Clearer/simpler code suggested by @fperez.

https://github.com/ipython/nbconvert/pull/43#discussion_r2114903

converters/base.py

@@ -1,357 +1,355 b''
 from __future__ import print_function, absolute_import
 from converters.utils import remove_fake_files_url
 
 # Stdlib
 import codecs
 import io
 import logging
 import os
 import pprint
 import re
 from types import FunctionType
 
 # From IPython
 from IPython.nbformat import current as nbformat
 
 # local
 
 def clean_filename(filename):
     """
     Remove non-alphanumeric characters from filenames.
 
     Parameters
     ----------
     filename : str
         The filename to be sanitized.
 
     Returns
     -------
     clean : str
         A sanitized filename that contains only alphanumeric
         characters and underscores.
     """
     filename = re.sub(r'[^a-zA-Z0-9_]', '_', filename)
     return filename
 
 #-----------------------------------------------------------------------------
 # Class declarations
 #-----------------------------------------------------------------------------
 
 class ConversionException(Exception):
     pass
 
 class DocStringInheritor(type):
     """
     This metaclass will walk the list of bases until the desired
     superclass method is found AND if that method has a docstring and only
     THEN does it attach the superdocstring to the derived class method.
 
     Please use carefully, I just did the metaclass thing by following
     Michael Foord's Metaclass tutorial
     (http://www.voidspace.org.uk/python/articles/metaclasses.shtml), I may
     have missed a step or two.
 
     source:
     http://groups.google.com/group/comp.lang.python/msg/26f7b4fcb4d66c95
     by Paul McGuire
     """
     def __new__(meta, classname, bases, classDict):
         newClassDict = {}
         for attributeName, attribute in classDict.items():
             if type(attribute) == FunctionType:
                 # look through bases for matching function by name
                 for baseclass in bases:
                     if hasattr(baseclass, attributeName):
                         basefn = getattr(baseclass, attributeName)
                         if basefn.__doc__:
                             attribute.__doc__ = basefn.__doc__
                             break
             newClassDict[attributeName] = attribute
         return type.__new__(meta, classname, bases, newClassDict)
 
 class Converter(object):
     __metaclass__ = DocStringInheritor
     default_encoding = 'utf-8'
     extension = str()
     figures_counter = 0
     infile = str()
     infile_dir = str()
     infile_root = str()
     files_dir = str()
     with_preamble = True
     user_preamble = None
     output = unicode()
     raw_as_verbatim = False
     # Which display data format is best? Subclasses can override if
     # they have specific requirements.
     display_data_priority = ['pdf', 'svg', 'png', 'jpg', 'text']
 
     def __init__(self, infile):
         self.infile = infile
         self.infile_dir, infile_root = os.path.split(infile)
         infile_root = os.path.splitext(infile_root)[0]
         self.clean_name = clean_filename(infile_root)
         files_dir = os.path.join(self.infile_dir, self.clean_name + '_files')
         if not os.path.isdir(files_dir):
             os.mkdir(files_dir)
         self.infile_root = infile_root
         self.files_dir = files_dir
         self.outbase = os.path.join(self.infile_dir, infile_root)
 
     def __del__(self):
         if os.path.isdir(self.files_dir) and not os.listdir(self.files_dir):
             os.rmdir(self.files_dir)
 
     def dispatch(self, cell_type):
         """return cell_type dependent render method,  for example render_code
         """
         return getattr(self, 'render_' + cell_type, self.render_unknown)
 
     def dispatch_display_format(self, format):
         """return output_type dependent render method,  for example render_output_text
         """
         return getattr(self, 'render_display_format_' + format, self.render_unknown_display)
 
     def convert(self, cell_separator='\n'):
         """
         Generic method to converts notebook to a string representation.
 
         This is accomplished by dispatching on the cell_type, so subclasses of
         Convereter class do not need to re-implement this method, but just
         need implementation for the methods that will be dispatched.
 
         Parameters
         ----------
         cell_separator : string
           Character or string to join cells with. Default is "\n"
 
         Returns
         -------
         out : string
         """
         lines = []
         lines.extend(self.optional_header())
         lines.extend(self.main_body(cell_separator))
         lines.extend(self.optional_footer())
         return u'\n'.join(lines)
 
     def main_body(self, cell_separator='\n'):
         converted_cells = []
         for worksheet in self.nb.worksheets:
             for cell in worksheet.cells:
                 #print(cell.cell_type)  # dbg
                 conv_fn = self.dispatch(cell.cell_type)
                 if cell.cell_type in ('markdown', 'raw'):
                     remove_fake_files_url(cell)
                 converted_cells.append('\n'.join(conv_fn(cell)))
         cell_lines = cell_separator.join(converted_cells).split('\n')
         return cell_lines
 
     def render(self):
         "read, convert, and save self.infile"
         if not hasattr(self, 'nb'):
             self.read()
         self.output = self.convert()
         assert(type(self.output) == unicode)
         return self.save()
 
     def read(self):
         "read and parse notebook into NotebookNode called self.nb"
         with open(self.infile) as f:
             self.nb = nbformat.read(f, 'json')
 
     def save(self, outfile=None, encoding=None):
         "read and parse notebook into self.nb"
         if outfile is None:
             outfile = self.outbase + '.' + self.extension
         if encoding is None:
             encoding = self.default_encoding
         with io.open(outfile, 'w', encoding=encoding) as f:
             f.write(self.output)
         return os.path.abspath(outfile)
 
     def optional_header(self):
         """
         Optional header to insert at the top of the converted notebook
 
         Returns a list
         """
         return []
 
     def optional_footer(self):
         """
         Optional footer to insert at the end of the converted notebook
 
         Returns a list
         """
         return []
 
     def _new_figure(self, data, fmt):
         """Create a new figure file in the given format.
 
