Python Language Exceptions

Internal

• Python Language
• Pytest Testing Idioms | Tested Code is Supposed to Throw Exception

TODO

• PROCESS PyOOP "Expecting the Unexpected", "Raising exceptions", "Raising an exception", "The effects of an exception", "Handling exceptions", "The exception hierarchy", "Defining our own exceptions", "Case study".
• PROCESS Google Python Style Guide, Exceptions: https://google.github.io/styleguide/pyguide.html#24-exceptions

Overview

Exceptions are Python's main error handling mechanism. try/except is a language-level mechanism to handle errors that may be caused by a section of the code. Exceptions can be triggered explicitly from the program code with raise. A convenient way to conditionally trigger an exception (AssertionError) in the code, use assert.

Exception State

An exception instance carries an args tuple.

try/except

try/except is a language-level mechanism to handle errors (traceback) that may be caused by a section of the code. This syntax eliminates tracebacks.

try:
  # do something
except <SomeException>:
  # execute if the previous block caused an error

The code provided under except is referred to as "exception handler". If an exception occurs in a try block and it is not caught in the corresponding except block, it bubbles up until is caught by a matching handler, in the calling layers. If no exception handler is found, Python prints and error message and some information about where the error occurred and then terminates the program.

Note that "bare" except: (the except keyword followed by colon and nothing else) is syntactically valid, but it is considered bad style. A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to interrupt a program with Control-C, and can disguise other problems. To catch all exceptions that signal program errors, use except Exception: (bare except is equivalent to except BaseException:). The use of bare except: is limited to two cases:

1. If the exception handler will be printing out or logging the traceback; at least the user will be aware that an error has occurred.
2. If the code needs to do some cleanup work, but then lets the exception propagate upwards with raise. try...finally can be a better way to handle this case.

If more than one type of exception could occur, it is best to provide a separate exception handler for each:

try:
  # do something
except SomeError as ex:
  # exception handler for 'SomeError'
except SomeOtherError as ex:
  # exception handler for 'SomeOtherError'
finally:
  # optional block to execute whether there are exceptions or not

The reserved word as is used to store the caught exception into a variable for further introspection.

Catching Multiple Exceptions on the Same Line

try:
  # do something
except (UserError, ValidationError) as e:
  # ...

finally

The finally block is optional. If specified, is executed regardless of whether there was an exception or not, and after the exception was handled, if there was an exceptions.

Introspecting Exceptions

TODO

raise

Avoid raising generic Exceptions, try to be specific as possible with the exception type and use the most specific exception type that semantically fits the issue.

raise ValueError('function invoked with no arguments')

assert

The assert statement can be used to trigger an AssertionError conditionally. It can be use with an optional error message, which is becoming part of the exception if the assertion proves false:

assert s is not None

assert s is not None, 'The string was supposed to be set!'

Exception Chaining

Exceptions can be chained, which preserves tracebacks:

raise RuntimeError('specific message') from error

Predefined Exceptions

https://docs.python.org/3/library/exceptions.html

Some exceptions are predefined in the Python Standard Library.

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
          ├─ EncodingWarning
          └─ ResourceWarning

Interesting Exceptions

NotImplementedError

Use NotImplementedError, not NotImplemented, NotImplemented cannot be caught.

Defining Custom Exceptions

A custom exception is a class, a child of class Exception.

class UserError(Exception):
    def __init__(self, msg):
        super().__init__(msg)

Access Exception State

Access the message: use str(e).