Address comments

IPython/nbconvert/preprocessors/csshtmlheader.py

@@ -1,74 +1,84 b''
 """Module that pre-processes the notebook for export to HTML.
 """
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 import os
 import io
+import hashlib
 
 from IPython.utils import path
 from IPython.utils.traitlets import Unicode
 from .base import Preprocessor
 
 class CSSHTMLHeaderPreprocessor(Preprocessor):
     """
     Preprocessor used to pre-process notebook for HTML output.  Adds IPython notebook
     front-end CSS and Pygments CSS to HTML output.
     """
     highlight_class = Unicode('.highlight', config=True,
                               help="CSS highlight class identifier")
 
+    def __init__(self, *pargs, **kwargs):
+        Preprocessor.__init__(self, *pargs, **kwargs)
+        self._default_css_hash = None
+
     def preprocess(self, nb, resources):
         """Fetch and add CSS to the resource dictionary
 
         Fetch CSS from IPython and Pygments to add at the beginning
         of the html files.  Add this css in resources in the 
         "inlining.css" key
         
         Parameters
         ----------
         nb : NotebookNode
             Notebook being converted
         resources : dictionary
             Additional resources used in the conversion process.  Allows
             preprocessors to pass variables into the Jinja engine.
         """
         resources['inlining'] = {}
         resources['inlining']['css'] = self._generate_header(resources)
         return nb, resources
 
     def _generate_header(self, resources):
         """ 
         Fills self.header with lines of CSS extracted from IPython 
         and Pygments.
         """
         from pygments.formatters import HtmlFormatter
         header = []
         
         # Construct path to IPy CSS
         from IPython.html import DEFAULT_STATIC_FILES_PATH
         sheet_filename = os.path.join(DEFAULT_STATIC_FILES_PATH,
             'style', 'style.min.css')
         
         # Load style CSS file.
         with io.open(sheet_filename, encoding='utf-8') as f:
             header.append(f.read())
 
+        # Add pygments CSS
+        formatter = HtmlFormatter()
+        pygments_css = formatter.get_style_defs(self.highlight_class)
+        header.append(pygments_css)
+
         # Load the user's custom CSS and IPython's default custom CSS.  If they
         # differ, assume the user has made modifications to his/her custom CSS
         # and that we should inline it in the nbconvert output.
         profile_dir = resources['profile_dir']
         custom_css_filename = os.path.join(profile_dir, 'static', 'custom', 'custom.css')
         if os.path.isfile(custom_css_filename):
-            with io.open(custom_css_filename, encoding='utf-8') as f:
-                custom_css = f.read()
-            with io.open(os.path.join(DEFAULT_STATIC_FILES_PATH, 'custom', 'custom.css'), encoding='utf-8') as f:
-                default_css = f.read()
-            if custom_css != default_css:
-                header.append(custom_css)
-
-        # Add pygments CSS
-        formatter = HtmlFormatter()
-        pygments_css = formatter.get_style_defs(self.highlight_class)
-        header.append(pygments_css)
+            if self._default_css_hash is None:
+                self._default_css_hash = self._hash(os.path.join(DEFAULT_STATIC_FILES_PATH, 'custom', 'custom.css'))
+            if self._hash(custom_css_filename) != self._default_css_hash:
+                with io.open(custom_css_filename, encoding='utf-8') as f:
+                    header.append(f.read())
         return header
 
+    def _hash(self, filename):
+        """Compute the hash of a file."""
+        md5 = hashlib.md5()
+        with open(filename) as f:
+            md5.update(f.read())
+        return md5.digest()