On Sat, Aug 15, 2020 at 3:36 PM Manfred Lotz <ml_n...@posteo.de> wrote:
>
> I have an object which I could initialize providind an int or a str.
>
> I am not sure which of the following is best to use
> - try/except
> - if type(int)...
> - if isinstance(v, int)
>
> Here a minimal example
>
> def get_id(fromname):
> # do something with `fromname`
> return 0
>
> def get_name(fromid):
> # do something with `fromid`
> return "something"
>
> """ For O1, O2, O3: self.myid is int
> self.name is str
> """
> class O1:
> def __init__(self, val):
> try:
> self.myid = int(val)
> self.name = get_name(self.myid)
> except:
> self.myid = get_id(val)
> self.name = val
Don't use a bare "except" - use "except ValueError" instead. But
otherwise, this is a perfectly reasonable way to say "anything that
can be interpreted as an integer will be".
> class O2:
> def __init__(self, val):
> if type(val) == int:
> self.myid = val
> self.name = get_name(self.myid)
> else:
> self.myid = get_id(val)
> self.name = val
Nope, don't do this. It's strictly worse than O3.
> class O3:
> def __init__(self, val):
> if isinstance(val, int):
> self.myid = val
> self.name = get_name(self.myid)
> else:
> self.myid = get_id(val)
> self.name = val
This is a perfectly reasonable way to say "integers will be treated as
IDs". Note that O1 and O3 are very different semantically; O1 will
treat the string "7" as an ID, but O3 will treat it as a name.
Here's an even better way:
class O4:
def __init__(self, id):
self.myid = id
self.name = get_name(id)
@classmethod
def from_name(cls, name):
return cls(get_id(name))
This makes the ID the main way you'd do things, and a name lookup as
an alternate constructor. Very good pattern, reliable, easy to use.
ChrisA
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