Modularity: subprograms, libraries and files

Kinds of module (spec 9.1.3)

• Blocked code: indented blocks under if, for, while, def.
• User-defined subprograms: procedures and functions you write.
• In-built subprograms: len(), print(), input(), int(), round().
• Libraries: collections of subprograms you import, such as random, math, copy, datetime, time, turtle and sqlite3.
• Files: code can be split across .py files and imported, and data kept in separate files.

Importing

The four import forms in the PLS:

import math
import random as rnd
from math import sqrt
from math import pi as PI_VALUE
print(math.floor(4.7), sqrt(81), round(PI_VALUE, 3))
rnd.seed(1)
print(rnd.randint(1, 1))

Output:

4 9.0 3.142
1

Dates and times

The PLS uses ISO 8601 text such as 2026-11-08 14:30:00, in UTC. time.time() and timestamp() count seconds since 1 January 1970 00:00:00 UTC (the epoch).

from datetime import datetime, timezone
FORMAT = "%Y-%m-%d %H:%M:%S"
start = datetime.fromtimestamp(0, timezone.utc)
print(start.strftime(FORMAT))
t = datetime.strptime("2026-11-08 14:30:00", FORMAT).replace(tzinfo=timezone.utc)
print(t.timestamp())
print(datetime.fromtimestamp(t.timestamp() + 3600, timezone.utc).strftime("%H:%M:%S"))
print(t.date())

Output:

1970-01-01 00:00:00
1794148200.0
15:30:00
2026-11-08

datetime.now(timezone.utc) gives the current time; time.sleep(sec) pauses the program.

Turtle graphics

The turtle library draws by moving a pen. In the default 'standard' mode the turtle starts facing east and left() turns anticlockwise. Common calls: turtle.Turtle(), forward(d), back(d), left(a), right(a), penup(), pendown(), pencolor(), fillcolor(), begin_fill(), end_fill(), circle(r), setposition(x, y), setheading(a), turtle.done().

import turtle
t = turtle.Turtle()
for side in range(6):
    t.forward(50)
    t.left(60)
turtle.done()

This draws a regular hexagon: to close a regular polygon with n sides, the turtle turns 360 / n degrees at each corner.

Worked example

How many seconds after the epoch is 1970-01-02 00:00:00 UTC, and what does the timestamp 90061 display as?

1. One day = 24 x 60 x 60 = 86400 seconds
2. datetime.fromtimestamp(90061, timezone.utc).strftime("%Y-%m-%d %H:%M:%S") gives 1970-01-02 01:01:01

Answer: 86400 seconds; 1970-01-02 01:01:01

Check yourself

1. What is displayed when this code runs? ```python from math import ceil, floor print(ceil(2.1) + floor(2.9)) ```
   Show answer

   5. ceil(2.1) = 3 and floor(2.9) = 2: 5.

2. Which exception is raised when this code runs? ```python import math print(sqrt(16)) ```
   • NameError
   • ValueError
   • TypeError
   • AttributeError
   Show answer

   NameError. import math only creates the name math; sqrt must be written math.sqrt, so NameError.

3. What is displayed when this code runs? ```python import math as m print(m.trunc(-3.8) + m.fabs(-2)) ```
   Show answer

   -1. m.trunc(-3.8) is -3 and m.fabs(-2) is 2.0, so the result is -1.0.

4. What is displayed when this code runs? ```python from datetime import datetime, timezone print(datetime.fromtimestamp(3600 * 5, timezone.utc).strftime("%H:%M:%S")) ```
   Show answer

   05:00:00. 18000 seconds after midnight on 1 January 1970 is 5 hours: 05:00:00.

5. Two readings were taken at 2026-11-08 09:15:00 and 2026-11-08 10:05:30 (UTC). Using timestamps, how many seconds apart are they?
   Show answer

   3030. Convert each to a timestamp and subtract: 50 minutes 30 seconds = 3030 seconds.

6. A turtle draws a regular pentagon with five forward() moves. Through what angle, in degrees, must it turn left at each corner?
   Show answer

   72 degrees. The turns add up to one full turn, so each is 360 / 5 = 72 degrees.

7. Why should you use a library function such as math.sqrt() instead of writing your own square root code?
   • Library functions never raise exceptions
   • Your own code would not be allowed in the exam
   • It makes the program use less memory in every case
   • It is already written and tested, so it saves time and is reliable
   Show answer

   It is already written and tested, so it saves time and is reliable. Reusing tested code is quicker and less error-prone.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (XCP01/YCP01), first teaching 2026
• Pearson IAL Computer Science Unit 2 Programming Language Subset (PLS), Python 3
• Pearson IAL Computer Science IAS sample assessment materials (Unit 2 paper and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.