<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def GetTypeChecker(field):
 """Returns a type checker for a message field of the specified types. Args: field: FieldDescriptor object for this field. Returns: An instance of TypeChecker which can be used to verify the types of values assigned to a field of the specified type. """

  if (field.cpp_type == _FieldDescriptor.CPPTYPE_STRING and
      field.type == _FieldDescriptor.TYPE_STRING):
    return UnicodeValueChecker()
  if field.cpp_type == _FieldDescriptor.CPPTYPE_ENUM:
    if SupportsOpenEnums(field):
      # When open enums are supported, any int32 can be assigned.
      return _VALUE_CHECKERS[_FieldDescriptor.CPPTYPE_INT32]
    else:
      return EnumValueChecker(field.enum_type)
  return _VALUE_CHECKERS[field.cpp_type]