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Re: dict.get_deep()

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From: hjp-python@hjp.at (Peter J. Holzer)
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Subject: Re: dict.get_deep()
Date: Sun, 3 Apr 2022 21:45:12 +0200
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by: Peter J. Holzer - Sun, 3 Apr 2022 19:45 UTC

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On 2022-04-03 17:58:09 +0300, Kirill Ratkin via Python-list wrote:
> 02.04.2022 23:44, Marco Sulla пишет:
> > A proposal. Very often dict are used as a deeply nested carrier of
> > data, usually decoded from JSON. Sometimes I needed to get some of
> > this data, something like this:
> >
> > data["users"][0]["address"]["street"]
> >
> > What about something like this instead?
> >
> > data.get_deep("users", 0, "address", "street")

Yup. I need something like this quite frequently, so I wrote a little
utility function (which I copy and paste into lots of code - I probably
should package that up, but a package with a single short function feels
weird).

[...]
> > data.get_deep("users", 0, "address", "street", default="second star")

Yep. Did that, too. Plus pass the final result through a function before
returning it.

I'm not sure whether I considered this when I wrote it, but a function
has the advantage of working with every class which can be indexed. A
method must be implemented on any class (so at least dict and list to be
useful).

> Recently I met same issue. A service I intergated with was documented badly
> and sent ... unpredictable jsons.
>
> And pattern matching helped me in first solution. (later I switched to
> Pydantic models)
>
> For your example I'd make match rule for key path you need. For example:
>
>
> data = {"users": [{"address": {"street": "Baker"}}]}
>
> match data:
>     case {"users": [{"address": {"street": street}}]}:
>         print(f"street: {street}")
>
>     case _:
>         print("unsupported message structure")

Neat. But that's 5 lines instead of one. I simple loop around try/except
also takes 5 lines, and the latter can be easily moved into a function,
like this:

def get_nested(coll, path, default=None, cast=None):
for i in path:
try:
coll = coll[i]
except (KeyError, IndexError, TypeError):
return default
if cast:
coll = cast(coll)
return coll

which can then be called in a single line.

> Structural matching gives you warranty you process exactly message you
> expect and explicitly discards messages with another structure.

True, and sometimes useful, I'm sure. Not sure whether it would have
helped me in the cases where I used the utility function above.

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"You can't make a program without broken egos."

devel / comp.lang.python / Re: dict.get_deep()

devel / comp.lang.python / Re: dict.get_deep()