Case insensitive 'in'

username = 'MICHAEL89'
if username.upper() in (name.upper() for name in USERNAMES):
    ...

Alternatively:

if username.upper() in map(str.upper, USERNAMES):
    ...

Or, yes, you can make a custom method.

str.casefold is recommended for case-insensitive string matching. @nmichaels's solution can trivially be adapted.

Use either:

if 'MICHAEL89'.casefold() in (name.casefold() for name in USERNAMES):

Or:

if 'MICHAEL89'.casefold() in map(str.casefold, USERNAMES):

As per the docs:

Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter 'ß' is equivalent to "ss". Since it is already lowercase, lower() would do nothing to 'ß'; casefold() converts it to "ss".

I would make a wrapper so you can be non-invasive. Minimally, for example...:

class CaseInsensitively(object):
    def __init__(self, s):
        self.__s = s.lower()
    def __hash__(self):
        return hash(self.__s)
    def __eq__(self, other):
        # ensure proper comparison between instances of this class
        try:
           other = other.__s
        except (TypeError, AttributeError):
          try:
             other = other.lower()
          except:
             pass
        return self.__s == other

Now, if CaseInsensitively('MICHAEL89') in whatever: should behave as required (whether the right-hand side is a list, dict, or set). (It may require more effort to achieve similar results for string inclusion, avoid warnings in some cases involving unicode, etc).

Usually (in oop at least) you shape your object to behave the way you want. name in USERNAMES is not case insensitive, so USERNAMES needs to change:

class NameList(object):
    def __init__(self, names):
        self.names = names

    def __contains__(self, name): # implements `in`
        return name.lower() in (n.lower() for n in self.names)

    def add(self, name):
        self.names.append(name)

# now this works
usernames = NameList(USERNAMES)
print someone in usernames

The great thing about this is that it opens the path for many improvements, without having to change any code outside the class. For example, you could change the self.names to a set for faster lookups, or compute the (n.lower() for n in self.names) only once and store it on the class and so on ...

Case insensitive 'in'

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