def have_chomp(program='homsimpl'):
    """
    Return True if this computer has ``program`` installed.

    The first time it is run, this function caches its result in the
    variable ``_have_chomp`` -- a dictionary indexed by program name
    -- and any subsequent time, it just checks the value of the
    variable.

    This program is used in the routine CHomP.__call__.

    If this computer doesn't have CHomP installed, you may obtain it
    from http://chomp.rutgers.edu/.

    EXAMPLES::

        sage: from sage.interfaces.chomp import have_chomp
        sage: have_chomp() # random -- depends on whether CHomP is installed
        True
        sage: 'homsimpl' in sage.interfaces.chomp._have_chomp
        True
        sage: sage.interfaces.chomp._have_chomp['homsimpl'] == have_chomp()
        True
    """
    global _have_chomp
    if program not in _have_chomp:
        from sage.misc.sage_ostools import have_program
        _have_chomp[program] = have_program(program)
    return _have_chomp[program]

def trac_create_instance(directory = 'sage_trac', easy_setup = False):
    """
    Create a new Trac project instance if Trac is installed.

    INPUT:

    - ``directory`` - a string (default: 'sage_trac'); the name of the
      project directory

    - ``easy_setup`` - a bool (default: False); whether to use the
      project name 'Sage', the default database, enable the webadmin
      plugin, and enable the source browser for the 'sage' Mercurial
      repository.

    .. note::

        To access the webadmin panel, first create a user, give that
        user admin permissions, and log in as that user.  Create new
        accounts using htdigest (part of Apache)::

            cd <directory>/conf
            htdigest passwd <server_address> <username>

        Grant a user administrative privileges with::

            trac-admin <directory> add <username> TRAC_ADMIN
    """
    from sage.misc.sage_ostools import have_program

    if not have_program('trac-admin'):
        raise RuntimeError("trac is not installed")

    if easy_setup:
        cmd = 'trac-admin "%s" initenv "Sage" "sqlite:db/trac.db" "" ""' % directory
    else:
        cmd = 'trac-admin "%s" initenv' % directory

    e = os.system(cmd)
    if e:
        raise RuntimeError("Error creating trac environment.")

    if easy_setup:
        conf_name = os.path.abspath(os.path.join(directory, 'conf/trac.ini'))

        __import__('trac.config')
        Configuration = sys.modules['trac.config'].Configuration
        conf = Configuration(conf_name)

        conf.set('trac', 'repository_dir', SAGE_LIB)
        conf.set('trac', 'repository_type', 'hg')

        conf.set('components', 'tracext.hg.*', 'enabled')
        conf.set('components', 'webadmin.*', 'enabled')
        conf.set('hg', 'node_format', 'short')
        conf.set('hg', 'show_rev', 'yes')

        conf.save()

    def _have_ffmpeg(self):
        """
        Return True if the program 'ffmpeg' is installed.  See
        www.ffmpeg.org to download ffmpeg.

        EXAMPLES::

            sage: a = animate([plot(sin, -1,1)], xmin=0, ymin=0)
            sage: a._have_ffmpeg() # random: depends on whether ffmpeg is installed
            False
        """
        from sage.misc.sage_ostools import have_program
        return have_program('ffmpeg')

    def blackbox(self, polys, input_ring, verbose = False):
        """
        Returns as a string the result of running PHC with the given polynomials
        under blackbox mode (the '-b' option).

        INPUT:

        - polys -- a list of multivariate polynomials (elements of a multivariate
          polynomial ring).
        - input_ring -- for coercion of the variables into the desired ring.
        - verbose -- print lots of verbose information about what this function does.

        OUTPUT:

        - a PHC_Object object containing the phcpack output string.

