title: How to select rows with only positive or negative values in Pandas
body: <p>I have the following DF:</p>
  
  <pre><code>df=pd.DataFrame({
                   'x':[1, 2, 3, -1, -2, -3],
                   'y':[-1, 3, 2, -4, 3, -2],
                   'z':[1 , 1, 5, 2, 1, -1]
  </code></pre>
  
  <p>})
  or </p>
  
  <pre><code>       x  y  z
      0  1 -1  1
      1  2  3  1
      2  3  2  5
      3 -1 -4  2
      4 -2  3  1
      5 -3 -2 -1
  </code></pre>
  
  <p>The goal is to find the rows that all of their elements have the same (either negative or positive) values.
  In this example, it means selecting rows 1, 2, and 5. </p>
  
  <p>I would appreciate any help.
  I am aware of this question: <a href="https://stackoverflow.com/questions/41596652/pandas-compare-positive-negative-values">Pandas - Compare positive/negative values</a>
  but it doesn't address the case where the values are negative.</p>
  
  <hr>
  
  <p>Thank you :)</p>