Computer Science · Class 11 · Types of Statements

Types of StatementsSequential⏱️ 9 min read

Sequential Statements

The first of the five, and the one you already use. A sequential statement is simply one that runs after the one above it — top to bottom, once each. There is no keyword to learn and nothing to type, because this is what Python does when you have not asked for anything else.

1A program to watch: the sum of two numbers

Four statements, and nothing in them you have not met. Read it first, then we will run it one line at a time.

sum.py

a = 25
b = 17
total = a + b

print('Sum =', total)

Output

Sum = 42

Store 25. Store 17. Add them and store that. Show it. Four instructions, carried out in the order they are written, each exactly once — and then the program ends, because there is no line 5.

2Watch it run, one statement at a time

Python is only ever doing one line at a time. Step through it below and watch two things: what each statement leaves behind in memory, and how long the screen stays blank.

⬇️ One line at a time

Step through sum.py and watch what each statement leaves behind.

sum.pystep 0 of 5

a = 25

b = 17

total = a + b

print('Sum =', total)

in memory

nothing yet

on the screen

still blank

Nothing has happened yet. Python is about to start at the top — memory is empty and the screen is blank.

Two things in that are worth saying out loud. First, nothing appeared on the screen until the last line. Three statements had already run and the output pane was still empty, because storing a value is not the same as showing it — only print() shows.

Second, each statement worked with what the ones above it left behind. By the time total = a + b ran, both a and b already existed, so it had something to add. That is not luck. It is the order.

3The order is the program

Statements are not a list of things to be done in any convenient order. Move one line and you have a different program — sometimes a broken one. Here is the same four statements with the last two swapped:

out_of_order.py

a = 25
b = 17

print('Sum =', total)
total = a + b

Output

Traceback (most recent call last):
  File "out_of_order.py", line 4, in <module>
    print('Sum =', total)
                   ^^^^^
NameError: name 'total' is not defined

Every statement here is correct on its own. The program still fails, because at the moment print() ran, total had not been worked out yet — the line that creates it is sitting one line below, waiting its turn that will never come. You met this error in the Errors chapter; here is where it comes from.

Watch Out

Not every wrong order gives you an error. That is the dangerous part. Sometimes the program runs perfectly and just answers the wrong question:

quietly_wrong.py

marks = 40
print(marks)
marks = marks + 10

Output

No traceback, no complaint — it printed 40 because that is genuinely what marks held at that moment. The 10 was added afterwards, to nobody's benefit. This is a logical error, and misordered statements are one of the commonest ways to write one.

4Try it

Run it as it stands, then move the print() line up one and run it again. Then try adding a third number.

sum.py

a = 25
b = 17
total = a + b

print('Sum =', total)

a = 25
b = 17
total = a + b

print('Sum =', total)

5Recap

Key Takeaway

A sequential statement runs after the one above it. No keyword, no indentation, nothing to type — it is Python's default, and it is what every program in this course has been made of so far. Each statement runs exactly once, and works with whatever the statements above it left behind. Change the order and you change the program.

Note

The next four kinds of statement are all defined by how they break this pattern. Selection lets Python skip statements. Iterative makes them run more than once. Jump leaves in the middle. And the empty statement holds a place where something is required but nothing needs doing. Sequential is the baseline all four are measured against — which is why it came first.

Quick Check

In sum.py, when does anything appear on the screen?

Quick Check

a = 25 / b = 17 / print('Sum =', total) / total = a + b — what happens?

Quick Check

How many times does a sequential statement run?

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🗺️ Introduction to Statements

What is a Selection Statement? 🛣️