        EXAMPLES::

            sage: from sage.interfaces.phc import *
            sage: R2.<x,y> = PolynomialRing(QQ,2)
            sage: start_sys = [x^6-y^2,y^5-1]
            sage: sol = phc.blackbox(start_sys, R2)        # optional -- phc
            sage: len(sol.solutions())                     # optional -- phc
            30
        """

        # Get three temporary file names (these will be in SAGE_HOME/.sage/tmp/pid)
        input_filename = tmp_filename()
        output_filename = input_filename + ".phc"
        log_filename = tmp_filename()

        # Get the input polynomial text
        input = self._input_file(polys)
        if verbose:
            print("Writing the input file to %s" % input_filename)
        open(input_filename, 'w').write(input)

        if verbose:
            print("The following file will be the input polynomial file to phc.")
            print(input)

        # Create the phc command line>
        cmd = 'phc -b %s %s'%(input_filename, output_filename)

        if verbose:
            print("The phc command line is:")
            print(cmd)

        # Do it -- make the system call.
        e = os.system(cmd)

        # Was there an error?
        if e:
            from sage.misc.sage_ostools import have_program
            if not have_program('phc'):
                print(os.system('which phc') + '  PHC needs to be installed and in your path')
                raise RuntimeError
            # todo -- why? etc.
            raise RuntimeError(open(log_filename).read() + "\nError running phc.")

        if not os.path.exists(output_filename):
            raise RuntimeError("The output file does not exist; something went wrong running phc.")

        # Read the output produced by PHC
        out = open(output_filename).read()

        # All done
        return PHC_Object(out, input_ring)

def default_viewer(viewer=None):
    """
    Set up default programs for opening web pages, PDFs, PNGs, and DVI files.

    INPUT:

    - ``viewer``: ``None`` or a string: one of 'browser', 'pdf', 'png',
      'dvi' -- return the name of the corresponding program.  ``None``
      is treated the same as 'browser'.

    EXAMPLES::

        sage: from sage.misc.viewer import default_viewer
        sage: default_viewer(None) # random -- depends on OS, etc.
        'sage-open'
        sage: default_viewer('pdf') # random -- depends on OS, etc.
        'xdg-open'
        sage: default_viewer('jpg')
        Traceback (most recent call last):
        ...
        ValueError: Unknown type of viewer: jpg.
    """
    import os
    from sage.misc.sage_ostools import have_program

    if isinstance(viewer, str):
        viewer = viewer.lower()

    if 'SAGE_BROWSER' in os.environ:
        BROWSER = os.environ['SAGE_BROWSER']
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif os.uname()[0] == 'Darwin':
        # Simple on OS X, since there is an open command that opens
        # anything, using the user's preferences.
        # sage-open -- a wrapper around OS X open that
        # turns off any of Sage's special library stuff.
        BROWSER = 'sage-open'
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif os.uname()[0][:6] == 'CYGWIN':
        # Windows is also easy, since it has a system for
        # determining what opens things.
        # Bobby Moreti provided the following.
        if not 'BROWSER' in os.environ:
            systemroot = os.environ['SYSTEMROOT'].replace(':','/').replace('\\','')
            systemroot = '/cygdrive/' + systemroot
            BROWSER = '%s/system32/rundll32.exe url.dll,FileProtocolHandler'%\
                      systemroot
        else:
            BROWSER = os.environ['BROWSER']
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif have_program('xdg-open'):
        # On other OS'es try xdg-open if present.
        # See http://portland.freedesktop.org/xdg-utils-1.0.
        BROWSER = 'xdg-open'
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    else:
        # If all fails try to get something from the environment.
        try:
            BROWSER = os.environ['BROWSER']
        except KeyError:
            BROWSER = 'less'  # silly default; lets hope it doesn't come to this!
            for cmd in ['firefox', 'konqueror', 'mozilla', 'mozilla-firefox']:
                if have_program(cmd):
                    BROWSER = cmd
                    break
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

        # Alternatives, if they are set in the environment or available.
        try:
            DVI_VIEWER = os.environ['DVI_VIEWER']
        except KeyError:
            for cmd in ['xdvi', 'kdvi']:
                if have_program(cmd):
                    DVI_VIEWER = cmd
                    break
        try:
            PDF_VIEWER = os.environ['PDF_VIEWER']
        except KeyError:
            for cmd in ['acroread', 'xpdf']:
                if have_program(cmd):
                    PDF_VIEWER = cmd
                    break

    if viewer is None or viewer.startswith('browse'):
        return BROWSER
    elif viewer.startswith('dvi'):
#.........这里部分代码省略.........

    def gif(self, delay=20, savefile=None, iterations=0, show_path=False,
            use_ffmpeg=False):
        r"""
        Returns an animated gif composed from rendering the graphics
        objects in self.

