Well, you can get the FuzzyText objects value by calling like this:

>> f = factory.fuzzy.FuzzyText(length=15)
>> f.fuzz()

But, in this text, the characters can be both uppercase or lower case. So if you want exclusive lower case characters, then you can override the FuzzyText like this:

from factory.fuzzy import FuzzyText

class CustomFuzzyText(FuzzyText):
    def fuzz(self, *args, **kwargs):
       return super(CustomFuzzyText, self).fuzz(*args, **kwargs).lower()

and use it in the factory like this:

underscore_15 = CustomFuzzyText(prefix="aa_", length=12)  # or CustomFuzzyText(length=15)