qiskit/releasenotes/notes/0.24/circuit_metadata_always_dic...

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upgrade:
- |
The :class:`.QuantumCircuit` :attr:`.QuantumCircuit.metadata` attribute now
always returns a dictionary, and can only be set to a dictionary. Previously,
its default value was ``None``, and could be manually set to ``None`` or a
dictionary.
- |
The default value of ``metadata`` in both :class:`.DAGCircuit` and
:class:`.DAGDependency` has been changed from ``None`` to ``{}`` for compatibility
with a similar attribute of :class:`.QuantumCircuit`.
deprecations:
- |
Setting the :class:`.QuantumCircuit` :attr:`.QuantumCircuit.metadata` attribute
to ``None`` has been deprecated. Instead, users should set it to an empty
dictionary if they want it to contain no data.