[Tutor] No module named player_data

[Michael bridges]

why does this happen?

server [folder]
       player_data [folder]
       server.py

Traceback (most recent call last):
  File "C:\Users\MySelf\Program\game_stuff\Game_28_02_11\Servers\server_client_begining\server.py", line 3, in <module>
    from player_data import PlayerArmy
ImportError: No module named player_data


from player_data import PlayerArmy
from player_data import PlayerBaseData
from player_data import PlayerEnabledFlags
from player_data import PlayerResearch
from player_data import PlayerResearchers
from player_data import PlayerTotalWeapons
from player_data import PlayerWeaponsEquipped
from player_data import PlayerWeaponsUnequipped

this worked before i tried:
from player_data import *
"""from player_data import PlayerArmy
from player_data import PlayerBaseData
from player_data import PlayerEnabledFlags
from player_data import PlayerResearch
from player_data import PlayerResearchers
from player_data import PlayerTotalWeapons
from player_data import PlayerWeaponsEquipped
from player_data import PlayerWeaponsUnequipped"""

got error about PlayerBaseData not found

then undid back to:
from player_data import PlayerArmy
from player_data import PlayerBaseData
from player_data import PlayerEnabledFlags
from player_data import PlayerResearch
from player_data import PlayerResearchers
from player_data import PlayerTotalWeapons
from player_data import PlayerWeaponsEquipped
from player_data import PlayerWeaponsUnequipped

now it does not work
why????????????????
using python 3 and tried with python 2.7