CamelCase Conversions

[perfringo]

Just playing with groupby:

from itertools import groupby

def pepify(_str):
    grouped = groupby(_str, lambda char: char.islower())
    keep = lambda seq: ''.join(seq)
    change = lambda seq: ''.join(['_'] + [*seq]).lower()
    replaced = (change(seq) if case is False else keep(seq) for case, seq in grouped)
    return ''.join(replaced).lstrip('_')

pepify('MyFunctionName') --> 'my_function_name'

EDIT:

PEP8 - Programming recommendations

Always use a def statement instead of an assignment statement that binds a lambda expression directly to an identifier

So in conformance with PEP8 it should be expressed:

from itertools import groupby

def pepify(_str):
    grouped = groupby(_str, lambda char: char.islower())
    def keep(seq):  return ''.join(seq)
    def change(seq): return ''.join(['_'] + [*seq]).lower()
    replaced = (change(seq) if case is False else keep(seq) for case, seq in grouped)
    return ''.join(replaced).lstrip('_')