        This method will only work if either (a) the ImageMagick
        software suite is installed, i.e., you have the ``convert``
        command or (b) ``ffmpeg`` is installed.  See
        [IM] for more about ImageMagick, and see
        [FF] for more about ``ffmpeg``.  By default, this
        produces the gif using ``convert`` if it is present.  If this
        can't find ``convert`` or if ``use_ffmpeg`` is True, then it
        uses ``ffmpeg`` instead.

        INPUT:

        -  ``delay`` - (default: 20) delay in hundredths of a
           second between frames

        -  ``savefile`` - file that the animated gif gets saved
           to

        -  ``iterations`` - integer (default: 0); number of
           iterations of animation. If 0, loop forever.

        -  ``show_path`` - boolean (default: False); if True,
           print the path to the saved file

        - ``use_ffmpeg`` - boolean (default: False); if True, use
          'ffmpeg' by default instead of 'convert'.

        If ``savefile`` is not specified: in notebook mode, display the
        animation; otherwise, save it to a default file name.

        EXAMPLES::

            sage: a = animate([sin(x + float(k)) for k in srange(0,2*pi,0.7)],
            ....:                xmin=0, xmax=2*pi, figsize=[2,1])
            sage: dir = tmp_dir()
            sage: a.gif()              # not tested
            sage: a.gif(savefile=dir + 'my_animation.gif', delay=35, iterations=3)  # optional -- ImageMagick
            sage: a.gif(savefile=dir + 'my_animation.gif', show_path=True) # optional -- ImageMagick
            Animation saved to .../my_animation.gif.
            sage: a.gif(savefile=dir + 'my_animation_2.gif', show_path=True, use_ffmpeg=True) # optional -- ffmpeg
            Animation saved to .../my_animation_2.gif.

        .. note::

           If neither ffmpeg nor ImageMagick is installed, you will
           get an error message like this::

              Error: Neither ImageMagick nor ffmpeg appears to be installed. Saving an
              animation to a GIF file or displaying an animation requires one of these
              packages, so please install one of them and try again.

              See www.imagemagick.org and www.ffmpeg.org for more information.
        """
        from sage.misc.sage_ostools import have_program
        have_convert = have_program('convert')
        have_ffmpeg = self._have_ffmpeg()
        if use_ffmpeg or not have_convert:
            if have_ffmpeg:
                self.ffmpeg(savefile=savefile, show_path=show_path,
                            output_format='.gif', delay=delay,
                            iterations=iterations)
            else:
                if not have_convert:
                    msg = """
Error: Neither ImageMagick nor ffmpeg appears to be installed. Saving an
animation to a GIF file or displaying an animation requires one of these
packages, so please install one of them and try again.

See www.imagemagick.org and www.ffmpeg.org for more information."""
                else:
                    msg = """
Error: ffmpeg does not appear to be installed.  Download it from
www.ffmpeg.org, or use 'convert' to produce gifs instead."""
                raise OSError(msg)
        else:
            if not savefile:
                savefile = tmp_filename(ext='.gif')
            if not savefile.endswith('.gif'):
                savefile += '.gif'
            savefile = os.path.abspath(savefile)
            d = self.png()
            cmd = ( 'cd "%s"; sage-native-execute convert -dispose Background '
                    '-delay %s -loop %s *.png "%s"' ) % ( d, int(delay),
                        int(iterations), savefile )
            from subprocess import check_call, CalledProcessError
            try:
                check_call(cmd, shell=True)
                if show_path:
                    print "Animation saved to file %s." % savefile
            except (CalledProcessError, OSError):
                msg = """
Error: Cannot generate GIF animation.  Verify that convert
(ImageMagick) or ffmpeg is installed, and that the objects passed to
the animate command can be saved in PNG image format.

#.........这里部分代码省略.........

def default_viewer(viewer=None):
    """
    Set up default programs for opening web pages, PDFs, PNGs, and DVI files.

    INPUT:

    - ``viewer``: ``None`` or a string: one of 'browser', 'pdf', 'png',
      'dvi' -- return the name of the corresponding program.  ``None``
      is treated the same as 'browser'.