         Returns a path relative to the input file.
         """
         figname = '%s_fig_%02i.%s' % (self.clean_name,
                                       self.figures_counter, fmt)
         self.figures_counter += 1
         fullname = os.path.join(self.files_dir, figname)
 
         # Binary files are base64-encoded, SVG is already XML
         if fmt in ('png', 'jpg', 'pdf'):
             data = data.decode('base64')
             fopen = lambda fname: open(fname, 'wb')
         else:
             fopen = lambda fname: codecs.open(fname, 'wb', self.default_encoding)
 
         with fopen(fullname) as f:
             f.write(data)
 
         return fullname
 
     def render_heading(self, cell):
         """convert a heading cell
 
         Returns list."""
         raise NotImplementedError
 
     def render_code(self, cell):
         """Convert a code cell
 
         Returns list."""
         raise NotImplementedError
 
     def render_markdown(self, cell):
         """convert a markdown cell
 
         Returns list."""
         raise NotImplementedError
 
     def _img_lines(self, img_file):
         """Return list of lines to include an image file."""
         # Note: subclasses may choose to implement format-specific _FMT_lines
         # methods if they so choose (FMT in {png, svg, jpg, pdf}).
         raise NotImplementedError
 
     def render_display_data(self, output):
         """convert display data from the output of a code cell
 
         Returns list.
         """
-        # Choose preferred format if available
-        preferred = [xx for xx in self.display_data_priority if xx in output]
-        if preferred:
-            fmt = preferred[0]
+        for fmt in self.display_data_priority:
+            if fmt in output:
+                break
         else:
-            # Choose a format randomly if preference can't be satisfied
-            available = [k for k in output.keys() if k != 'output_type']
-            if available:
-                fmt = available[0]
+            for fmt in output:
+                if fmt != 'output_type':
+                    break
             else:
                 raise RuntimeError('no display data')
 
         # Is it an image?
         if fmt in ['png', 'svg', 'jpg', 'pdf']:
             img_file = self._new_figure(output[fmt], fmt)
             # Subclasses can have format-specific render functions (e.g.,
             # latex has to auto-convert all SVG to PDF first).
             lines_fun = getattr(self, '_%s_lines' % fmt, None)
             if not lines_fun:
                 lines_fun = self._img_lines
             lines = lines_fun(img_file)
         else:
             lines_fun = self.dispatch_display_format(fmt)
             lines = lines_fun(output)
 
         return lines
 
     def render_raw(self, cell):
         """convert a cell with raw text
 
         Returns list."""
         raise NotImplementedError
 
     def render_unknown(self, cell):
         """Render cells of unkown type
 
         Returns list."""
         data = pprint.pformat(cell)
         logging.warning('Unknown cell: %s' % cell.cell_type)
         return self._unknown_lines(data)
 
     def render_unknown_display(self, output, type):
         """Render cells of unkown type
 
         Returns list."""
         data = pprint.pformat(output)
         logging.warning('Unknown output: %s' % output.output_type)
         return self._unknown_lines(data)
 
     def render_stream(self, output):
         """render the stream part of an output
 
         Returns list.
 
         Identical to render_display_format_text
         """
         return self.render_display_format_text(output)
 
     def render_pyout(self, output):
         """convert pyout part of a code cell
 
         Returns list."""
         raise NotImplementedError
 
 
     def render_pyerr(self, output):
         """convert pyerr part of a code cell
 
         Returns list."""
         raise NotImplementedError
 
     def _unknown_lines(self, data):
         """Return list of lines for an unknown cell.
 
         Parameters
         ----------
         data : str
           The content of the unknown data as a single string.
         """
         raise NotImplementedError
 
     # These are the possible format types in an output node
 
     def render_display_format_text(self, output):
         """render the text part of an output
 
         Returns list.
         """
         raise NotImplementedError
 
     def render_display_format_html(self, output):
         """render the html part of an output
 
         Returns list.
         """
         raise NotImplementedError
 
     def render_display_format_latex(self, output):
         """render the latex part of an output
 
         Returns list.
         """
         raise NotImplementedError
 
     def render_display_format_json(self, output):
         """render the json part of an output
 
         Returns list.
         """
         raise NotImplementedError
 
     def render_display_format_javascript(self, output):
         """render the javascript part of an output
 
         Returns list.
         """
         raise NotImplementedError