Hi,
Here's something I wanted in Python for many years. If this has been
discussed in the past, please refer me to that discussion.
On one hand, it's something that I can't imagine the python-dev community
supporting. On the other hand, it would maintain backward compatibility.
I wish there were a 100 more built-in exceptions in Python, that will be
very specific about what went wrong.
If I do this:
>>> x, y = range(3)
I know it'll raise a ValueError, because I've memorized that, but it did
take me a few years to remember where I should expect ValueError and where
I should expect TypeError.
It would be nice if the operation above raised UnpackingOverflowError,
which will be a subclass of UnpackingError, along with
UnpackingUnderflowError. UnpackingError can be a subclass of ValueError,
for backward compatibility.
Similarly, if I did this:
>>> def f(x, y): return x + y
>>> f(1)
I would get a TypeError. Would be a lot cooler if I got
MissingArgumentsError, which would be a subclass of SignatureError, which
would be a subclass of TypeError.
There are 2 reasons I want this:
1. When I'm writing a try..except clause, I want to catch a specific
exception like MissingArgumentsError rather than ValueError or TypeError.
They're too ubiquitous. I don't want some other unexpected failure
producing the same ValueError and triggering my except clause.
2. When I get an error, especially from some shitty corporate system that
truncates the traceback, I want to get as many hints as possible about what
went wrong.
It's true that today, most Python exceptions have good text in their
message, like "TypeError: f() missing 1 required positional argument: 'y'".
But that isn't guaranteed everywhere, and specific exception types could
help.
What do you think?
Ram.
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