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
start val: []
initializingstart val: []
initializing[<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
start val: []
initializing[<eDatabase instance at 0x87>, <eHandle instance at 0x8755e>]


t

 

[Steve Holden]

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?

Because an assignment to an argument inside the method merely rebinds
the local variable for the remainder of that execution. Mutating the
referenced object ensures that the changed value is available to further
calls.

regards
Steve