Z gate implementation using Qiskit. The Z gate applied to $\lvert 0\rangle$ is invisible at measurement (it leaves $\lvert 0\rangle$ unchanged), so this example prepares $\lvert +\rangle$ first to make the phase flip observable via interference.

from qiskit import QuantumCircuit
from qiskit.quantum_info import Statevector
 
qc = QuantumCircuit(1)
qc.h(0)  # |0> -> |+>
qc.z(0)  # |+> -> |->
print(Statevector(qc))
# Statevector([ 0.70710678+0.j, -0.70710678+0.j], dims=(2,))