latex_to_png returns None when the backend cannot be used

When matplotlib/dvipng is not present, other formatter will be used.

IPython/lib/latextools.py

@@ -1,167 +1,174 b''
 # -*- coding: utf-8 -*-
 """Tools for handling LaTeX.
 
 Authors:
 
 * Brian Granger
 """
 #-----------------------------------------------------------------------------
 # Copyright (C) 2010-2011, 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 StringIO import StringIO
 from base64 import encodestring
 import os
 import tempfile
 import shutil
 import subprocess
 
 #-----------------------------------------------------------------------------
 # Tools
 #-----------------------------------------------------------------------------
 
 
 def latex_to_png(s, encode=False, backend='mpl'):
     """Render a LaTeX string to PNG.
 
     Parameters
     ----------
     s : str
         The raw string containing valid inline LaTeX.
     encode : bool, optional
         Should the PNG data bebase64 encoded to make it JSON'able.
     backend : {mpl, dvipng}
         Backend for producing PNG data.
 
+    None is returned when the backend cannot be used.
+
     """
     if backend == 'mpl':
         f = latex_to_png_mpl
     elif backend == 'dvipng':
         f = latex_to_png_dvipng
     else:
         raise ValueError('No such backend {0}'.format(backend))
     bin_data = f(s)
-    if encode:
+    if encode and bin_data:
         bin_data = encodestring(bin_data)
     return bin_data
 
 
 def latex_to_png_mpl(s):
-    from matplotlib import mathtext
+    try:
+        from matplotlib import mathtext
+    except ImportError:
+        return None
     
     mt = mathtext.MathTextParser('bitmap')
     f = StringIO()
     mt.to_png(f, s, fontsize=12)
     return f.getvalue()
 
 
 def latex_to_png_dvipng(s):
     try:
         workdir = tempfile.mkdtemp()
         tmpfile = os.path.join(workdir, "tmp.tex")
         dvifile = os.path.join(workdir, "tmp.dvi")
         outfile = os.path.join(workdir, "tmp.png")
 
         with open(tmpfile, "w") as f:
             f.write(_latex_header)
             f.write(s)
             f.write(_latex_footer)
 
         subprocess.check_call(
             ["latex", "-halt-on-errror", tmpfile], cwd=workdir)
 
         subprocess.check_call(
             ["dvipng", "-T", "tight", "-x", "1500", "-z", "9",
              "-bg", "transparent", "-o", outfile, dvifile], cwd=workdir)
 
         with open(outfile) as f:
             bin_data = f.read()
+    except subprocess.CalledProcessError:
+        bin_data = None
     finally:
         shutil.rmtree(workdir)
     return bin_data
 
 
-
 _latex_header = r'''
 \documentclass{article}
 \usepackage{amsmath}
 \usepackage{amsthm}
 \usepackage{amssymb}
 \usepackage{bm}
 \pagestyle{empty}
 \begin{document}
 '''
 
 _latex_footer = r'\end{document}'
 
 
 _data_uri_template_png = """<img src="data:image/png;base64,%s" alt=%s />"""
 
 def latex_to_html(s, alt='image'):
     """Render LaTeX to HTML with embedded PNG data using data URIs.
 
     Parameters
     ----------
     s : str
         The raw string containing valid inline LateX.
     alt : str
         The alt text to use for the HTML.
     """
     base64_data = latex_to_png(s, encode=True)
-    return _data_uri_template_png  % (base64_data, alt)
+    if base64_data:
+        return _data_uri_template_png  % (base64_data, alt)
 
 
 # From matplotlib, thanks to mdboom. Once this is in matplotlib releases, we
 # will remove.
 def math_to_image(s, filename_or_obj, prop=None, dpi=None, format=None):
     """
     Given a math expression, renders it in a closely-clipped bounding
     box to an image file.
 
     *s*
        A math expression.  The math portion should be enclosed in
        dollar signs.
 
     *filename_or_obj*
        A filepath or writable file-like object to write the image data
        to.
 
     *prop*
        If provided, a FontProperties() object describing the size and
        style of the text.
 
     *dpi*
        Override the output dpi, otherwise use the default associated
        with the output format.
 
     *format*
        The output format, eg. 'svg', 'pdf', 'ps' or 'png'.  If not
        provided, will be deduced from the filename.
     """
     from matplotlib import figure
     # backend_agg supports all of the core output formats
     from matplotlib.backends import backend_agg
     from matplotlib.font_manager import FontProperties
     from matplotlib.mathtext import MathTextParser
 
     if prop is None:
         prop = FontProperties()
 
     parser = MathTextParser('path')
     width, height, depth, _, _ = parser.parse(s, dpi=72, prop=prop)
 
     fig = figure.Figure(figsize=(width / 72.0, height / 72.0))
     fig.text(0, depth/height, s, fontproperties=prop)
     backend_agg.FigureCanvasAgg(fig)
     fig.savefig(filename_or_obj, dpi=dpi, format=format)
 
     return depth