Fix blank cell collapsing.

IPython/nbconvert/filters/highlight.py

@@ -1,123 +1,123 b''
 """
 Module containing filter functions that allow code to be highlighted
 from within Jinja templates.
 """
 #-----------------------------------------------------------------------------
 # 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
 #-----------------------------------------------------------------------------
 
 # pygments must not be imported at the module level
 # because errors should be raised at runtime if it's actually needed,
 # not import time, when it may not be needed.
 
 # Our own imports
 from IPython.nbconvert.utils.base import NbConvertBase
 
 #-----------------------------------------------------------------------------
 # Globals and constants
 #-----------------------------------------------------------------------------
 
 MULTILINE_OUTPUTS = ['text', 'html', 'svg', 'latex', 'javascript', 'json']
 
 #-----------------------------------------------------------------------------
 # Utility functions
 #-----------------------------------------------------------------------------
 
 __all__ = [
     'Highlight2Html',
     'Highlight2Latex'
 ]
 
 
 class Highlight2Html(NbConvertBase):
 
     def __call__(self, source, language=None, metadata=None):
         """
         Return a syntax-highlighted version of the input source as html output.
 
         Parameters
         ----------
         source : str
             source of the cell to highlight
         language : str
             language to highlight the syntax of
         metadata : NotebookNode cell metadata
             metadata of the cell to highlight
         """
         from pygments.formatters import HtmlFormatter
         if not language:
             language=self.default_language
 
-        return _pygments_highlight(source, HtmlFormatter(), language, metadata)
+        return _pygments_highlight(source if len(source) > 0 else ' ', HtmlFormatter(), language, metadata)
 
 
 class Highlight2Latex(NbConvertBase):
 
     def __call__(self, source, language=None, metadata=None, strip_verbatim=False):
         """
         Return a syntax-highlighted version of the input source as latex output.
 
         Parameters
         ----------
         source : str
             source of the cell to highlight
         language : str
             language to highlight the syntax of
         metadata : NotebookNode cell metadata
             metadata of the cell to highlight
         strip_verbatim : bool
             remove the Verbatim environment that pygments provides by default
         """
         from pygments.formatters import LatexFormatter
         if not language:
             language=self.default_language
 
         latex = _pygments_highlight(source, LatexFormatter(), language, metadata)
         if strip_verbatim:
             latex = latex.replace(r'\begin{Verbatim}[commandchars=\\\{\}]' + '\n', '')
             return latex.replace('\n\\end{Verbatim}\n', '')
         else:
             return latex
 
 
 
 def _pygments_highlight(source, output_formatter, language='ipython', metadata=None):
     """
     Return a syntax-highlighted version of the input source
 
     Parameters
     ----------
     source : str
         source of the cell to highlight
     output_formatter : Pygments formatter
     language : str
         language to highlight the syntax of
     metadata : NotebookNode cell metadata
         metadata of the cell to highlight
     """
     from pygments import highlight
     from pygments.lexers import get_lexer_by_name
     from IPython.nbconvert.utils.lexers import IPythonLexer
 
     # If the cell uses a magic extension language,
     # use the magic language instead.
     if language == 'ipython' \
         and metadata \
         and 'magics_language' in metadata:
 
         language = metadata['magics_language']
 
     if language == 'ipython':
         lexer = IPythonLexer()
     else:
         lexer = get_lexer_by_name(language, stripall=True)
 
     return highlight(source, lexer, output_formatter)