COMP1531

2.6 - Python - Exceptions

Python - Exceptions

An exception is an action that disrupts the normal flow of a program. This action is often representative of an error being thrown. Exceptions are ways that we can elegantly recover from errors

import sys

def sqrt(x):
    if x < 0:
        sys.stderr.write("Error Input < 0\n")
        sys.exit(1)
    return x**0.5

if __name__ == '__main__':
    print("Please enter a number: ",)
    inputNum = int(sys.stdin.readline())
    print(sqrt(inputNum))

The simplest way to deal with problems...

 

Just crash

exception_1.py

Python - Exceptions

import sys

def sqrt(x):
    if x < 0:
        raise Exception(f"Error, sqrt input {x} < 0")
    return x**0.5

if __name__ == '__main__':
    print("Please enter a number: ",)
    inputNum = int(sys.stdin.readline())
    print(sqrt(inputNum))

Now instead, let's raise an exception

 

However, this just gives us more information, and doesn't help us handle it

exception_2.py

Python - Exceptions

import sys

def sqrt(x):
    if x < 0:
        raise Exception(f"Error, sqrt input {x} < 0")
    return x**0.5

if __name__ == '__main__':
    try:
        print("Please enter a number: ",)
        inputNum = int(sys.stdin.readline())
        print(sqrt(inputNum))
    except Exception as e:
        print(f"Error when inputting! {e}. Please try again:")
        inputNum = int(sys.stdin.readline())
        print(sqrt(inputNum))

If we catch the exception, we can better handle it

exception_3.py

Python - Exceptions

import sys

def sqrt(x):
    if x < 0:
        raise Exception(f"Error, sqrt input {x} < 0")
    return x**0.5

if __name__ == '__main__':
    print("Please enter a number: ",)
    while True:
        try:
            inputNum = int(sys.stdin.readline())
            print(sqrt(inputNum))
            break
        except Exception as e:
            print(f"Error when inputting! {e}. Please try again:")

Or we could make this even more robust

exception_4.py

Python - Exceptions

import sys

def sqrt(x):
    if x < 0:
        raise Exception(f"Input {x} is less than 0. Cannot sqrt a number < 0")
    return x**0.5

if __name__ == '__main__':
    if len(sys.argv) == 2:
        try:
            print(sqrt(int(sys.argv[1])))
        except Exception as e:
            print(f"Got an error: {e}")

Key points:

  • Exceptions carry data
  • When exceptions are thrown, normal code execution stops

throw_catch.py

Python - Exceptions

Examples with pytest (very important for project)

import pytest

def sqrt(x):
    if x < 0:
        raise Exception(f"Input {x} is less than 0. Cannot sqrt a number < 0")
    return x**0.5

def test_sqrt_ok():
    assert sqrt(1) == 1
    assert sqrt(4) == 2
    assert sqrt(9) == 3
    assert sqrt(16) == 4

def test_sqrt_bad():
    with pytest.raises(Exception):
        sqrt(-1)
        sqrt(-2)
        sqrt(-3)
        sqrt(-4)
        sqrt(-5)

pytest_except_1.py

Python - Exceptions

Other basic exceptions can be caught with the "Exception" type

import pytest

def sqrt(x):
    if x < 0:
        raise ValueError(f"Input {x} is less than 0. Cannot sqrt a number < 0")
    return x**0.5

def test_sqrt_ok():
    assert sqrt(1) == 1
    assert sqrt(4) == 2
    assert sqrt(9) == 3
    assert sqrt(16) == 4

def test_sqrt_bad():
    with pytest.raises(Exception):
        sqrt(-1)
        sqrt(-2)
        sqrt(-3)
        sqrt(-4)
        sqrt(-5)

pytest_except_2.py

Python - Exception Sub-types

COMP1531 21T1 - 2.6 - Python - Exceptions

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COMP1531 21T1 - 2.6 - Python - Exceptions

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