CODEWITHSUNDEEP
pythonpython-2.7error-handling
brandones
brandones

Reputation: 1937

Print dict on KeyError

I'm working with several enormous lists of dicts, some of which might be missing items. I'm getting a lot of KeyErrors. They tell me what the bad key is, but they don't tell me what the dict in question is. I'd like something that does the equivalent of

foos = [{'abc': 0, 'bcd': 1}, {'abc': 2}, {'abc': 4, 'bcd': 0}]
for foo in foos:
    try:
        print foo['bcd']
    except KeyError as err:
        print 'bcd not found in ' + str(foo)

But where I'm subscripting foos, bars, bazes and quxes. Put another way, I'd like it if my error print statement could infer that 'foo' is the dict in question without me having to be explicit about it. When dealing with lots of dictionaries, I'd really rather not give each subscripting its own try/except block.

Upvotes: 0

Views: 502

Answers (2)

Eithos
Eithos

Reputation: 2491

If what you want is to print the looked-up key and its dictionary each time you try to access a nonexistent key, it seems to me you'd have to subclass dict.

class ErrorDict(dict):

    def __getitem__(self, key):
        try:
            val = dict.__getitem__(self, key)
        except KeyError:
            val = None
            print '{0} not found in {1}'.format(key, self)

        return val


foos = [ErrorDict(i) for i in ({'abc': 0, 'bcd': 1}, {'abc': 2}, {'abc': 4, 'bcd': 0})]
for foo in foos:
    if foo['bcd']: # If invalid, None is returned and condition will fail.
        #... all the code
        pass

# Output: bcd not found in {'abc': 2}

This seems to fulfill your requirement which, from what I understood, is to avoid having to use try/except blocks everywhere in your code. I can't say I recommend going this way because I don't have much experience subclassing dict and so I'm not familiar with its potential pitfalls.

In any case, it works for what you've asked. If you've any other questions, shoot.

Upvotes: 1

dave
dave

Reputation: 12806

If you are looking for the index of the dictionary within the list you can use enumerate: 

foos = [{'abc': 0, 'bcd': 1}, {'abc': 2}, {'abc': 4, 'bcd': 0}]
for idx, foo in enumerate(foos):
    try:
        print foo['bcd']
    except KeyError as err:
        print 'bcd not found in dictionary #' + idx

Upvotes: 2

Related Questions