Converts a string to snake case.

Break the string into words and combine them adding _ as a separator, using a regexp.

from re import sub

def snake(s):
  return '_'.join(
    sub('([A-Z][a-z]+)', r' \1',
    sub('([A-Z]+)', r' \1',
    s.replace('-', ' '))).split()).lower()

snake('camelCase') # 'camel_case'
snake('some text') # 'some_text'
snake('some-mixed_string With spaces_underscores-and-hyphens') # 'some_mixed_string_with_spaces_underscores_and_hyphens'
snake('AllThe-small Things') # "all_the_small_things"