Confusion regarding constructor as default value

[Steve Holden]

aine_canby at yahoo.com wrote:
> On 27 Sep, 15:03, aine_ca... at yahoo.com wrote:
>> Why are the following different?
>>
>> def AddRow(self, rowName, tableRow=TableRow(ReleaseDate(""),
>> ExpiryDate(""))):
>>                 # check to see if the row already exists, if not add it to the
>> container
>>
>>                 if not self.dict.has_key(rowName):
>>                         self.dict[rowName] = tableRow
>>
>> def AddRow(self, rowName, tableRow):
>>                 # check to see if the row already exists, if not add it to the
>> container
>>
>>                 if not self.dict.has_key(rowName):
>>                         self.dict[rowName] = TableRow(ReleaseDate(""), ExpiryDate(""))
>>
>> It seems that when I use the first function that I'm getting duplicate
>> objects in self.dict
>>
>> Thanks for your help,
>>
>> Aine.
> 
> I've just tried the following:
> 
> Function A:
> 
> def AddRow(self, rowName, tableRow=TableRow(ReleaseDate(""),
> ExpiryDate(""))):
> 
>     print tableRow
>     if not self.dict.has_key(rowName):
>         self.dict[rowName] = tableRow
> 
> Function B:
> 
> def AddRow(self, rowName, tableRow=None):
> 
>     if tableRow==None:
>     	tableRow = TableRow(ReleaseDate(""), ExpiryDate(""))
>     # check to see if the row already exists, if not add it to the
> container
>     print tableRow
>     if not self.dict.has_key(rowName):
>         self.dict[rowName] = tableRow
> 
> Function A is giving:
> 
> <DatabaseExamination.TableRow instance at 0x011D4468>
> <DatabaseExamination.TableRow instance at 0x011D42B0> same!
> <DatabaseExamination.TableRow instance at 0x011D42B0> same!
> <DatabaseExamination.TableRow instance at 0x011D42B0> same!
> <DatabaseExamination.TableRow instance at 0x011D42B0> same!
> 
> Function B is giving:
> 
> <DatabaseExamination.TableRow instance at 0x011D0670>
> <DatabaseExamination.TableRow instance at 0x011D0760>
> <DatabaseExamination.TableRow instance at 0x011D07D8>
> <DatabaseExamination.TableRow instance at 0x011D0850>
> 
> 
> So at least I know know what is causing the problem. But I'm still not
> understanding why I can't use TableRow() as the default value in order
> to geterate new instances.
> 
Because the value of the default is computed as the funciton declaration 
is processed, whereas you seem to expect to see a new call to ExpiryDate 
each time the function is called.

> How would you implement this?
> 
> Thanks,
> 
> Aine.
> 
Function B is the canonical solution to this problem.

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
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