title: TypeError: must be string without null bytes, not str
body: <p>I'm trying to run this code, to run the same command (with little changes) with every frame that I have:</p>
  
  <pre><code>traj.reset()
  import os
  #os.chdir(outname)
  for i, frame in enumerate(traj):
      frame.superpose()
      comando = "python hollow.py -c constraint -o hollow_%s.pdb urei%s.pdb" % (i, i)
      os.system(comando)
      pml_cmd = "pymol urei%s.pdb hollow_%s.pdb -c -d 'as cartoon, urei%s;color gray90, urei%s;center chain A;set_view (\-0.605158150,0.089404292,0.791067421,\0.795849979,0.093013920,0.598304033,\-0.020089993,0.991641700,-0.127439827,\0.000000000,0.000000000,-202.017959595,\-28.771762848,-7.683309555,10.745590210,\-568.485290527,972.520690918,-20.000000000);bg white;as sphere, hollow_%s;color cyan, hollow_%s;ray;save urei%s.png' " % (i, i, i, i, i, i, i)
      os.system(pml_cmd)
      #remove = "rm urei%s.pdb hollow_%s.pdb" % (i, i)
      #os.system(remove)
  os.chdir("../")
  </code></pre>
  
  <p>I run this and I get this error:</p>
  
  <hr>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-8-53cd3e7bd107&gt; in &lt;module&gt;()
        7     os.system(comando)
            8     pml_cmd = "pymol urei%s.pdb hollow_%s.pdb -c -d 'as cartoon, urei%s;color gray90, urei%s;center chain A;set_view (\-0.605158150,0.089404292,0.791067421,\0.795849979,0.093013920,0.598304033,\-0.020089993,0.991641700,-0.127439827,\0.000000000,0.000000000,-202.017959595,\-28.771762848,-7.683309555,10.745590210,\-568.485290527,972.520690918,-20.000000000);bg white;as sphere, hollow_%s;color cyan, hollow_%s;ray;save urei%s.png' " % (i, i, i, i, i, i, i)
  ----&gt; 9     os.system(pml_cmd)
       10     #remove = "rm urei%s.pdb hollow_%s.pdb" % (i, i)
       11     #os.system(remove)
  
  TypeError: must be string without null bytes, not str
  </code></pre>
  
  <p>I searched in internet, but I can't find a good answer.</p>