Create a cosine wave with 30 points as follows:

x =  np.linspace(0, 2 * np.pi, 30)
wave = np.cos(x)

Transform the cosine wave with the fft() function:

transformed = np.fft.fft(wave)

Apply the inverse transform with the ifft() function. It should approximately return the original signal. Check with the following line:

print(np.all(np.abs(np.fft.ifft(transformed) - wave) < 10 ** -9))

The result appears as follows:

True

Plot the transformed signal with matplotlib:

plt.plot(transformed)
plt.title('Transformed cosine')
plt.xlabel('Frequency')
plt.ylabel('Amplitude')
plt.grid()
plt.show()

The following resulting diagram shows the FFT result: