Replace usage of os.devnull with subprocess.DEVNULL (#13932)

Refactoring one file in the latex library

Closes https://github.com/ipython/ipython/issues/13906

# Code changes

Replaced one occurence of `open(os.devnull)` with using directly the
`subprocess.DEVNULL` special value, removing a with/open block along the
way.

# User-facing changes

None

IPython/lib/latextools.py

@@ -1,258 +1,257 b''
 # -*- coding: utf-8 -*-
 """Tools for handling LaTeX."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from io import BytesIO, open
 import os
 import tempfile
 import shutil
 import subprocess
 from base64 import encodebytes
 import textwrap
 
 from pathlib import Path
 
 from IPython.utils.process import find_cmd, FindCmdError
 from traitlets.config import get_config
 from traitlets.config.configurable import SingletonConfigurable
 from traitlets import List, Bool, Unicode
 from IPython.utils.py3compat import cast_unicode
 
 
 class LaTeXTool(SingletonConfigurable):
     """An object to store configuration of the LaTeX tool."""
     def _config_default(self):
         return get_config()
 
     backends = List(
         Unicode(), ["matplotlib", "dvipng"],
         help="Preferred backend to draw LaTeX math equations. "
         "Backends in the list are checked one by one and the first "
         "usable one is used.  Note that `matplotlib` backend "
         "is usable only for inline style equations.  To draw  "
         "display style equations, `dvipng` backend must be specified. ",
         # It is a List instead of Enum, to make configuration more
         # flexible.  For example, to use matplotlib mainly but dvipng
         # for display style, the default ["matplotlib", "dvipng"] can
         # be used.  To NOT use dvipng so that other repr such as
         # unicode pretty printing is used, you can use ["matplotlib"].
         ).tag(config=True)
 
     use_breqn = Bool(
         True,
         help="Use breqn.sty to automatically break long equations. "
         "This configuration takes effect only for dvipng backend.",
         ).tag(config=True)
 
     packages = List(
         ['amsmath', 'amsthm', 'amssymb', 'bm'],
         help="A list of packages to use for dvipng backend. "
         "'breqn' will be automatically appended when use_breqn=True.",
         ).tag(config=True)
 
     preamble = Unicode(
         help="Additional preamble to use when generating LaTeX source "
         "for dvipng backend.",
         ).tag(config=True)
 
 
 def latex_to_png(s, encode=False, backend=None, wrap=False, color='Black',
                  scale=1.0):
     """Render a LaTeX string to PNG.
 
     Parameters
     ----------
     s : str
         The raw string containing valid inline LaTeX.
     encode : bool, optional
         Should the PNG data base64 encoded to make it JSON'able.
     backend : {matplotlib, dvipng}
         Backend for producing PNG data.
     wrap : bool
         If true, Automatically wrap `s` as a LaTeX equation.
     color : string
         Foreground color name among dvipsnames, e.g. 'Maroon' or on hex RGB
         format, e.g. '#AA20FA'.
     scale : float
         Scale factor for the resulting PNG.
     None is returned when the backend cannot be used.
 
     """
     s = cast_unicode(s)
     allowed_backends = LaTeXTool.instance().backends
     if backend is None:
         backend = allowed_backends[0]
     if backend not in allowed_backends:
         return None
     if backend == 'matplotlib':
         f = latex_to_png_mpl
     elif backend == 'dvipng':
         f = latex_to_png_dvipng
         if color.startswith('#'):
             # Convert hex RGB color to LaTeX RGB color.
             if len(color) == 7:
                 try:
                     color = "RGB {}".format(" ".join([str(int(x, 16)) for x in
                                                       textwrap.wrap(color[1:], 2)]))
                 except ValueError as e:
                     raise ValueError('Invalid color specification {}.'.format(color)) from e
             else:
                 raise ValueError('Invalid color specification {}.'.format(color))
     else:
         raise ValueError('No such backend {0}'.format(backend))
     bin_data = f(s, wrap, color, scale)
     if encode and bin_data:
         bin_data = encodebytes(bin_data)
     return bin_data
 
 
 def latex_to_png_mpl(s, wrap, color='Black', scale=1.0):
     try:
         from matplotlib import figure, font_manager, mathtext
         from matplotlib.backends import backend_agg
         from pyparsing import ParseFatalException
     except ImportError:
         return None
 
     # mpl mathtext doesn't support display math, force inline
     s = s.replace('$$', '$')
     if wrap:
         s = u'${0}$'.format(s)
 
     try:
         prop = font_manager.FontProperties(size=12)
         dpi = 120 * scale
         buffer = BytesIO()
 
         # Adapted from mathtext.math_to_image
         parser = mathtext.MathTextParser("path")
         width, height, depth, _, _ = parser.parse(s, dpi=72, prop=prop)
         fig = figure.Figure(figsize=(width / 72, height / 72))
         fig.text(0, depth / height, s, fontproperties=prop, color=color)
         backend_agg.FigureCanvasAgg(fig)
         fig.savefig(buffer, dpi=dpi, format="png", transparent=True)
         return buffer.getvalue()
     except (ValueError, RuntimeError, ParseFatalException):
         return None
 
 
 def latex_to_png_dvipng(s, wrap, color='Black', scale=1.0):
     try:
         find_cmd('latex')
         find_cmd('dvipng')
     except FindCmdError:
         return None
 
     startupinfo = None
     if os.name == "nt":
         # prevent popup-windows
         startupinfo = subprocess.STARTUPINFO()
         startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
 
     try:
         workdir = Path(tempfile.mkdtemp())
         tmpfile = "tmp.tex"
         dvifile = "tmp.dvi"
         outfile = "tmp.png"
 
         with workdir.joinpath(tmpfile).open("w", encoding="utf8") as f:
             f.writelines(genelatex(s, wrap))
 
-        with open(os.devnull, 'wb') as devnull:
             subprocess.check_call(
                 ["latex", "-halt-on-error", "-interaction", "batchmode", tmpfile],
                 cwd=workdir,
-                stdout=devnull,
-                stderr=devnull,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
                 startupinfo=startupinfo,
             )
 
             resolution = round(150*scale)
             subprocess.check_call(
                 [
                     "dvipng",
                     "-T",
                     "tight",
                     "-D",
                     str(resolution),
                     "-z",
                     "9",
                     "-bg",
                     "Transparent",
                     "-o",
                     outfile,
                     dvifile,
                     "-fg",
                     color,
                 ],
                 cwd=workdir,
-                stdout=devnull,
-                stderr=devnull,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
                 startupinfo=startupinfo,
             )
 
         with workdir.joinpath(outfile).open("rb") as f:
             return f.read()
     except subprocess.CalledProcessError:
         return None
     finally:
         shutil.rmtree(workdir)
 
 
 def kpsewhich(filename):
     """Invoke kpsewhich command with an argument `filename`."""
     try:
         find_cmd("kpsewhich")
         proc = subprocess.Popen(
             ["kpsewhich", filename],
             stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         (stdout, stderr) = proc.communicate()
         return stdout.strip().decode('utf8', 'replace')
     except FindCmdError:
         pass
 
 
 def genelatex(body, wrap):
     """Generate LaTeX document for dvipng backend."""
     lt = LaTeXTool.instance()
     breqn = wrap and lt.use_breqn and kpsewhich("breqn.sty")
     yield r'\documentclass{article}'
     packages = lt.packages
     if breqn:
         packages = packages + ['breqn']
     for pack in packages:
         yield r'\usepackage{{{0}}}'.format(pack)
     yield r'\pagestyle{empty}'
     if lt.preamble:
         yield lt.preamble
     yield r'\begin{document}'
     if breqn:
         yield r'\begin{dmath*}'
         yield body
         yield r'\end{dmath*}'
     elif wrap:
         yield u'$${0}$$'.format(body)
     else:
         yield body
     yield u'\\end{document}'
 
 
 _data_uri_template_png = u"""<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).decode('ascii')
     if base64_data:
         return _data_uri_template_png  % (base64_data, alt)