Incorrect error message when conversion of ParameterExpression to float fails.

[jlapeyre]

Environment

• Qiskit Terra version: main branch
• Python version: 3.11
• Operating system: Linux

What is happening?

When using SymEngine, sometimes a ParameterExpression wraps a SymEngine expression that is equivalent to a float. But trying to convert the ParameterExpression to a float fails with an error claiming that there are unbound parameters, when in fact all parameters are bound.

How can we reproduce the issue?

The following prints True three times.

from qiskit import QuantumCircuit
from qiskit.circuit import Parameter, ParameterExpression

x = Parameter("x")
pex = 1j * x
pexb = pex.bind({x : 1j})
print(str(pexb) == '-1.0')
print(complex(pexb) == -1)
print(isinstance(complex(pexb), complex))

But this fails:

float(pexb)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
File ~/qiskit/qiskit-terra/qiskit/circuit/parameterexpression.py:474, in ParameterExpression.__float__(self)
    473 try:
--> 474     return float(self._symbol_expr)
    475 # TypeError is for sympy, RuntimeError for symengine

File symengine_wrapper.pyx:1151, in symengine.lib.symengine_wrapper.Basic.__float__()

File symengine_wrapper.pyx:976, in symengine.lib.symengine_wrapper.Basic.n()

File symengine_wrapper.pyx:4346, in symengine.lib.symengine_wrapper.evalf()

RuntimeError: Not Implemented

The above exception was the direct cause of the following exception:

TypeError                                 Traceback (most recent call last)
Cell In[17], line 1
----> 1 float(pexb)

File ~/qiskit/qiskit-terra/qiskit/circuit/parameterexpression.py:477, in ParameterExpression.__float__(self)
    475 # TypeError is for sympy, RuntimeError for symengine
    476 except (TypeError, RuntimeError) as exc:
--> 477     raise TypeError(
    478         "ParameterExpression with unbound parameters ({}) "
    479         "cannot be cast to a float.".format(self.parameters)
    480     ) from exc

TypeError: ParameterExpression with unbound parameters (set()) cannot be cast to a float.

The error message is misleading. The attempt made to cast to a float failed, but for a different reason.

What should happen?

Maybe

• Try to avoid calling float on the wrapped expression. Trap the attempt and print or forward the underlying reason (not implemented)
• Call complex instead, check if the result is a float. If so return it, else raise error. This is the approach in Assign values directly to fully bound parameters in quantum circuits #10183

Any suggestions?

No response

[jlapeyre]

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