ParamResolver is hashable but mutable

This is bad!

pdict = {'beta': 0.11, 'gamma': 0.3}
pdict
Out[11]: {'beta': 0.11, 'gamma': 0.3}
p = cirq.ParamResolver(pdict)
p
Out[13]: cirq.ParamResolver({'beta': 0.11, 'gamma': 0.3})
hash(p)
Out[14]: 9055359463938249742
pdict['beta'] = 5
p
Out[16]: cirq.ParamResolver({'beta': 5, 'gamma': 0.3})
hash(p)
Out[17]: 9055359463938249742

[95-martin-orion]

This is already documented: https://github.com/quantumlib/Cirq/blob/f1ea0bd78ce3b287ca925c2306919134650c2607/cirq-core/cirq/study/resolver.py#L53 I’m loath to introduce performance reductions to ParamResolver specifically as it’s part of the cirq-to-qsim translation hot path, which we’re actively trying to speed up.

[95-martin-orion]

Lessons from un-hashing experiment: more pain than it’s worth, likely introduces new and exciting issues.

The chosen resolution for this at the Cirq sync is outlined in #5507: we’ll make the relevant fields “private” and use properties to at least let mypy identify sketchy behavior.