Error Handling

11. Error Handling#

By now, everyone has likely encountered a Python error or two…

For example:

print(this)  # => NameError: name 'this' is not defined

or:

else = 5  # => SyntaxError: invalid syntax

or:

open("nonexistent_file")  # => FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent_file'

or:

5 / 0  # => ZeroDivisionError: division by zero

Some errors, such as else = 5, indicate genuine mistakes in the code that need to be corrected (and luckily Python prevents this). However, errors can also occur during program execution and aren’t always a reason to stop the entire program.

def divider(number, divide_by):
    """Divides 'number' by 'divide_by'.
    
    Returns None for division by zero.
    """
    if divide_by == 0:
        return None
    return number / divide_by

print(divider(5, 0))

The issue: Not all errors are as easily anticipated. A better approach is to catch errors using Python’s try: except mechanism:

try:
    print(this)
except:
    print("The program continued running...")

Or like this:

try:
    print(this)
except Exception as error:
    print(error)
print("The program continued running...")

Back to the divider function:

def divider(number, divide_by):
    """Divides 'number' by 'divide_by'.
    
    Returns None for division by zero.
    """
    try:
        return number / divide_by
    except ZeroDivisionError:
        return None

print(divider(5, 0))

Mini Quiz:

What happens if the function is called with: print(divider("text", 2))

a) None
b) te
c) TypeError

11.1. Common Error Types#

  • IOError – For example, when a file cannot be opened.

  • ImportError – When a Python module cannot be found or loaded.

  • ValueError – When a function receives an argument of the correct type but with an invalid value.

  • TypeError – When a function receives an argument of an invalid type.

  • KeyboardInterrupt – When the user interrupts execution using the keyboard (e.g., Ctrl+C).

Sometimes, we intentionally want to raise errors. This can be done using raise:

raise Exception("Something went wrong")

Or:

x = "hello"

if not isinstance(x, int):
    raise TypeError("Only integers are allowed")

11.2. assert#

Often, it’s simpler to use the assert keyword to handle such cases:

selected_number = "hello"
assert isinstance(selected_number, int), "Expected an integer"

Example:

def hour_to_am_pm(hour):
    """Convert a 24-hour time to 12-hour clock format."""
    assert 0 <= hour <= 24, "Hour must be between 0 and 24"
    
    if 1 <= hour <= 12:
        print(f"{hour} a.m.")
    elif hour == 0:
        print("12 a.m.")
    else:
        print(f"{hour - 12} p.m.")


hour_to_am_pm(13)  # => 1 p.m.

If the input doesn’t meet the conditions, an assertion error is raised:

hour_to_am_pm(25)  # => AssertionError: Hour must be between 0 and 24
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