Python: correct Exception to raise [closed]

Question

In Python, if a set of if and elif conditions are not satisfied, the else statement is reached and executed.

How, and raise which exception type, should a developer notify that such point in the code should not be reached?

Example:

if condition1:
    # do something
elif condition2:
    # do something
else:
    raise ShouldVeNeverGotHereException

This is to ensure that at least one of the above if conditions actually evaluates to True. If there is a pattern more elegant to achieve what I describe, please suggest.

Answer 1

You should never write code that can result in loose ends. But assuming you must, probably the best type of Exception to raise is a ValueError.

>>> foo = 3
>>> if foo == 1:
    pass
elif foo == 2:
    pass
else:
    raise ValueError('foo is unexpected value: %s' % foo)


Traceback (most recent call last):
  File "<pyshell#41>", line 6, in <module>
    raise ValueError('foo is unexpected value: %s' % foo)
ValueError: foo is unexpected value: 3