    EXAMPLES::

        sage: from sage.misc.viewer import default_viewer
        sage: default_viewer(None) # random -- depends on OS, etc.
        'sage-open'
        sage: default_viewer('pdf') # random -- depends on OS, etc.
        'xdg-open'
        sage: default_viewer('jpg')
        Traceback (most recent call last):
        ...
        ValueError: Unknown type of viewer: jpg.
    """
    import os
    from sage.misc.sage_ostools import have_program

    if isinstance(viewer, str):
        viewer = viewer.lower()

    if "SAGE_BROWSER" in os.environ:
        BROWSER = os.environ["SAGE_BROWSER"]
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif os.uname()[0] == "Darwin":
        # Simple on OS X, since there is an open command that opens
        # anything, using the user's preferences.
        # sage-open -- a wrapper around OS X open that
        # turns off any of Sage's special library stuff.
        BROWSER = "sage-open"
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif os.uname()[0][:6] == "CYGWIN":
        # Windows is also easy, since it has a system for
        # determining what opens things.
        # Bobby Moreti provided the following.
        if not "BROWSER" in os.environ:
            systemroot = os.environ["SYSTEMROOT"].replace(":", "/").replace("\\", "")
            systemroot = "/cygdrive/" + systemroot
            BROWSER = "%s/system32/rundll32.exe url.dll,FileProtocolHandler" % systemroot
        else:
            BROWSER = os.environ["BROWSER"]
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif have_program("xdg-open"):
        # On other OS'es try xdg-open if present.
        # See http://portland.freedesktop.org/xdg-utils-1.0.
        BROWSER = "xdg-open"
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    else:
        # If all fails try to get something from the environment.
        try:
            BROWSER = os.environ["BROWSER"]
        except KeyError:
            BROWSER = "less"  # silly default; lets hope it doesn't come to this!
            for cmd in ["firefox", "konqueror", "mozilla", "mozilla-firefox"]:
                if have_program(cmd):
                    BROWSER = cmd
                    break
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

        # Alternatives, if they are set in the environment or available.
        try:
            DVI_VIEWER = os.environ["DVI_VIEWER"]
        except KeyError:
            for cmd in ["xdvi", "kdvi"]:
                if have_program(cmd):
                    DVI_VIEWER = cmd
                    break
        try:
            PDF_VIEWER = os.environ["PDF_VIEWER"]
        except KeyError:
            for cmd in ["acroread", "xpdf"]:
                if have_program(cmd):
                    PDF_VIEWER = cmd
                    break

    if viewer is None or viewer.startswith("browse"):
        return BROWSER
    elif viewer.startswith("dvi"):
        return DVI_VIEWER
#.........这里部分代码省略.........

def install_scripts(directory=None, ignore_existing=False):
    r"""
    Running ``install_scripts(directory)`` creates scripts in the
    given directory that run various software components included with
    Sage.  Each of these scripts essentially just runs ``sage --CMD``
    where ``CMD`` is also the name of the script:

    - 'gap' runs GAP
    - 'gp' runs the PARI/GP interpreter
    - 'hg' runs Mercurial
    - 'ipython' runs IPython
    - 'maxima' runs Maxima
    - 'mwrank' runs mwrank
    - 'R' runs R
    - 'singular' runs Singular
    - 'sqlite3' runs SQLite version 3
    - 'kash' runs Kash if it is installed (Kash is an optional Sage
      package)
    - 'M2' runs Macaulay2 if it is installed (Macaulay2 is an
      experimental Sage package)
    
    This command:
    
    -  verbosely tells you which scripts it adds, and
    
    -  will *not* overwrite any scripts you already have in the given
       directory.
    
    INPUT:

    - ``directory`` - string; the directory into which to put the
      scripts.  This directory must exist and the user must have write
      and execute permissions.

    - ``ignore_existing`` - bool (optional, default False): if True,
      install script even if another version of the program is in your
      path.

    OUTPUT: Verbosely prints what it is doing and creates files in
    ``directory`` that are world executable and readable.
    
