Qiskit / qiskit / 18711961920

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Committed by web-flow

Commit Message

Document subclass considerations for `Target` (#15220)

* Document subclass considerations for `Target`

The documentation of `Target` previously implied that subclassing it is
ok, and we have users that have done that.  However, subclassing it is
fragile, does not affect any transpiler pass that reads from the
`Target` in Rust space, and requires the subclass to arrange for
`__new__` to be called correctly.  I re-added a `**kwargs` variadic in
`Target.__new__` as a minor convenience; subclasses that only add
keyword-only arguments (which are generally the safest things to add)
will remain functional.  Had `Target.__new__` been defined in terms of
keyword-only arguments, we could also have added an `*args` munch too,
but as it is, adding or modifying the positional arguments to a `Target`
subclass constructor _always_ requires overriding `__new__` as well to
avoid clobbering positional `description`, `num_qubits`, etc, so there's
no benefit to having `*args`.

We should be discouraging users from subclassing `Target`; even our own
subclass of the Python-space `Target` from the Rust-space `Target` is
rather fragile with how we attempt to keep data in sync between the two.
Most APIs exposing a `Target` to their users would probably be better as
constructor functions; this is supposed to be the default way of
building a `Target`.

* Reexplain constructors vs subclassing

---------

Co-authored-by: Shelly Garion <46566946+ShellyGarion@users.noreply.github.com>

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