Removing duplicate characters from a string

If order does not matter, you can use

"".join(set(foo))

set() will create a set of unique letters in the string, and "".join() will join the letters back to a string in arbitrary order.

If order does matter, you can use a dict instead of a set, which since Python 3.7 preserves the insertion order of the keys. (In the CPython implementation, this is already supported in Python 3.6 as an implementation detail.)

foo = "mppmt"
result = "".join(dict.fromkeys(foo))

resulting in the string "mpt". In earlier versions of Python, you can use collections.OrderedDict, which has been available starting from Python 2.7.

If order does matter, how about:

>>> foo = 'mppmt'
>>> ''.join(sorted(set(foo), key=foo.index))
'mpt'

If order is not the matter:

>>> foo='mppmt'
>>> ''.join(set(foo))
'pmt'

To keep the order:

>>> foo='mppmt'
>>> ''.join([j for i,j in enumerate(foo) if j not in foo[:i]])
'mpt'