title: Limitation in output generated by a class
body: <p>Let's say that I want to create a class <strong>circle</strong> that, provided a value <strong>r</strong> and a value <strong>delta_r</strong>, can calculate the annulus area of a couple of circles.
  Afterwards, using this class, I want to create a list with several couples of circles.</p>
  <p>How can I add a condition, built in the class, that allows to append values <strong>only if r &lt; 7</strong>? So that, in the example, the list C is still created but includes only results till r=6.</p>
  <pre><code>import numpy as np
  from numpy import pi
  
  class circle:
      def __init__(self, r=1, delta_r=0.1):
          self.r = r
          self.delta_r = delta_r
  
          self.area1 = pi*r**2
          self.area2 = pi*(r+delta_r)**2
          self.annulus = self.area2 - self.area1
  
  C_range = np.arange(1, 10)
  C = []
  for n in C_range:
      C.append(circle(n, 0.2))
  </code></pre>