Conditions: if, elif, else — and the number that is not there

Why this is needed

So far the program has done the same arithmetic whatever the numbers were. Real work with data starts where the numbers differ and the road depends on them: skip a year that has no value instead of counting it as zero, mark a figure above the norm, set aside a row that arrived as rubbish.

All of that is one construct. It compares and it chooses, and it has three words: if, elif, else.

Today also brings a rule that is worth more than syntax in a course about data: a missing number and a zero are not the same thing. A report that counts a gap as zero is wrong silently, and you find out from a reader.

The whole thing at once

The file is shart.py. Run it with python shart.py from inside the environment.

"""Lesson 7: conditions, on inflation that has something to be compared with.

A single number means nothing until there is something to compare it against.
A condition is that comparison, after which the program takes one road and not
the other.
"""

# Inflation for the year, %. The figures come from lesson one, the World Bank.
# 2026 has no value: the year is not over. That is a gap, not a zero.
kz = {2022: 15.0, 2023: 14.5, 2024: 8.7, 2025: 11.4, 2026: None}
world = {2022: 8.1, 2023: 5.8, 2024: 3.0, 2025: 3.0}

here = kz[2025]
there = world[2025]

print("== the year 2025")
print(f"Kazakhstan: {here} | the world: {there}")
if here is None:
    print("there is no figure for this year yet")
elif here < there:
    print("below the world's")
elif here < 2 * there:
    print("above the world's, but less than twice")
else:
    print("twice the world's or more")

print()
print("== the year 2026")
future = kz[2026]
if future is None:
    print("no figure yet — nothing to compare with")
else:
    print("there is a figure:", future)

print()
print("== two comparisons in one condition")
if 2.0 <= there < 4.0:
    print(f"the world: {there} — between two and four")
if here > there and kz[2024] > world[2024]:
    print("Kazakhstan: above the world's for the second year running")

It prints:

== the year 2025
Kazakhstan: 11.4 | the world: 3.0
twice the world's or more

== the year 2026
no figure yet — nothing to compare with

== two comparisons in one condition
the world: 3.0 — between two and four
Kazakhstan: above the world's for the second year running

Taking it apart

A colon and an indent instead of brackets

if here < there:
    print("below the world's")

The colon at the end of the line means “a block follows”. The block itself is marked by the indent — four spaces. Most languages need curly brackets for this; in Python the empty space on the left does the job.

Hence the one beginner’s mistake you can catch by eye: a line drifts to the wrong level and runs at a moment you did not intend. An editor puts the four spaces in for you; mixing them with tabs is not allowed, and Python complains about that separately.

Picture it. A paragraph in a letter. You do not write “paragraph begins” and “paragraph ends” — you indent, and it shows.

elif is not “another if”

The difference people trip over is visible in our own output. The chain is checked from the top down: 11.4 is not None, not below the world’s, not less than twice the world’s. No condition matched, so it reached the else, and one line was printed.

An if — elif — else chain is checked from the top down and stops at the first match. The rest are not checked at all.

Three separate if statements are a different thing entirely:

if here > there:        → matches
if here > 2 * there:    → matches too

Two lines instead of one. That is not a fault of the language; they are different tools. A chain picks one of the options; separate if statements ask independent questions.

The else at the end means “in every other case”. It is optional, but without it nothing happens when no condition matched — and that, too, is a decision to make deliberately.

A gap is not a zero

The most expensive line of the lesson:

if future is None:

The series holds None for 2026 — not because inflation was zero but because the year has not happened. Those are different things, and in a report they give different answers.

The way to ask is is None, not this:

if future:          → False for None and for 0.0 alike

Remember from the lesson on types: bool(0.0) is false. A year of zero inflation would pass such a check as “no data” and vanish from the calculation without a word. is None asks exactly what is meant: the value is absent.

The word is is not accidental here. It compares not values but whether this is the same object, and None exists in a single copy in Python — which is what makes is None the right check.

Two comparisons in one condition

if 2.0 <= there < 4.0:

It reads as it does in mathematics: from two inclusive up to four. Most languages will not take that and make you write there >= 2.0 and there < 4.0; Python understands a chain of comparisons directly and evaluates there once.

and, or, and the order of the checks

if here > there and kz[2024] > world[2024]:

and is true when both are; or when at least one is; not turns it over.

