Error Handling in Python

Catching Errors in Python

In the topics covered so far we never talked about errors, how to catch them, or how to continue execution based on the error encountered — but this is an important topic and you will learn it here.

Here is a small example:

get_n = float(input("Enter a number >> "))
result = 10 / get_n
print(f"result {result}")

What happens if the user enters 0 instead of 1 or 2? An error occurs and our program does not produce the result we want. You could control the zero-entry case with if, or you can use Python’s error handling mechanism.

The script below shows how we find which error occurred:

try:
    1 / 0
except Exception as exc:
    print(f"Oh no! We caught a {exc.__class__.__name__} error")

Output:

Oh no! We caught a ZeroDivisionError error

As you can see, dividing a number by zero gives us ZeroDivisionError.

Catching Specific Errors

Try, Except

General Structure

try:
   # some code
   pass
except ValueError:
   # runs if a ValueError occurs
   pass
except (TypeError, ZeroDivisionError):
   # catching multiple errors at once
   # runs if TypeError or ZeroDivisionError occurs
   pass
except Exception:
   # runs if any other error occurs
   pass

Try, Except, As

The as keyword assigns the caught exception instance to a name.

General Structure 1

try:
   pass
except ValueError as error:
   # caught a ValueError — receive it as "error" and print it
   print(error)

General Structure 2

try:
   pass
except Exception as e:
   # any error caught — receive it as "e" and print it
   print(e)

Try, Except, Else

General Structure

try:
   pass
except:
   # runs if any error occurs
   pass
else:
    # runs if NO error occurred
    pass

Try, Except, Else, Finally

General Structure

try:
   pass
except:
   pass
else:
    pass
finally:
    # runs whether or not an error occurred
    pass

Creating Our Own Exception Class

To do this we use the Exception class and the raise keyword.

raise

raise is a keyword used to raise (trigger) any exception class.

Let us redo the example from the beginning:

class InputError(Exception):
    """An exception class for input errors.

    Attributes:
        expression -- input expression in which the error occurred
        message    -- explanation of the error
    """

    def __init__(self, expression, message):
        self.expression = expression
        self.message = message

input_ = float(input("Enter a number other than 0 >> "))
if input_ == 0:
    raise InputError(input_, "do not enter 0")

Result:

Traceback (most recent call last):
  File "myapp.py", line 28, in <module>
    raise InputError(input_, "do not enter 0")
__main__.InputError: (0.0, 'do not enter 0')

Now let us refine it and also catch the error we raise:

class InputError(Exception):
    def __init__(self, expression, message):
        self.expression = expression
        self.message = message

def get_input():
    input_ = float(input("Enter a number other than 0 >> "))
    if input_ == 0:
        raise InputError(input_, "do not enter 0")
    return input_

try:
    input_ = get_input()
except InputError as e:   # if the returned error is our InputError
    print(e)
else:
    print(input_)

Result:

Enter a number other than 0 >> 0
(0.0, 'do not enter 0')

Python Built-in Exceptions

The tree below shows which exception inherits from which:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      |    +-- ModuleNotFoundError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      |    +-- RecursionError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning

References

• https://docs.python.org/3/library/exceptions.html
• https://docs.python.org/3/tutorial/errors.html