Python Style Question

Roy Smith roy at panix.com
Fri Oct 31 01:34:27 CET 2014


In article <54521c8f$0$12982$c3e8da3$5496439d at news.astraweb.com>,
 Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:

> Anton wrote:
> 
> > Let's say I have an incoming list of values *l*. Every element of *l* can
> > be one of the following options: 
> > 1) an integer value 
> > 2) a string in form of '<int_value>', e.g. '7'
> > 3) a string with a json serialization of an integer value, e.g. '"7"'
> > 4) something else that should be ignored
> > 
> > I need to transform this list into another list with values from options
> > 1)-3) coerced to int. The code below should do this.
> 
> I don't particularly like either version. I prefer this:
> 
> def load_int(obj):
>     if isinstance(obj, int):
>         # Case 1), an int, e.g. 7
>         return obj
>     elif isinstance(obj, str):
>         # Case 2) and 3), a str or JSON serialised int.
>         # E.g. '7' or '"7"'.
>         try:
>             return int(obj)
>         except ValueError:
>             return int(json.loads(obj))
>     raise TypeError('require int or str, got %s' % type(obj).__name__)

Depending on how strictly you're trying to do input validation, the 
int(json.loads(obj)) may not be what you want.  It allows well-formed 
json encoding floats, for example.

And, of course, since

>>> isinstance(True, int)
True

this code accepts booleans.  Oh, but wait, that's by design :-)



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