Exceptions

The Python language actually can handle exceptions (Perl and C do not where as Java or C++ do), what this means is that if a error happens in a program (for example database or file connection issue) you can write code that will handle this error so that the program does not crash, you also might want to perform some tidy up before the program continues. I will have a more advanced example later but below is the basic concept of exceptions

Try to use exception in code that may fial due to external issues, database connections, file i/o, networking, etc.

try-except block

try:
    # do stuff
except Exception:
    # handle exceptions

Python uses object-oriented, an exception is a OOP object, it is automatically generated by Python functions with a raise statement. When an exception is raised the program will then look for a handler to handle the issue, if the handler is found the code is executed in that handler, if no handler is found then the program aborts.

Exception Types

Python can handle many types of exceptions, the baseException type contains all exceptions, you can even define your own. Because the exception set is hierarchical to can use higher exception type to capture many types of exceptions, so using the Exception type would be a good idea.

Built-in exception types

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

Raising Exceptions

Exceptions are raised by many of Pythons built-in functions, but you can raise your own if something goes wrong

Raise exception

raise exception(args)

Catching and Handling Exceptions

You can handle multiple exceptions each with its handler code, the below code is executed in the following way

try statement is executed first, if successful then the else_body is executed and the finally_body is also executed
if exception is throw in try block the except blocks are searched in order to find a handler,if a handler is found the code in that block is executed
if no handler can be found then the exception is thrown up the stack looking for an handler, however you can have a default handler which is the except block.

Catch and handle exceptions

try:
    body
except exception_type1 as var1:          # handles a specific type of exception
    exception_code1
except exception_type2 as var2:          # handles a specific type of exception
    exception_code2
except:                                  # catch-all handler
    default_exception_code
else:
    else_body                             # only executed if try block is successful
finally:
    finally_body                          # only executed after try, except and else blocks have executed, its always executed

Defining New Eceptions

You can easy define your own exceptions, you create a class that inherits from the Exception class

Defining your own custom exception

class MyError(Exception):
    pass

try:
    raise MyError("Some information about what went wrong")
except MyError as error:
    print("Situation:", error)

Assert Statement

assert is a specialized form of a raise statement, the AssertError exception with the optional argument is raised if the expression evaluates to False and the system variable __debug__ is True. Assert can be used for debugging programs, code will be obmitted if the __debug__ statement is set to False.

Assert statement

x = (1, 2, 3)
assert len(x) > 5, "len(x) not > 5"

Note: an assertion exception will be throw with the message