Environment

• Qiskit version: any that has SparsePauliOp.from_sparse_list
• Python version: 3.11
• Operating system: macOS

What is happening?

From @BryceFuller:

Another question, what is the proper way to instantiate a SparsePauliOp with parameterized coefficients?
I'm running into a curious snag where I have an already parameterized operator, which I then want to apply transformations to.
I'm exporting to a sparse list, making some changes, then trying to re-instantiate the operator, but I observe an error with the parameterized coefficients.

How can we reproduce the issue?

Here's a MWE

param = Parameter('a')

# In my situation, this parameterized operator is given to me after being 
# made through composition of smaller parameterized operators 
op1 = SparsePauliOp.from_sparse_list([('IX', [0,1], 2),('ZI', [0,1], 3)], num_qubits = 2)
op1 *= param

# This breaks because there are parameter expressions in the coeffs
op2 = SparsePauliOp.from_sparse_list(op1.to_sparse_list(), num_qubits=2)

What should happen?

Like the SparsePauliOp initializer, we should determine the dtype from the input coefficients. The dtype can already be set manually, via

op2 = SparsePauliOp.from_sparse_list(op1.to_sparse_list(), num_qubits=2, dtype=object)

but it would be nice to automate this, given that the class initializer does the same thing.

Any suggestions?

No response