seed_simulator parameter accepts inconsistent and undocumented input types

[vili-1]

Informations

• Qiskit Aer version: 0.15.1
• Python version: 3.12.6
• Operating system: 24.1.0 Darwin Kernel Version 24.1.0

What is the current behavior?

The seed_simulator parameter in Qiskit’s Aer simulator backend accepts various input types (e.g., integers, floats, strings) with undocumented and inconsistent behaviour. This could lead to unexpected results or user confusion.

Steps to reproduce the problem

from qiskit import QuantumCircuit, Aer, transpile

qc = QuantumCircuit(2, 2)

qc.u(np.pi, 1, 1, 0)  # U gate with angle np.pi
qc.rx(np.pi**50, 0)    # RX gate with a large angle
qc.ry(np.pi**50, 0)    # RY gate with a large angle
qc.u(np.random.rand() * 2 * np.pi, np.random.rand() * 2 * np.pi, np.random.rand() * 2 * np.pi, 0)
qc.u(np.random.rand() * 2 * np.pi, np.random.rand() * 2 * np.pi, np.random.rand() * 2 * np.pi, 1)
qc.cx(0, 1)

qc.measure_all()

simulator = Aer.get_backend("statevector_simulator")
transpiled_qc = transpile(qc, simulator)

 , 

# Test seed_simulator with various types
seeds = ["42", "-42", "42.3", "-42.3", -42, 42.4, 42, 2**63-1, None, 0, 0.0, -0.0, 1.0, -1.0, 1e3, -1e3, "\t42\n", True, b"42", None, "abc", "4ab", 1 + 2j, -3j, "inf", "nan", "-inf", (x for x in range(42)), "9" * 1000, object(), {"seed": 42}, [42], (42,), {42}]
for seed in seeds:
    try:
        job = simulator.run(transpiled_qc, seed_simulator=seed)
        print(f"Seed {seed} accepted.")
    except Exception as e:
        print(f"Seed {seed} rejected with error: {e}")

Characteristic Observed Results

• Integer (42, -42): Accepted ✅
• String ("42", "-42"): Accepted (probably, implicitly converted to integer) ✅
• Float (42.4): Accepted (probably, implicitly truncated to integer) ✅
• None: Accepted (likely defaulting to internal seeding) ✅
• True: Accepted - Likely to convert to 1 and 0 (implicit integer conversion) ✅
• "\t42\n": Accepted (likely stripped and converted to 42) ✅
• (42): Accepted ✅
• b"42": Accepted ✅
• [42], {42}, {"seed": 42}, object(): Rejected with TypeError ❌
• Invalid strings ("abc", "4ab", "42.3", "-42.3"): Rejected with TypeError ❌
• "9" * 1000: Rejected (likely attempting to interpret the excessively large value of "9" * 1000 as an integer. The resulting integer exceeds the maximum size supported by the underlying implementation, likely causing the function to reject it outright with a TypeError) ❌

What is the expected behavior?

• The parameter should only accept integers or None, and strict type checking should enforce this.
• If implicit conversions are intended, this behaviour must be documented.

Suggested solutions

1. Enforce Type Checking: Ensure only valid input types (e.g., int, None, bool, string representations of integers) are accepted, with clear error messages for invalid inputs.
2. Clarify Documentation: Update documentation to clearly specify acceptable input types for seed_simulator.
3. Prevent Implicit Conversion: Avoid automatic conversions (e.g., from float or string to integer) and provide explicit error messages when needed.