Simplify return type of DAGCircuit::control_flow_op_nodes
#13892
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Technically this is a breaking change, not because of the type change (we're returning a subset of what we previously did), but because the docstring previously _explicitly_ said that `None` was used for the empty list. In practice, this is a bit of a footgun (as I found), and inconsistent with the other `DAGCircuit` methods. It's not clear that it has a performance advantage; technically it can save a single Python empty-list allocation (`None` is a singleton), but this cost is pretty trivial.
jakelishman
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Changelog: API Change
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Technically this is a breaking change, not because of the type change (we're returning a subset of what we previously did), but because the docstring previously explicitly said that
Nonewas used for the empty list. In practice, this is a bit of a footgun (as I found), and inconsistent with the otherDAGCircuitmethods. It's not clear that it has a performance advantage; technically it can save a single Python empty-list allocation (Noneis a singleton), but this cost is pretty trivial.Summary
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I'll not be distraught if we choose not to do this because it's technically a breaking change, but I can say from experience that I managed to write a buggy transpiler pass because of the unexpected behaviour here, and
if not op_nodesis naturally supported on both 1.x and 2.x.