why cannot I use sp.signal by import scipy as sp? [duplicate]

Question

I would like to use scipy.signal.lti and scipy.signal.impulse function to calculate the transfer function. I import the scipy module in the following way.

import scipy as sp
import numpy as np
import matplotlib.pyplot as plt
from math import *

However, when I type the following scripts,

tf = sp.signal.lti(numH, denH)

The Kernel gives an error:

---> 10 tf = sp.signal.lti(numH, denH)
AttributeError: 'module' object has no attribute 'signal' 

I tried another way to import the signal module,

from scipy.signal import lti, step, impulse

Then, the script works,

tf = lti(numH, denH)

So, my questions is, must we import every subpackage in the script? Then what's the point of importing the scipy package?

Thanks.

Answer 1

From the scipy doc:

Using any of these subpackages requires an explicit import. For example, import scipy.cluster.

or from scipy import cluster.

There isn't much point to doing a simple

import scipy

Look at the site-packages/scipy/__init__.py file for more details. Compare it with the numpy init.

numpy is an integrated package, scipy is a collection of loosely integrated packages. numpy is the basic numeric package that everyone uses. The scipy subpackages are relatively independent of each other. I can load and use sparse without knowning anything about the signal or integrate packages.