modifying static (local) method variables

[t scytale]

i am using a mutable default paramater to simulate a static method variable.
i notice that 
- assigning to the var inside the method causes it to return to the
default value at each call.
- appending to the var gives the expected behaviour (the value is
preserved to the next call

why is this?

def GetCachedVal(val=[]):
  print 'start val:', val
  if val:
    return val
  print  'initializing'
  val = TimeConsumingSetup()           # assignment
  return val

>> GetCachedVal()
start val: []
initializing
>> print val
[<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]
>>GetCachedVal()
start val: []
initializing
>>print val
[<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]


def GetCachedVal(val=[]):
  print 'start val:', val
  if val:
    return val
  print  'initializing'
  val += TimeConsumingSetup()           # append
  return val

>> GetCachedVal()
start val: []
initializing
>> print val
[<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]
>>GetCachedVal()
start val: [<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]
>>print val
[<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]


t