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Fix sympy error #5930

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Merged
merged 9 commits into from
Oct 30, 2022
Merged

Fix sympy error #5930

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eb59ca2
Fix sympy error
dstrain115 Oct 25, 2022
7bff7a0
Fix coverage.
dstrain115 Oct 25, 2022
28e3036
more coverage ignore.
dstrain115 Oct 25, 2022
be327af
Change to raise error.
dstrain115 Oct 26, 2022
966d380
Fix lint.
dstrain115 Oct 26, 2022
e4874d3
Move raises to correct place
dstrain115 Oct 26, 2022
d6d2d3b
Address review comments.
dstrain115 Oct 28, 2022
205e72b
Fix docs
dstrain115 Oct 28, 2022
592b4e1
Merge branch 'master' into fix_future_sympy_issue
dstrain115 Oct 30, 2022
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7 changes: 6 additions & 1 deletion cirq-core/cirq/study/resolver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@

"""Resolves ParameterValues to assigned values."""
import numbers
import warnings
from typing import Any, cast, Dict, Iterator, Mapping, Optional, TYPE_CHECKING, Union

import numpy as np
Expand Down Expand Up @@ -179,7 +180,11 @@ def value_of(
if not recursive:
# Resolves one step at a time. For example:
# a.subs({a: b, b: c}) == b
v = value.subs(self.param_dict, simultaneous=True)
try:
v = value.subs(self.param_dict, simultaneous=True)
except sympy.SympifyError:
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What do you mean by "not parsable"? An actual syntax error? If so, then this is probably a user mistake and it's better to let the exception propagate out of here. Then the PR could catch it in resolver_test.py rather than in resolver.py. WDYT?

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The actual sympy bug is here:
sympy/sympy#23887

Their definition is "sympifiable" whatever that means. I think that it is either a sympy object or a basic python type, possibly a few other cases. I think it is talking about objects foreign to the sympy library. In our case, that is the symbol 'NotImplemented'.

I am hesitant to change the existing behavior. The test currently asserts that resolving {'b': NotImplemented} would just leave sympy.Symbol('b') alone. I am loathe to change this behavior right now, since there is a lot of special handling around NotImplemented.

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I agree we should preserve existing behavior.

I have two conerns:

  • the fact that if one of the values is unsympifiable then none of the substitutions are made,
  • the fact that we're failing to communicate a potential mistake to the user as soon as we discover it (we could be passing an unsympifiable object as a result of a simple typo in the initialization of the ParamDict).

Is NotImplemented the only offender? If so, could we just filter them out by saying something like

v = value.subs({k: w for k, w in self.param_dict.values() if w is not NotImplemented}, simultaneous=True)

?

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I changed this to raise a ValueError instead, explaining what the problem substitution is.

We cannot do your suggestion, because the test case is a function that defines resolved_value as NotImplemented. This is indistinguishable from something like Sympy.Symbol('b')+Sympy.Symbol('c') which also has an unimplemented resolved_value.

This does change the behavior of the function slightly, but, as you pointed out, the previous PR would also change the value as some additional variables would not be substituted.

I think this is the best solution we can do, since this preserves the behavior 'the variable could not be substituted' but makes it explicit as a ValueError rather than silently refusing to substitute it. It's also quite a buried case, because it involves the user specifying something that was unresolvable in the first place.

warnings.warn(f'Could not resolve parameter {value}')
return value
if v.free_symbols:
return v
elif sympy.im(v):
Expand Down