Question

I can use map to implement the case insensitive list search with Python.

a = ['xyz', 'wMa', 'Pma'];

b = map(string.lower, a)
if 'Xyz'.lower() in b:
    print 'yes'

How can I do the same thing with dictionary?

I tried the following code, but ap has the list of [‘a’,’b’,’c’], not the case insensitive dictionary.

a = {'a':1, 'B':2, 'c':3}
ap = map(string.lower, a)

Answer 1

Note that making a dictionary case-insensitive, by whatever mean, may well lose information: for example, how would you “case-insensitivize” {'a': 23, 'A': 45}?! If all you care is where a key is in the dict or not (i.e., don’t care about what value corresponds to it), then make a set instead — i.e.

theset = set(k.lower() for k in thedict)

(in every version of Python, or {k.lower() for k in thedict} if you’re happy with your code working only in Python 2.7 or later for the sake of some purely decorative syntax sugar;-), and check with if k.lower() in theset: ....

Or, you could make a wrapper class, e.g., maybe a read-only one…:

import collections

class CaseInsensitiveDict(collections.Mapping):
    def __init__(self, d):
        self._d = d
        self._s = dict((k.lower(), k) for k in d)
    def __contains__(self, k):
        return k.lower() in self._s
    def __len__(self):
        return len(self._s)
    def __iter__(self):
        return iter(self._s)
    def __getitem__(self, k):
        return self._d[self._s[k.lower()]]
    def actual_key_case(self, k):
        return self._s.get(k.lower())

This will keep (without actually altering the original dictionary, so all precise information can still be retrieve for it, if and when needed) an arbitrary one of possibly-multiple values for keys that “collapse” into a single key due to the case-insensitiveness, and offer all read-only methods of dictionaries (with string keys, only) plus an actual_key_case method returning the actual case mix used for any given string key (or None if no case-alteration of that given string key matches any key in the dictionary).