Case insensitive 'in' - Python

I love using the expression

if 'MICHAEL89' in USERNAMES:
    ...

where USERNAMES is a list

Is there any way to match items with case insensitivity or do I need to use a custom method? Just wondering if there is need to write extra code for this.

Thanks to everyone!

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

Alternatively:

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

Or, yes, you can make a custom method.

Alex Martelli

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 ...

Manoj Govindan

I think you have to write some extra code. For example:

if 'MICHAEL89' in map(lambda name: name.upper(), USERNAMES):
   ...

In this case we are forming a new list with all entries in USERNAMES converted to upper case and then comparing against this new list.

Update

As @viraptor says, it is even better to use a generator instead of map. See @Nathon's answer.

Here's one way:

if string1.lower() in string2.lower(): 
    ...

For this to work, both string1 and string2 objects must be of type string.

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".

You could do

matcher = re.compile('MICHAEL89', re.IGNORECASE)
filter(matcher.match, USERNAMES)

Update: played around a bit and am thinking you could get a better short-circuit type approach using

matcher = re.compile('MICHAEL89', re.IGNORECASE)
if any( ifilter( matcher.match, USERNAMES ) ):
    #your code here

The ifilter function is from itertools, one of my favorite modules within Python. It's faster than a generator but only creates the next item of the list when called upon.

My 5 (wrong) cents