On Mon, Oct 18, 2021 at 9:35 AM Paul Bryan <pbr...@anode.ca> wrote:
> NoneType is just another type, and in type checking scenarios should be
> expressed with `Optional[type]` or more preferably in the future `type |
> None`; `None` is not a non-value. Assuming what I just wrote is true, I
> don't get what the basis of this thread is; what am I missing?
>
To me the thread isn't about type checking. It is about APIs that are built
into the language that special-case None, in particular dict.get(). In
certain cases, encountered commonly in certain styles of coding (data
science? web programming?), users encounter data that is structured as
dicts nested multiple levels. This is often out of the user's control, as
the dict is returned by reading a JSON value whose structure is controlled
by some other framework (often not specific to Python).
For example, if we have a config structure like this:
config = {
"timeout": 0.1,
"handler: {
"timeout-override": 0.4,
"method-name": "plot",
"parameters": {
"x": 10,
"y": "auto",
}
}
}
where the convention is that keys at any level may be omitted altogether
and config itself may be NOne, then to safely access the value of
config["handler"]["parameters"]["y"] we would have to write
y = None # Default
if config is not None:
handler = config.get("handler")
if handler is not None:
parameters = handler.get("parameters")
if parameters is not None:
y = parameters.get("y")
This kind of pattern (and all the various other ways of writing it, e.g.
using the walrus or passing {} as the second argument to dict.get()) can be
*very* common if that's the kind of data you're given and that's the kind
of app you have to write, and you can't control the format of the data.
Using ?. this can be written as
y = config?.get("handler")?.get("parameters")?.get("y")
More examples are in PEP 505 itself, see
https://www.python.org/dev/peps/pep-0505/#examples
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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