Handling multiple KeyErrors in Python

Question

I get this error when I try to catch a KeyError twice. Is there anything in python which prevents you trying to catch the same error twice?

$ ./scratch.py
try getting a
can't get a try getting b
Traceback (most recent call last):
  File "./scratch.py", line 13, in <module>
    print dict['b']
KeyError: 'b'

The simplified code is below

dict={}
dict['c'] = '3'

try:
        print 'try getting a'
        print dict['a']
except KeyError:
        print 'can\'t get a try getting b'
        print dict['b']
except:
        print 'can\'t get a or b'

why not use `dict.get`? you also don't have the `print dict['b']` in a try. — Padraic Cunningham, Jul 24 '14 at 10:34
Hi Padriac- dict.get doesn't seem to throw an exception and fails silently? I would like to do something if I can't get a key. — nev, Jul 24 '14 at 11:42

wim · Answer 1 · 2014-07-24T12:53:39.063

2

I would do this with a simple for loop:

>>> d = {'c': 1}
>>> keys = ['a', 'b', 'c']
>>> for key in keys:
...     try:
...         value = d[key]
...         break
...     except KeyError:
...         pass
... else:
...     raise KeyError('not any of keys in dict')
... 
>>> value
1
>>> key
'c'

If you want to do it in one line:

key, value = next((k, d[k]) for k in keys if k in d)

edited Jul 24 '14 at 12:53

answered Jul 24 '14 at 12:47

wim

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score 0 · Answer 2 · answered Jul 24 '14 at 10:33

0

You need an extra try...except:

dict={}
dict['c'] = '3'

try:
        print 'try getting a'
        print dict['a']
except KeyError:
        print 'can\'t get a try getting b'
        try:
            print dict['b']
        except KeyError as e:
            print "Got another exception", e
except:
        print 'can\'t get a or b'

answered Jul 24 '14 at 10:33

Anthony Kong

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I don't think the indent is right for the second try, logically. Because if dict['a'] exists, there is no error and it never tries to check for dict['b'] – Kasisnu Jul 24 '14 at 11:23
I believe @nev is using the code snippet to understand the python exception handling mechanism. The key point I want to make is: if you want to handle an exception thrown within an exception handler, you need to add another try...except block. Here is a similar question in OS: http://stackoverflow.com/questions/17015230/are-nested-try-except-blocks-in-python-a-good-programming-practice – Anthony Kong Jul 24 '14 at 12:30

Handling multiple KeyErrors in Python

2 Answers2