Cast CamelCase to "underscore_case"

main_re.py

import re
import json

def add_underscore(letter):
  return "_"+letter.group(0).lower()

def to_lower(letter):
  return letter.group(0).lower()

def cast_to_underscore_case(value):
  if isinstance(value, str):
    return re.sub(r'([A-Z])', add_underscore, value)
  
  elif isinstance(value, dict):
    _values = {}
    for (k, v) in value.items():
      _k = re.sub(r'([A-Z]){2,}', to_lower, k)
      _k = re.sub(r'\A([A-Z])', to_lower, _k)
      _k = re.sub(r'(?<=.)([A-Z])[a-z]', add_underscore, _k)
      _k = re.sub(r'(?<=.)([A-Z]$)', add_underscore, _k)
      _values[_k] = v
    return _values
  
  else:
    raise TypeError("Unexpected type {}".format(type(value)))

if __name__ == "__main__":
  values = [
    {"IsCustomer": True, "OBID": 12345}, 
    {"isCustomer": True, "obid": 12345},
    {"iscustomer": True, "obid": 12345},
    {"is_customer": True, "obid": 12345},
    {"isCustomer": True, "oBiD": 12345},
    {"isCustomer": True, "Obid": 12345}
  ]

  for val in values:
    print "-------------\n"
    print val
    print "->" 
    print cast_to_underscore_case(val)

Result:

-------------

{'IsCustomer': True, 'OBID': 12345}
->
{'is_customer': True, 'obid': 12345}
-------------

{'isCustomer': True, 'obid': 12345}
->
{'is_customer': True, 'obid': 12345}
-------------

{'iscustomer': True, 'obid': 12345}
->
{'iscustomer': True, 'obid': 12345}
-------------

{'is_customer': True, 'obid': 12345}
->
{'is_customer': True, 'obid': 12345}
-------------

{'isCustomer': True, 'oBiD': 12345}
->
{'is_customer': True, 'o_bi_d': 12345}
-------------

{'isCustomer': True, 'Obid': 12345}
->
{'is_customer': True, 'obid': 12345}