"D'Arcy Cain" wrote in message
news:6b4b8587-46c0-19b0-c538-efdf396f0...@vybenetworks.com...
On 2018-08-14 04:58 AM, Frank Millman wrote:
> As an example, I have a master class defining a unit of data (i.e. the
> value of a column) retrieved from a database. I have separate
> sub-classes for each data type - text, integer, date, etc. To ensure
> that a value is valid before storing it as an instance attribute, I call
> a method called 'check_value'. The details of check_value vary according
> to the data type, but that is transparent to the piece of code that
> calls check_value().
class classA:
DATATYPE = None # Or default type
def check_value(self, v)
if not isinstance(v, self.DATATYPE):
raise RuntimeError("Invalid data type for '%s'" % v)
class classB(classA):
DATATYPE = int
Very simplistic and untested but does that give you any ideas?
Hopefully your email client doesn't mess up the formatting. You can
fill out check_value to do more than simply check the the type matches
and you can also do further checks based on the type. Also, you can
have more than one sub-class doing the same check without having to cut
and paste code from another class.
Thanks, D'Arcy. That is a neat idea if all you want to do is check the data
type, but I do a lot more than that.
How would you extend it without a long chain of
if isinstance(v, str):
[perform checks for str]
elif isinstance(v, int)
[perform checks for int]
etc
etc
I find that using a separate method per subclass does exactly what I want,
and that part of my project has been working stably for some time.
The only thing that has changed is that I recently started using pylint (as
a result of switching my editor to VS Code).
My main class has the following method (simplified) -
class Field:
def setval(self, value): # handle value received from external source
"""
checkval is a method defined in each subclass
it does a bit of typecasting, so value is replaced on return
it can raise an exception, which is caught elsewhere
"""
value = self.checkval(value) # this is the line that pylint
complains about
[various other checks]
if all checks are passed:
self._value = value
Frank
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