How to suppress irrelevant errors in a try block

Question

Suppose I want to check that a certain entry is in a Series. I would like to try to access that entry, and if that fails, raise a simple, short ValueError.

For example, I have a series that doesn't have entry C - I want a check to halt the script. Example:

s = {'A': 1, 'B': 2}
s = pd.Series(s)

try:
    s['C']
except:
    raise ValueError('C is missing.')

But this code throws a long KeyError before spitting out the ValueError. It works, but is verbose.

(I know that I can use an assert statement instaead.)

Why doesn't the try block suppress the KeyError - isn't that part of its purpose? Is there a way to get my intended behavior?

Answer 1

You are seeing exception chaining. This extra information can be suppressed with a from None clause in your raise statement. Consider this (totally contrived) case where I am suppressing a ZeroDivisionError and raising a KeyError:

>>> try:
...     1/0
... except ZeroDivisionError:
...     raise KeyError
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
KeyError

But if I use from none:

>>> try:
...     1/0
... except ZeroDivisionError:
...     raise KeyError from None
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
KeyError
>>>

Also note, you really should not use a bare except clause. Catch as specific an error as possible.