Python: User-Defined Exception That Proves The Rule

Question

Python documentations states:

Exceptions should typically be derived from the Exception class, either directly or indirectly.

the word 'typically' leaves me in an ambiguous state.

consider the code:

class good(Exception): pass
class bad(object): pass

Heaven = good()
Hell = bad()

>>> raise Heaven

Traceback (most recent call last):
  File "<pyshell#163>", line 1, in <module>
    raise Heaven
good

>>> raise Hell

Traceback (most recent call last):
  File "<pyshell#171>", line 1, in <module>
    raise Hell
TypeError: exceptions must be classes or instances, not bad

so when reading the python docs, should i replace 'typically' with ''?

what if i have a class hierarchy that has nothing to do with the Exception class, and I want to 'raise' objects belonging to the hierarchy?

I can always raise an exception with an argument:

raise Exception, Hell

This seems slightly awkward to me

What's so special about the Exception (EDIT: or BaseException) class, that only its family members can be raised?

Answer 1

There are other valid classes you can inherit from apart from Exception, for example BaseException.

See the documentation for the exception hierarchy.

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StandardError
      etc..

In older versions of Python it was possible to throw things other than exceptions. For example in Python 2.5:

>>> raise "foo"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
foo

But you get this deprecation warning:

DeprecationWarning: raising a string exception is deprecated

In newer versions this is not allowed. Everything you raise must derive from BaseException.

Answer 2

"so when reading the python docs, should i change 'typically' with ''?"

No.

Typically, you inherit from Exception. Period. That's what it says.

Sometimes, you might inherit from BaseException. That's what it doesn't say. You might extend BaseExcetion because you want to defeat except Exception handlers.

What's so special about ...

They're subclasses of BaseException. What more do you need to know? The source is readily available. You can read the source code for the raise statement to see exactly what it checks before it throws the TypeError.

http://svn.python.org/view/python/trunk/Python/ceval.c?annotate=80817

Lines 3456 to 3563.

However, all that matters from a practical stand-point is "subclasses of BaseException."