How to get Case Insensitive Python SET

If you need to preserve case, you could use a dictionary instead. Case-fold the keys, then extract the values to a set:

 set({v.casefold(): v for v in l}.values())

The str.casefold() method uses the Unicode case folding rules ^(pdf) to normalize strings for case-insensitive comparisons. This is especially important for non-ASCII alphabets, and text with ligatures. E.g. the German ß sharp S, which is normalised to ss, or, from the same language, the s long s:

>>> print(s := 'Waſſerſchloß', s.lower(), s.casefold(), sep=" - ")
Waſſerſchloß - waſſerſchloß - wasserschloss

You can encapsulate this into a class.

If you don't care about preserving case, just use a set comprehension:

{v.casefold() for v in l}

Note that Python 2 doesn't have this method, use str.lower() in that case.

Demo:

>>> l = ['#Trending', '#Trending', '#TrendinG', '#Yax', '#YAX', '#Yax']
>>> set({v.casefold(): v for v in l}.values())
{'#Yax', '#TrendinG'}
>>> {v.lower() for v in l}
{'#trending', '#yax'}

Wrapping the first approach into a class would look like:

try:
    # Python 3
    from collections.abc import MutableSet
except ImportError:
    # Python 2
    from collections import MutableSet

class CasePreservingSet(MutableSet):
    """String set that preserves case but tests for containment by case-folded value

    E.g. 'Foo' in CasePreservingSet(['FOO']) is True. Preserves case of *last*
    inserted variant.

    """
    def __init__(self, *args):
        self._values = {}
        if len(args) > 1:
            raise TypeError(
                f"{type(self).__name__} expected at most 1 argument, "
                f"got {len(args)}"
            )
        values = args[0] if args else ()
        try:
            self._fold = str.casefold  # Python 3
        except AttributeError:
            self._fold = str.lower     # Python 2
        for v in values:
            self.add(v)

    def __repr__(self):
        return '<{}{} at {:x}>'.format(
            type(self).__name__, tuple(self._values.values()), id(self))

    def __contains__(self, value):
        return self._fold(value) in self._values

    def __iter__(self):
        try:
            # Python 2
            return self._values.itervalues()
        except AttributeError:
            # Python 3
            return iter(self._values.values())

    def __len__(self):
        return len(self._values)

    def add(self, value):
        self._values[self._fold(value)] = value

    def discard(self, value):
        try:
            del self._values[self._fold(value)]
        except KeyError:
            pass

Usage demo:

>>> cps = CasePreservingSet(l)
>>> cps
<CasePreservingSet('#TrendinG', '#Yax') at 1047ba290>
>>> '#treNdinG' in cps
True

You can use lower() :

>>> set(i.lower() for i in l)
set(['#trending', '#yax'])

You could convert the entire list to lowercase before creating a set.

l = map(lambda s: s.lower(), l)
set(l)

Create a case-insensitive set class of your own.

class CaseInsensitiveSet(set):

    def add(self, item):
         try:
             set.add(self, item.lower())
         except Exception:                # not a string
             set.add(self, item)

    def __contains__(self, item):
        try:
            return set.__contains__(self, item.lower())
        except Exception:
            return set.__contains__(self, item)

    # and so on... other methods will need to be overridden for full functionality