What matters more: and stops at the first false, or at the first true. The right-hand side is not evaluated at all. Hence the habit the rest of the course will need constantly:

if value is not None and value > 5:

First make sure there is a number, only then compare. Swap the two and the program falls over on a gap, because None > 5 cannot be compared.

Why the same thing is written three times

Look at the program again: three blocks, and each of them has another print and another comparison. If there were thirty years instead of three, writing it this way would be impossible.

That is what the next two lessons are for: a loop will walk all the years by itself, and a function will give the comparison a name so it can be called as often as needed. For now it is done by hand, so that what they grow out of is visible.

The map of the lesson

Say it in your own words

Without looking, answer out loud or on paper. The answers are at the end of the lesson.

1. Why does an if — elif — else chain print one line while three separate if statements print two?
2. How does if value is None differ from if not value, and why does it matter in data?
3. What does and do when the left-hand side turns out to be false?

Warm-up

Three short steps before the exercise: predict, fill in, fix. The answers are at the end of the lesson, but answer them yourself first.

1. Predict. What does this program print? Say it out loud, then run it.

value = None
if value == 0:
    print("zero")
elif value is None:
    print("no number")
else:
    print("number:", value)

2. Fill in the gap. In place of ... put the condition “differs from the average by no more than 5% either way”.

value = 8.3
average = 8.0
if ...:
    print("about the average")
else:
    print("far from the average")

3. Fix it. This program falls over on the second pair. Find the line that causes it and make both lines print.

kz = {2024: 8.7, 2025: None}
for year, value in kz.items():
    if value > 8:
        print(year, "above eight")
    elif value is None:
        print(year, "no number")

Exercise

Required. Given:

prices = {"January": 520.0, "February": 580.0, "March": None, "April": 498.0}

Print the average over the months that have a number, and then each month with a label: “about the average” if the price differs from it by no more than 5% (exactly 5% counts as “about”), otherwise “above the average” or “below the average”; a month without a number is “no data”. The checks go in this order: the gap first, then “about”, then above and below.

The expected output:

average: 532.67
January: 520.0 — about the average
February: 580.0 — above the average
March: no data
April: 498.0 — below the average

Done when: the output matches line by line; the average is worked out over three months rather than four; no comparison ever meets a None.

On your own data. Take three numbers of your own — the price of one item in different shops, or your own inflation from your receipts, or anything you can measure. Compare each with the average and print an if — elif — else chain: “below average”, “about average” (within 5%), “above average”. Make one of the three None and check for it first, before any comparison.

Optional.

• Replace the chain with three separate if statements and see how many lines get printed.
• Write a condition with and whose right-hand side would fall over if it came first.
• Check it on your own series: how many of the five years were above world inflation?

Where this goes in the project

The digest stops counting everything indiscriminately. From this lesson on it has judgement: a year with a gap stays out of the average, a figure above the norm is marked, and a row of rubbish is set aside instead of spoiling the total.

Still open. The conditions are still written out one per year — with a loop that becomes three lines. And “above the norm” is decided by eye: where the norm runs is written down nowhere.

The answers

To the questions

1. Because the chain is checked from the top down and stops at the first condition that matched: the rest are not checked at all. Separate if statements are independent questions, and each of them can match.
2. is None asks whether the value is absent. not value is true for None, for zero and for an empty string alike — a year of zero inflation would disappear from the calculation as “no data”.
3. Nothing: the right-hand side is not evaluated. That is why value is not None and value > 5 is safe while the other order falls over on a gap.

To the warm-up

1. no number. None == 0 is False: a gap is not equal to zero, so the first branch does not match, and the second one asks is None and does.

no number

2. abs(value - average) <= average * 0.05. The absolute difference is what “either way” means, and <= includes exactly 5%: the boundary has to be decided once and written down, or two people will get different answers from the same data.

value = 8.3
average = 8.0
if abs(value - average) <= average * 0.05:
    print("about the average")
else:
    print("far from the average")

about the average

3. The line with None is not the culprit, the order is: value > 8 is checked first, and on 2025 that comparison meets a None — TypeError. The gap is checked before any comparison:

kz = {2024: 8.7, 2025: None}
for year, value in kz.items():
    if value is None:
        print(year, "no number")
    elif value > 8:
        print(year, "above eight")

2024 above eight
2025 no number

Sources

• Python: the if statement
• Python: comparisons and their chains
• Python: truth value testing
• World Bank: inflation, annual %