Only check for node on first call, not on import.

IPython/nbconvert/filters/markdown.py

@@ -1,131 +1,139 b''
 """Markdown filters
 This file contains a collection of utility filters for dealing with 
 markdown 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
 #-----------------------------------------------------------------------------
 from __future__ import print_function
 
 # Stdlib imports
 import os
 import subprocess
+import warnings
 from io import TextIOWrapper, BytesIO
 
 # IPython imports
 from IPython.nbconvert.utils.pandoc import pandoc
 from IPython.nbconvert.utils.exceptions import ConversionException
 from IPython.utils.process import get_output_error_code
 from IPython.utils.py3compat import cast_bytes
 from IPython.utils.version import check_version
 
 #-----------------------------------------------------------------------------
 # Functions
 #-----------------------------------------------------------------------------
 marked = os.path.join(os.path.dirname(__file__), "marked.js")
+_node = None
 
 __all__ = [
     'markdown2html',
     'markdown2html_pandoc',
     'markdown2html_marked',
     'markdown2latex',
     'markdown2rst',
 ]
 
 class NodeJSMissing(ConversionException):
     """Exception raised when node.js is missing."""
     pass
 
 def markdown2latex(source):
     """Convert a markdown string to LaTeX via pandoc.
 
     This function will raise an error if pandoc is not installed.
     Any error messages generated by pandoc are printed to stderr.
 
     Parameters
     ----------
     source : string
       Input string, assumed to be valid markdown.
 
     Returns
     -------
     out : string
       Output as returned by pandoc.
     """
     return pandoc(source, 'markdown', 'latex')
 
+def markdown2html(source):
+    """Convert a markdown string to HTML"""
+    global _node
+    if _node is None:
+        # prefer md2html via marked if node.js >= 0.9.12 is available
+        # node is called nodejs on debian, so try that first
+        _node = 'nodejs'
+        if not _verify_node(_node):
+            _node = 'node'
+            if not _verify_node(_node):
+                warnings.warn(  "Node.js 0.9.12 or later wasn't found.\n" +
+                                "Nbconvert will try to use Pandoc instead.")
+                _node = False
+    if not _node:
+        return markdown2html_pandoc(source)
+    else:
+        return markdown2html_marked(source)
+
 def markdown2html_pandoc(source):
     """Convert a markdown string to HTML via pandoc"""
     return pandoc(source, 'markdown', 'html', extra_args=['--mathjax'])
 
 def markdown2html_marked(source, encoding='utf-8'):
     """Convert a markdown string to HTML via marked"""
-    command = [node_cmd, marked]
+    command = [_node, marked]
     try:
         p = subprocess.Popen(command,
                              stdin=subprocess.PIPE, stdout=subprocess.PIPE
         )
     except OSError as e:
         raise NodeJSMissing(
             "The command '%s' returned an error: %s.\n" % (" ".join(command), e) +
             "Please check that Node.js is installed."
         )
     out, _ = p.communicate(cast_bytes(source, encoding))
     out = TextIOWrapper(BytesIO(out), encoding, 'replace').read()
     return out.rstrip('\n')
 
 def markdown2rst(source):
     """Convert a markdown string to LaTeX via pandoc.
 
     This function will raise an error if pandoc is not installed.
     Any error messages generated by pandoc are printed to stderr.
 
     Parameters
     ----------
     source : string
       Input string, assumed to be valid markdown.
 
     Returns
     -------
     out : string
       Output as returned by pandoc.
     """
     return pandoc(source, 'markdown', 'rst')
 
 def _verify_node(cmd):
     """Verify that the node command exists and is at least the minimum supported
     version of node.
 
     Parameters
     ----------
     cmd : string
         Node command to verify (i.e 'node')."""
     try:
         out, err, return_code = get_output_error_code([cmd, '--version'])
     except OSError:
         # Command not found
         return False
     if return_code:
         # Command error
         return False
     return check_version(out.lstrip('v'), '0.9.12')
-
-# prefer md2html via marked if node.js >= 0.9.12 is available
-# node is called nodejs on debian, so try that first
-node_cmd = 'nodejs'
-if _verify_node(node_cmd):
-    markdown2html = markdown2html_marked
-else:
-    node_cmd = 'node'
-    if _verify_node(node_cmd):
-        markdown2html = markdown2html_marked
-    else:
-        markdown2html = markdown2html_pandoc