gh-141984: Reword and reorganize Subscription (and Slicing) docs

[encukou]

This merges the Subscriptions and Slicings sections in the language reference. From the parser's point of view, slicing is a special case of subscription, and it makes sense to document it that way too.

The PR also touches Data model documentation: I've tried to move details of the runtime behaviour there, and to de-duplicate (and cross-link) as needed.
Similar for the slice attribute docs.

Todo:

• Add a glossary entry for subscript -- the term this uses consistently for the key/index/whatever in the square brackets.

• Issue: Reword the Expressions chapter of the docs #141984

---

📚 Documentation preview 📚: https://cpython-previews--141985.org.readthedocs.build/

[StanFromIreland]

I find using examples that require bits from several examples ago and are in a different section a tad odd. We are also missing colons.

[StanFromIreland]

Same issue as above.

[StanFromIreland]

Maybe add an example of a negative index here (the more the merrier ;-)?

[encukou]

And deletion perhaps? :-3

Suggested change

	['zero', 'two', 'four']
	['zero', 'two', 'four']
	>>> number_names[:-3]
	['zero', 'one', 'two']
	>>> del number_names[4:]
	>>> number_names
	['zero', 'one', 'two', 'three']

[StanFromIreland]

And deletion perhaps? :-3

My previous comment still applies :-)

[StanFromIreland]

Suggested change

	Subscriptions and slicing
	-------------------------
	Subscription and slicing
	------------------------

[encukou]

All the other section names are plural¹. What do you think about continuing that?

Suggested change

	Subscriptions and slicing
	-------------------------
	Subscriptions and slicings
	--------------------------

¹ except about two, let's fix them when their time comes?

[StanFromIreland]

To me, this sounds like a section about subscripting tuples specifically. Maybe "Comma-separated subscription" instead?

[encukou]

Suggested change

	Tuple subscription
	^^^^^^^^^^^^^^^^^^
	Comma-separated subscripts
	^^^^^^^^^^^^^^^^^^^^^^^^^^

[encukou]

Thank you for going through this!

[encukou]

All the other section names are plural¹. What do you think about continuing that?

Suggested change

	Subscriptions and slicing
	-------------------------
	Subscriptions and slicings
	--------------------------

¹ except about two, let's fix them when their time comes?

[encukou]

And deletion perhaps? :-3

Suggested change

	['zero', 'two', 'four']
	['zero', 'two', 'four']
	>>> number_names[:-3]
	['zero', 'one', 'two']
	>>> del number_names[4:]
	>>> number_names
	['zero', 'one', 'two', 'three']

[encukou]

Suggested change

	Tuple subscription
	^^^^^^^^^^^^^^^^^^
	Comma-separated subscripts
	^^^^^^^^^^^^^^^^^^^^^^^^^^

[encukou]

I find using examples that require bits from several examples ago and are in a different section a tad odd.

True, but the SubscriptionDemo is a bit too big to define every time it's used. Would the following work? (added whenever demo is first used in a section).

Suggested change

	>>> demo[1, 2, 3]
	subscripted with (1, 2, 3)
	>>> demo[1:2, 3]
	subscripted with (slice(1, 2, None), 3)
	# continuing with the SubscriptionDemo instance defined above:
	>>> demo[1, 2, 3]
	subscripted with (1, 2, 3)
	>>> demo[1:2, 3]
	subscripted with (slice(1, 2, None), 3)

We are also missing colons.

What do you mean?

[StanFromIreland]

What do you mean?

Like in my above comments, the demo actually prints subscripted with: and not subscripted with .

[StanFromIreland]

True, but the SubscriptionDemo is a bit too big to define every time it's used. Would the following work? (added whenever demo is first used in a section).

I agree, this is also a bit like the issue with imports. The comment is an improvement, although still not ideal.