    .. note::

       You may need to run ``sage`` as ``root`` in order to run
       ``install_scripts`` successfully, since the user running
       ``sage`` needs write permissions on ``directory``.  Note
       that one good candidate for ``directory`` is
       ``'/usr/local/bin'``, so from the shell prompt, you could run ::

           sudo sage -c "install_scripts('/usr/local/bin')"

    .. note::

       Running ``install_scripts(directory)`` will be most helpful if
       ``directory`` is in your path.

    AUTHORS:

    - William Stein: code / design

    - Arthur Gaer: design

    - John Palmieri: revision, 2011-07 (trac ticket #11602)

    EXAMPLES::

        sage: install_scripts(str(SAGE_TMP), ignore_existing=True)
        Checking that Sage has the command 'gap' installed
        ...
    """
    if directory is None:
        # We do this since the intended user of install_scripts
        # will likely be pretty clueless about how to use Sage or
        # its help system.
        import sagedoc
        print sagedoc.format(install_scripts.__doc__)
        print "USAGE: install_scripts('directory')"
        return
    
    if not os.path.exists(directory):
        print "Error: '%s' does not exist." % directory
        return

    if not os.path.isdir(directory):
        print "Error: '%s' is not a directory." % directory
        return

    if not (os.access(directory, os.W_OK) and os.access(directory, os.X_OK)):
        print "Error: you do not have write permission for '%s'." % directory
        return

    from sage.misc.sage_ostools import have_program
    from sage.env import SAGE_LOCAL
    script_created = False
    SAGE_BIN = os.path.join(SAGE_LOCAL, 'bin')
    # See if 'directory' is already in PATH, and then remove
    # SAGE_LOCAL/bin from PATH so that we can later check whether
    # cmd is available outside of Sage.
    PATH = os.environ['PATH'].split(os.pathsep)
    PATH = [d for d in PATH if os.path.exists(d)]
#.........这里部分代码省略.........

def viewer():
    from sage.misc.sage_ostools import have_program

    global BROWSER, DVI_VIEWER, PDF_VIEWER, PNG_VIEWER

    if not (BROWSER is None):
        return BROWSER

    if os.environ.has_key('SAGE_BROWSER'):
        BROWSER = os.environ['SAGE_BROWSER']
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER
        return BROWSER
    
    if os.uname()[0] == 'Darwin':
        # Simple on OS X, since there is an open command that opens
        # anything, using the user's preferences.
        # sage-open -- a wrapper around OS X open that
        # turns off any of Sage's special library stuff.

        BROWSER = 'sage-open'
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif os.uname()[0][:6] == 'CYGWIN':
        # Windows is also easy, since it has a system for
        # determining what opens things.
        # Bobby Moreti provided the following. 

        if not os.environ.has_key('BROWSER'):
            systemroot = os.environ['SYSTEMROOT'].replace(':','/').replace('\\','')
            systemroot = '/cygdrive/' + systemroot
            BROWSER = '%s/system32/rundll32.exe url.dll,FileProtocolHandler'%\
                      systemroot

        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

    elif have_program('xdg-open'):

        # On other OS'es try xdg-open if present.
        # See http://portland.freedesktop.org/xdg-utils-1.0.

            BROWSER = 'xdg-open'
            DVI_VIEWER = BROWSER
            PDF_VIEWER = BROWSER
            PNG_VIEWER = BROWSER

    else:

        # If all fails try to get something from the environment.

        try:
            BROWSER = os.environ['BROWSER']
        except KeyError:
            BROWSER = 'less'  # silly default; lets hope it doesn't come to this!
            for cmd in ['firefox', 'konqueror', 'mozilla', 'mozilla-firefox']:
                if have_program(cmd):
                    BROWSER = cmd
                    break
        DVI_VIEWER = BROWSER
        PDF_VIEWER = BROWSER
        PNG_VIEWER = BROWSER

        # Alternatives, if they are set in the environment or available. 
        try:
            DVI_VIEWER = os.environ['DVI_VIEWER']
        except KeyError:
            for cmd in ['xdvi', 'kdvi']:
                if have_program(cmd):
                    DVI_VIEWER = cmd
                    break
        try:
            PDF_VIEWER = os.environ['PDF_VIEWER']
        except KeyError:
            for cmd in ['acroread', 'xpdf']:
                if have_program(cmd):
                    PDF_VIEWER = cmd
                    break

    return BROWSER