruction on the second line only happens when the sentence on the first line is true. Otherwise it is skipped. Code that makes the computer do different things in different situations is called selection, and every piece of selection starts with the part that does the deciding: the condition.

A condition is a true or false check

“It is raining” is a check with exactly two possible answers. Either it is raining, or it is not. There is no third option and no maybe. A check like that is called a Boolean condition, and its answer is one of Python’s two Boolean values: True or False.

In the umbrella example, the condition is “it is raining”. If it comes back true, the instruction below it is carried out. If it comes back false, that instruction is skipped and the computer carries on with whatever comes next.

Comparison operators

Conditions are written with comparison operators. Each one takes the two pieces of data on either side of it and answers one question about them with True or False.

The first two, == and !=, work on numbers and on text. The other four are for comparing numbers, because they ask which of the two is bigger, and that is a question about size.

Either side of a comparison can be a variable rather than a value typed out in full, which is what makes conditions useful. The umbrella rule written with a variable looks like this:

IF weather == "rain" THEN
take your umbrella.

The computer looks inside weather, compares whatever it finds with "rain", and gets True or False back.

Checking a range

A single comparison only checks one side of a number: whether it is big enough, or whether it is small enough. To check that a number lies inside a range, for example between 1 and 10, you need two comparisons, one for the bottom of the range and one for the top, joined together with and:

num >= 1 and num <= 10

The whole condition is only True when both of the comparisons around the and are True:

One False is enough to make the whole condition False, so 15 is turned away for being too big and 0 for being too small.

Use >= and <= when the numbers at both ends of the range belong in it themselves, the way 1 and 10 do here.

One equals stores, two equals compare

This is the mistake that catches almost everybody at least once.

A single = is the assignment symbol. It stores the data on its right in the variable on its left, and it changes what the variable holds:

weather = "rain"

A double == is a comparison. It changes nothing at all. It looks at the two pieces of data and answers one question about them:

weather == "rain"

Read them out loud and the difference is clear. The first one says “let the weather be rain”. The second one asks “is the weather rain?”. A condition always needs the question, so a condition always needs ==.

SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

Comparing text is case sensitive

When == compares two pieces of text, the match has to be exact. Every character has to be the same, and a capital letter is a different character from the small letter that looks like it.

So if the user types rain, the condition weather == "rain" is True. If they type Rain, that same condition is False, because R is not r. So is RAIN, and so is rain with a stray space on the end.

Try it yourself

Run the code below. Every line outputs True or False, and nothing else, because that is all a condition ever produces.

weather = "rain"

print(weather == "rain")
print(weather == "Rain")
print(weather != "rain")

temperature = 12

print(temperature > 20)
print(temperature < 20)
print(temperature >= 12)
print(temperature >= 10 and temperature <= 20)

Work out what each of the seven answers should be before you run it. Then change weather to "sun" and temperature to 25, and work them all out again.

Python also accepts the shorter 10 <= temperature <= 20, and the editor may suggest it on the last line. Both work the same; this course uses and, the way the exam writes it.