add comment clarifying that `out is None` when verbose=True

IPython/nbconvert/post_processors/pdf.py

@@ -1,58 +1,60 b''
 """
 Contains writer for writing nbconvert output to PDF.
 """
 #-----------------------------------------------------------------------------
 #Copyright (c) 2013, the IPython Development Team.
 #
 #Distributed under the terms of the Modified BSD License.
 #
 #The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 import subprocess
 import os
 
 from IPython.utils.traitlets import Integer, List, Bool
 
 from .base import PostProcessorBase
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 class PDFPostProcessor(PostProcessorBase):
     """Writer designed to write to PDF files"""
 
     iteration_count = Integer(3, config=True, help="""
         How many times pdflatex will be called.
         """)
 
     command = List(["pdflatex", "{filename}"], config=True, help="""
         Shell command used to compile PDF.""")
 
     verbose = Bool(False, config=True, help="""
         Whether or not to display the output of the compile call.
         """)
 
     def call(self, input):
             """
             Consume and write Jinja output a PDF.  
             See files.py for more...
             """        
             command = [c.format(filename=input) for c in self.command]
             self.log.info("Building PDF: %s", command)
             with open(os.devnull, 'rb') as null:
                 stdout = subprocess.PIPE if not self.verbose else None
                 for index in range(self.iteration_count):
                     p = subprocess.Popen(command, stdout=stdout, stdin=null)
                     out, err = p.communicate()
                     if p.returncode:
                         if self.verbose:
+                            # verbose means I didn't capture stdout with PIPE,
+                            # so it's already been displayed and `out` is None.
                             out = u''
                         else:
                             out = out.decode('utf-8', 'replace')
                         self.log.critical(u"PDF conversion failed: %s\n%s", command, out)
                         return