Errors, Modules and Next Steps

Reading a traceback

When Python hits an error it can't handle, it stops and prints a traceback: the trail of lines it was running. Read it from the bottom:

def average(scores):
    return sum(scores) / len(scores)

print(average([]))

Traceback (most recent call last):
  File "<ipython-input-1>", line 4, in <module>
    print(average([]))
  File "<ipython-input-1>", line 2, in average
    return sum(scores) / len(scores)
ZeroDivisionError: division by zero

1. The last line says what went wrong: ZeroDivisionError: division by zero.
2. The lines above show where: line 2, inside average, called from line 4.
3. So: average was given an empty list, and len([]) is 0.

The errors you've met in this course:

Handling errors with try and except

Some errors aren't bugs in your code: they come from the outside world. A user types "forty" instead of 40; a file is missing. try and except let your program handle the problem instead of crashing:

'forty' isn't a number. Please type digits, like 40.

1. Python runs the try block.
2. If an error happens, it jumps to the matching except block instead of stopping.
3. If no error happens, the except block is skipped.

Try it with a valid value:

Next year you'll be 41

Catch specific errors

Name the error you expect. You can have several except blocks for different errors:

67.33333333333333
0
The list must contain numbers only.
None

else and finally

Two optional extras:

• else runs only if no error happened.
• finally runs always, error or not, which is useful for clean-up.

Recorded ₦2,500
Check complete

A loop that keeps asking

Combined with a loop, try lets a program keep asking until it gets a valid answer. (This uses input, so run it in Colab rather than reading it here.)

while True:
    try:
        age = int(input("How old are you? "))
        break
    except ValueError:
        print("Please type a whole number.")
print(f"Thanks. You're {age}.")

while True loops forever, and break leaves the loop once int() succeeds.

Raising your own errors

Your functions can raise an error when they're given something that makes no sense, rather than quietly returning a wrong answer:

34000.0
Error: Discount must be between 0 and 100, not 150

except ValueError as error gives you the error object, so you can print its message.

Modules: using code others have written

A module is a file of ready-made functions. Python comes with hundreds (the standard library). Bring one in with import, then use its functions with a dot:

63.8
67

Ways to import:

random

Musa
5

random.choice picks an item; random.randint(1, 6) rolls a die. Without random.seed, you'd get different results each run, which is the point. random.shuffle(names) mixes a list up.

datetime

83 days until the exam
Monday 07 December 2026

Subtracting two dates gives the time between them; strftime formats a date for people. date.today() gives today's date.

Installing more: pip

Beyond the standard library, thousands of free packages are published for Python. You install them with pip. In Colab, run it in a cell with ! in front:

!pip install requests

The most important ones for data work, pandas (tables), matplotlib (charts) and numpy (numbers), are already installed in Colab:

import pandas as pd
data = pd.DataFrame({"name": ["Adaeze", "Musa"], "score": [67, 81]})
print(data)

You'll use them in the next course.

What to learn next

You now know the core of Python: values and types, text and numbers, decisions, lists, loops, dictionaries, functions, errors and modules. That's the same foundation every Python programmer builds on. Where to go from here:

Summary

Try it