Don't take an instance of a Python exception class directly as an argument to the exception class!

• Note: The code below only works with Python 3.3 because it uses the raise from syntax, but the basic story remains the same with Python 2.7. *

This time, I would like to talk about using an instance of a Python exception class directly as an argument of the exception class, but in the first place

I'm not sure about the situation "** Make an instance of the exception class directly as an argument of the exception class **".

I'm sure there are people who say that, so let's set a little more specific situation.

First, let's say you created a spam module in Python, prepared SpamError as the base class for module exceptions, and created a custom exception classSpamIndexError that inherited SpamError and the Python standard ʻIndexError`.

It looks like this when you actually write it.

spam.py

class SpamError(Exception):
    pass


class SpamIndexError(SpamError, IndexError):
    pass

Next, because of the requirement that users who use the spam module can catch Python standard errors that occur in the spam module with ʻexcept spam.SpamError as exception:` etc., in the spam module

spam.py

try:
    [][0]  #Index Error occurs
except IndexError as original:
    raise SpamIndexError(original) from original

I decided to write code like this. The reason that the argument of SpamError in the last line is ʻoriginal, which is an exception supplemented by the except statement, is intended to display a message equivalent to the original ʻIndexError. This is a common practice in many sources.

When I run the above code,

python

Traceback (most recent call last):
  File "./spam.py", line 16, in <module>
    [][0]  #Index Error occurs
IndexError: list index out of range

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./spam.py", line 18, in <module>
    raise SpamIndexError(original) from original
spam.SpamIndexError: list index out of range

Is displayed. Both ʻIndexError and SpamIndexErrorare displayed aslist index out of range`. There is no problem so far.

But unfortunately if this was a KeyError instead of a ʻIndexError`

python

Traceback (most recent call last):
  File "./spam.py", line 16, in <module>
    {}[0] #KeyError occurs
KeyError: 0

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./spam.py", line 18, in <module>
    raise SpamKeyError(original) from original
spam.SpamKeyError: KeyError(0,)

Will be displayed like this. The original KeyError is displayed as KeyError: 0, but the SpamKeyError is displayed asspam.SpamKeyError: KeyError (0,). ʻIndexError evaluates the argument with str () , but KeyErrorevaluates withrepr ()` if there is one argument, so this is the case.

To avoid this

python

    raise SpamKeyError(original) from original

Line,

python

    raise SpamKeyError(*original.args) from original

And instead of ʻoriginal, take the argument of the original exception, * original.args`. Then

python

Traceback (most recent call last):
  File "./spam.py", line 16, in <module>
    {}[0] #KeyError occurs
KeyError: 0

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./spam.py", line 18, in <module>
    raise SpamKeyError(*original.args) from original
spam.SpamKeyError: 0

Is displayed, and SpamKeyError is displayed in the same way as the original KeyError.

However, it's okay to take * original.args as an argument only if the exception you raise inherits from the class of the original exception. If not,

python

    raise SpamError(str(original)) from original

It is safe to enclose the supplemented exception with str () and forcibly take a string as an argument.

If you want to raise an exception class that simply inherits from the ʻException` class,

python

    raise SpamError(original) from original

It's okay to say that you don't have to enclose it in str () like, but in that case, the exception that was displayed as KeyError: 0 is displayed as spam.SpamError: 0. Therefore, it becomes difficult to understand what kind of error it is from the message.

In any case, it would be a bad idea to just use the exception class instance ʻoriginal` as an argument.