[BUG]

[Mehdisoley]

Title: NameError: name 'np' is not defined during circuit evaluation (missing numpy in eval namespace)

Description:

impedance versions ≥1.x produce a NameError during equivalent-circuit fitting when evaluating the circuit string.
The error occurs because np (numpy) is used in the generated circuit expression but is not included in the circuit_elements dictionary passed to eval().

Error traceback:

NameError: name 'np' is not defined
File ".../impedance/models/circuits.py", line XXX
return eval(circuit_string, circuit_elements)

Cause:

eval() is executed like this:

eval(circuit_string, circuit_elements)

But the circuit string contains np (e.g., np.power, np.sqrt).
Since np is not present in circuit_elements, the evaluation fails.

Older versions of the library did not show this problem.

Minimal reproducible example
from impedance.models.circuits import CustomCircuit

circuit = 'R0-p(CPE1,R1)'
initial_guess = [10, 1e-5, 0.9, 50]

frequencies = [...]
Z = [...]

circuit_model = CustomCircuit(circuit, initial_guess)
circuit_model.fit(frequencies, Z)

Produces:

NameError: name 'np' is not defined

Proposed fix

Modify circuits.py to inject numpy into the evaluation namespace:

import numpy as np
circuit_elements['np'] = np

This ensures circuit expressions that rely on numpy evaluate properly.

Workaround

Users can manually patch it:

from impedance.models.elements import circuit_elements
import numpy as np
circuit_elements['np'] = np

Circuit expressions should evaluate without raising NameError.