The canonical Pythonic way of doing this is
>>> 'Kilometers'.lower()
'kilometers'
However, if the purpose is to do case insensitive matching, you should use case-folding:
>>> 'Kilometers'.casefold()
'kilometers'
Here's why:
>>> "Maße".casefold()
'masse'
>>> "Maße".lower()
'maße'
>>> "MASSE" == "Maße"
False
>>> "MASSE".lower() == "Maße".lower()
False
>>> "MASSE".casefold() == "Maße".casefold()
True