title: why cannot I use sp.signal by import scipy as sp?
body: <p>I would like to use <code>scipy.signal.lti</code> and <code>scipy.signal.impulse</code> function to calculate the transfer function. I import the <code>scipy</code> module in the following way.</p>
  
  <pre><code>import scipy as sp
  import numpy as np
  import matplotlib.pyplot as plt
  from math import *
  </code></pre>
  
  <p>However, when I type the following scripts,</p>
  
  <pre><code>tf = sp.signal.lti(numH, denH)
  </code></pre>
  
  <p>The Kernel gives an error:</p>
  
  <pre><code>---&gt; 10 tf = sp.signal.lti(numH, denH)
  AttributeError: 'module' object has no attribute 'signal' 
  </code></pre>
  
  <p>I tried another way to import the signal module,</p>
  
  <pre><code>from scipy.signal import lti, step, impulse
  </code></pre>
  
  <p>Then, the script works,</p>
  
  <pre><code>tf = lti(numH, denH)
  </code></pre>
  
  <p>So, my questions is, must we import every subpackage in the script? Then what's the point of importing the <code>scipy</code> package?</p>
  
  <p>Thanks.</p>