Elegant, pythonic solution for forcing all keys and values to lower case in nested dictionaries of Unicode strings?

Question

Hi! I'm curious how the Python Ninjas around here would do the following, elegantly and pythonically:

I've got a data structure that's a dict from unicode strings to dicts from unicode strings to unicode string lists. So:

>>> type(myDict)
<type 'dict'>
>>> type(myDict[u'myKey'])
<type 'dict'>
>>> type(myDict[u'myKey'][u'myKey2'])
<type 'list'>
>>> type(myDict[u'myKey'][u'myKey2'][0])
<type 'unicode'>

I want to go through and make every string in it lowercase, i.e. every key and every string in every list.

How would you do this?

Answer 1

The following works for your example input:

>>> myDict = {'myKey': {'myKey2': [u'Abc']}}
>>> newDict = dict([
        (k1.lower(), dict([
            (k2.lower(), [x.lower() for x in myDict[k1][k2]])
                for k2 in myDict[k1].keys()
        ])) for k1 in myDict.keys()
    ])
>>> newDict
{'mykey': {'mykey2': [u'abc']}}

Answer 2

A recursive variant:

def make_lowercase(obj):
    if hasattr(obj,'iteritems'):
        # dictionary
        ret = {}
        for k,v in obj.iteritems():
            ret[make_lowercase(k)] = make_lowercase(v)
        return ret
    elif isinstance(obj,basestring):
        # string
        return obj.lower()
    elif hasattr(obj,'__iter__'):
        # list (or the like)
        ret = []
        for item in obj:
            ret.append(make_lowercase(item))
        return ret
    else:
        # anything else
        return obj

Example:

>>> make_lowercase({'Foo': [{1: 'Baz', 'Bar': 2}]})
{'foo': [{1: 'baz', 'bar': 2}]}

Answer 3

Really simple way, though I'm not sure you'd call it Pythonic:

newDict = eval(repr(myDict).lower())

Saner way:

newDict = dict((k1.lower(),
                dict((k2.lower(),
                      [s.lower() for s in v2]) for k2, v2 in v1.iteritems()))
               for k1, v1 in myDict.iteritems())

Answer 4

If you don't mind modifying your input:

def make_lower(x):
    try:
        return x.lower()    # x is a string
    except AttributeError:
        # assume x is a dict of the appropriate form
        for key in x.keys():
            x[key.lower()] = make_lower(x[key])
            if key != key.lower():
                del x[key]

    return x

make_lower(myDict)