HighlightMagics: merge default languages with the user config languages

* When configuring HighlightMagics, instead of replacing fully the
  language dict by the user provided one, we merge both.

IPython/nbconvert/preprocessors/highlightmagics.py

@@ -1,104 +1,111
 """This preprocessor detect cells using a different language through
 magic extensions such as `%%R` or `%%octave`. Cell's metadata is marked
 so that the appropriate highlighter can be used in the `highlight`
 filter.
 """
 
 #-----------------------------------------------------------------------------
 # 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
 #-----------------------------------------------------------------------------
 
 from __future__ import print_function, absolute_import
 
 import re
 
 # Our own imports
 # Needed to override preprocessor
 from .base import (Preprocessor)
 from IPython.utils.traitlets import Dict
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 
 
 class HighlightMagicsPreprocessor(Preprocessor):
     """
     Detects and tags code cells that use a different languages than Python.
     """
 
     # list of magic language extensions and their associated pygment lexers
-    languages = Dict(
+    default_languages = Dict(
         default_value={
             '%%R': 'r',
             '%%bash': 'bash',
             '%%octave': 'octave',
             '%%perl': 'perl',
             '%%ruby': 'ruby'},
+        config=False)
+
+    # user defined language extensions
+    languages = Dict(
         config=True,
         help=("Syntax highlighting for magic's extension languages. "
          "Each item associates a language magic extension such as %%R, "
          "with a pygments lexer such as r."))
 
     def __init__(self, config=None, **kw):
         """Public constructor"""
 
         super(HighlightMagicsPreprocessor, self).__init__(config=config, **kw)
 
+        # Update the default languages dict with the user configured ones
+        self.default_languages.update(self.languages)
+
         # build a regular expression to catch language extensions and choose
         # an adequate pygments lexer
-        any_language = "|".join(self.languages.keys())
+        any_language = "|".join(self.default_languages.keys())
         self.re_magic_language = re.compile(
             r'^\s*({0})\s+'.format(any_language))
 
     def which_magic_language(self, source):
         """
         When a cell uses another language through a magic extension,
         the other language is returned.
         If no language magic is detected, this function returns None.
 
         Parameters
         ----------
         source: str
             Source code of the cell to highlight
         """
 
         m = self.re_magic_language.match(source)
 
         if m:
             # By construction of the re, the matched language must be in the
             # languages dictionnary
-            assert(m.group(1) in self.languages)
-            return self.languages[m.group(1)]
+            assert(m.group(1) in self.default_languages)
+            return self.default_languages[m.group(1)]
         else:
             return None
 
     def preprocess_cell(self, cell, resources, cell_index):
         """
         Tags cells using a magic extension language
 
         Parameters
         ----------
         cell : NotebookNode cell
             Notebook cell being processed
         resources : dictionary
             Additional resources used in the conversion process.  Allows
             preprocessors to pass variables into the Jinja engine.
         cell_index : int
             Index of the cell being processed (see base.py)
         """
 
         # Only tag code cells
         if hasattr(cell, "input") and cell.cell_type == "code":
             magic_language = self.which_magic_language(cell.input)
             if magic_language:
                 cell['metadata']['magics_language'] = magic_language
         return cell, resources