Confusion regarding constructor as default value

[aine_canby]

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.

 

[Diez B. Roggisch]

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,

It's a FAQ.

http://effbot.org/pyfaq/why-are-default-values-shared-between-objects.htm

Diez

 

[aine_canby]

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.

How would you implement this?

Thanks,

Aine.

 

[aine_canby]

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,

It's a FAQ.

http://effbot.org/pyfaq/why-are-default-values-shared-between-objects...

Diez- Dölj citerad text -

- Visa citerad text -

Cool, I understand now. It was my understanding of how Python works
that was at fault.

 

[Steve Holden]

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
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden

Sorry, the dog ate my .sigline