Dictionary to lowercase in Python

I wish to do this but for a dictionary:

"My string".lower()

Is there a built in function or should I use a loop?

Solution 1:

You will need to use either a loop or a list/generator comprehension. If you want to lowercase all the keys and values, you can do this::

dict((k.lower(), v.lower()) for k,v in {'My Key':'My Value'}.iteritems())

If you want to lowercase just the keys, you can do this::

dict((k.lower(), v) for k,v in {'My Key':'My Value'}.iteritems())

Generator expressions (used above) are often useful in building dictionaries; I use them all the time. All the expressivity of a loop comprehension with none of the memory overhead.

Solution 2:

The following is identical to Rick Copeland's answer, just written without a using generator expression:

outdict = {}
for k, v in {'My Key': 'My Value'}.iteritems():
    outdict[k.lower()] = v.lower()

Generator-expressions, list comprehension's and (in Python 2.7 and higher) dict comprehension's are basically ways of rewriting loops.

In Python 2.7+, you can use a dictionary comprehension (it's a single line of code, but you can reformat them to make it more readable):

{k.lower():v.lower()
    for k, v in
    {'My Key': 'My Value'}.items()
}

They are quite often tidier than the loop equivalent, as you don't have to initialise an empty dict/list/etc.. but, if you need to do anything more than a single function/method call they can quickly become messy.

Solution 3:

If you want keys and values of multi-nested dictionary (json format) lowercase, this might help. Need to have support for dict comprehensions what should be in Python 2.7

dic = {'A':['XX', 'YY', 'ZZ'],
       'B':(u'X', u'Y', u'Z'),
       'C':{'D':10,
            'E':('X', 'Y', 'Z'),
            'F':{'X', 'Y', 'Z'}
           },
       'G':{'X', 'Y', 'Z'}
      }

PYTHON2.7 -- also supports OrderedDict

def _lowercase(obj):
    """ Make dictionary lowercase """
    if isinstance(obj, dict):
        t = type(obj)()
        for k, v in obj.items():
            t[k.lower()] = _lowercase(v)
        return t
    elif isinstance(obj, (list, set, tuple)):
        t = type(obj)
        return t(_lowercase(o) for o in obj)
    elif isinstance(obj, basestring):
        return obj.lower()
    else:
        return obj 

PYTHON 3.6

def _lowercase(obj):
    """ Make dictionary lowercase """
    if isinstance(obj, dict):
        return {k.lower():_lowercase(v) for k, v in obj.items()}
    elif isinstance(obj, (list, set, tuple)):
        t = type(obj)
        return t(_lowercase(o) for o in obj)
    elif isinstance(obj, str):
        return obj.lower()
    else:
        return obj

Solution 4:

Shorter way in python 3: {k.lower(): v for k, v in my_dict.items()}

Solution 5:

This will lowercase all your dict keys. Even if you have nested dict or lists. You can do something similar to apply other transformations.

def lowercase_keys(obj):
  if isinstance(obj, dict):
    obj = {key.lower(): value for key, value in obj.items()}
    for key, value in obj.items():         
      if isinstance(value, list):
        for idx, item in enumerate(value):
          value[idx] = lowercase_keys(item)
      obj[key] = lowercase_keys(value)
  return obj 

json_str = {"FOO": "BAR", "BAR": 123, "EMB_LIST": [{"FOO": "bar", "Bar": 123}, {"FOO": "bar", "Bar": 123}], "EMB_DICT": {"FOO": "BAR", "BAR": 123, "EMB_LIST": [{"FOO": "bar", "Bar": 123}, {"FOO": "bar", "Bar": 123}]}}

lowercase_keys(json_str)


Out[0]: {'foo': 'BAR',
 'bar': 123,
 'emb_list': [{'foo': 'bar', 'bar': 123}, {'foo': 'bar', 'bar': 123}],
 'emb_dict': {'foo': 'BAR',
  'bar': 123,
  'emb_list': [{'foo': 'bar', 'bar': 123}, {'foo': 'bar', 'bar': 123}]}}