Errors

The two types of errors in Python are syntax errors and exceptions.

Syntax Errors

Syntax errors (also known as parsing errors) occur when a sequence of characters, or tokens, violates the syntax of the Python programming language:

File "script.py", line 1
  while True print("Hello world!")
                   ^
SyntaxError: invalid syntax

The parser repeats the offending line and displays a little arrow ^ pointing at the earliest point in the line where the error was detected.

The error is caused by (or at least detected at) the token preceding the arrow in the example, the error is detected at the print() function, since a colon : is missing before it.

File name and line number are printed to state where the error originated.

Exceptions

Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal. Most exceptions are not handled by programs, however, and result in error messages as shown here:

Value Error

ValueError is thrown when a function’s argument is of the correct type, but an inappropriate value, such as being out of range.

Traceback (most recent call last):
File "script.py", line 1, in <module>
int('xyz')
ValueError: invalid literal for int() with base 10: 'xyz'

Name Error

NameError is thrown when an object could not be found.

Traceback (most recent call last):
File "script.py", line 1, in <module>
age
NameError: name 'age' is not defined

Index Error

IndexError is thrown when trying to access an item at an invalid index.

Traceback (most recent call last):
File "script.py", line 1, in <module>
employees[3]
IndexError: list index out of range

Module Not Found Error

ModuleNotFoundError is thrown when a module could not be found.

Traceback (most recent call last):
File "script.py", line 1, in <module>
import notamodule
ModuleNotFoundError: No module named 'notamodule'

Type Error

TypeError is thrown when a function’s argument is of an inappropriate type.

Traceback (most recent call last):
File "script.py", line 1, in <module>
max(True)
TypeError: 'bool' object is not iterable

Zero Division Error

ZeroDivisionError is thrown when the second operator in the division is zero.

Traceback (most recent call last):
File "script.py", line 1, in <module>
ratio = 100 / 0
ZeroDivisionError: division by zero

Custom Exceptions

Custom exceptions can be used to define and handle application-specific errors. This can be done by creating a sub-class of the built-in Exception class.

Custom exceptions help provide clear feedback to users about unexpected behavior, aid in debugging, and can be used throughout the program.

Defining Custom Exceptions

To define a custom exception, a new class can be created by inheriting Exception and the constructor can be optionally overridden to provide additional information:

# Custom exception for invalid age input
class InvalidAgeError(Exception):
  def __init__(self, message, age=None):
    super().__init__(message)
    self.age = age

Using Custom Exceptions

Once a custom exception has been created, it can raise custom error messages for invalid inputs. For example, the following function raises an error if the user inputs an age not between 0 and 130:

def validate_age(age):
  if not isinstance(age, int):
    raise InvalidAgeError("Age must be an integer.", age)
  if age < 0 or age > 130:
    raise InvalidAgeError(f"{age} is not a valid age. Please choose an age between 0 and 130.", age)

Example

Once the custom exception is defined, it can be used to handle errors in a try-except block. The following example demonstrates how to raise the custom exception when an invalid age is provided:

class InvalidAgeError(Exception):
  def __init__(self, message, age=None):
    super().__init__(message)
    self.age = age

def validate_age(age):
  if not isinstance(age, int):
    raise InvalidAgeError("Age must be an integer.", age)
  if age < 0 or age > 130:
    raise InvalidAgeError(f"{age} is not a valid age. Please choose an age between 0 and 130.", age)

try:
  validate_age(-5)
except InvalidAgeError as e:
  print(f"Error: {e}")

Here is the output:

Error: -5 is not a valid age. Please choose an age between 0 and 130.