On 2018-09-07 20:51, Michael F. Stemper wrote:
On 2018-09-06 16:00, MRAB wrote:
On 2018-09-06 21:24, Michael F. Stemper wrote:
On 2018-09-06 09:35, Rhodri James wrote:
Is it worth creating the superclass in Python? It sounds like it's a
bit marginal in your case. I'm not that seasoned in object-oriented
design either, but my yardstick would be how much common code does it
eliminate?
About half a dozen lines. Here's the common part:
def __init__( self, xmlmodel, V, id ):
try:
P_0s = xmlmodel.findall( 'RatedPower' )[0].text
self.P_0 = float( P_0s )
except:
Utility.DataErr( "Power not specified for %s load" % (id) )
if self.P_0<=0.0:
Utility.DataErr( "Power for %s load must be postive, not %s" \
% (id,P_0s) )
A word of advice: don't use a "bare" except, i.e. one that doesn't
specify what exception(s) it should catch.
Given that I moved the first line ("P_0s = ...") out of the "try"
clause, does this align with your advice?
# If pre-validation has been run, guaranteed to have RatedPower
P_0s = xmlmodel.findall( 'RatedPower' )[0].text
try:
self.P_0 = float( P_0s )
except ValueError:
Utility.DataErr( "Power for %s load, '%s', not parsable" \
% (id,P_0s) )
if self.P_0<=0.0:
Utility.DataErr( "Power for %s load must be postive, not %s" \
% (id,P_0s) )
That's better.
However, if P_0s can't be parse as a float, then self.P_0 will be
unchanged (assuming that it already has a value, of course).
Is it OK that if there's no rated power, or it's invalid, it'll report
it and continue with whatever value, if any, that it already has?
Your try...except above will catch _any_ exception. If you misspelled a
name in the 'try' part, it would raise NameError, which would also be
caught.
In another case where I had a "bare exception", I was using it to see if
something was defined and substitute a default value if it wasn't. Have
I cleaned this up properly?
try
id = xmlmodel.attrib['name']
except KeyError:
id = "constant power"
(Both changes appear to meet my intent, I'm more wondering about how
pythonic they are.)
There's an alternative that's recommended when the key is often absent:
id = xmlmodel.attrib.get('name', "constant power")
Catching an exception has a cost, so the usual advice is to use .get
when the key is often absent, or catch KeyError when the key is usually
present but occasionally is absent and you can provide a default value.
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