def use(arg, warn=True):
    """
    Set the matplotlib backend to one of the known backends.

    The argument is case-insensitive.

    .. note::

        This function must be called *before* importing pyplot for
        the first time; or, if you are not using pyplot, it must be called
        before importing matplotlib.backends.  If warn is True, a warning
        is issued if you try and call this after pylab or pyplot have been
        loaded.  In certain black magic use cases, e.g.
        :func:`pyplot.switch_backends`, we are doing the reloading necessary to
        make the backend switch work (in some cases, e.g. pure image
        backends) so one can set warn=False to supporess the warnings.

    To find out which backend is currently set, see
    :func:`matplotlib.get_backend`.

    """
    if 'matplotlib.backends' in sys.modules:
        if warn: warnings.warn(_use_error_msg)
        return
    if arg.startswith('module://'):
        name = arg
    else:
        # Lowercase only non-module backend names (modules are case-sensitive)
        arg = arg.lower()
        name = validate_backend(arg)
    rcParams['backend'] = name

def use(arg, warn=True, force=False):
    """
    Set the matplotlib backend to one of the known backends.

    The argument is case-insensitive. *warn* specifies whether a
    warning should be issued if a backend has already been set up.
    *force* is an **experimental** flag that tells matplotlib to
    attempt to initialize a new backend by reloading the backend
    module.

    .. note::

        This function must be called *before* importing pyplot for
        the first time; or, if you are not using pyplot, it must be called
        before importing matplotlib.backends.  If warn is True, a warning
        is issued if you try and call this after pylab or pyplot have been
        loaded.  In certain black magic use cases, e.g.
        :func:`pyplot.switch_backend`, we are doing the reloading necessary to
        make the backend switch work (in some cases, e.g., pure image
        backends) so one can set warn=False to suppress the warnings.

    To find out which backend is currently set, see
    :func:`matplotlib.get_backend`.

    """
    # Lets determine the proper backend name first
    if arg.startswith('module://'):
        name = arg
    else:
        # Lowercase only non-module backend names (modules are case-sensitive)
        arg = arg.lower()
        name = validate_backend(arg)

    # Check if we've already set up a backend
    if 'matplotlib.backends' in sys.modules:
        # Warn only if called with a different name
        if (rcParams['backend'] != name) and warn:
            warnings.warn(_use_error_msg)

        # Unless we've been told to force it, just return
        if not force:
            return
        need_reload = True
    else:
        need_reload = False

    # Store the backend name
    rcParams['backend'] = name

    # If needed we reload here because a lot of setup code is triggered on
    # module import. See backends/__init__.py for more detail.
    if need_reload:
        reload(sys.modules['matplotlib.backends'])

def use(arg, warn=True):
    """
    Set the matplotlib backend to one of the known backends.

    The argument is case-insensitive.  For the Cairo backend,
    the argument can have an extension to indicate the type of
    output.  Example:

        use('cairo.pdf')

    will specify a default of pdf output generated by Cairo.

    .. note::

        This function must be called *before* importing pyplot for
        the first time; or, if you are not using pyplot, it must be called
        before importing matplotlib.backends.  If warn is True, a warning
        is issued if you try and call this after pylab or pyplot have been
        loaded.  In certain black magic use cases, e.g.
        :func:`pyplot.switch_backends`, we are doing the reloading necessary to
        make the backend switch work (in some cases, e.g. pure image
        backends) so one can set warn=False to supporess the warnings.

    To find out which backend is currently set, see
    :func:`matplotlib.get_backend`.

    """
    if 'matplotlib.backends' in sys.modules:
        if warn: warnings.warn(_use_error_msg)
        return
    if arg.startswith('module://'):
        name = arg
    else:
        # Lowercase only non-module backend names (modules are case-sensitive)
        arg = arg.lower()
        be_parts = arg.split('.')
        name = validate_backend(be_parts[0])
        if len(be_parts) > 1:
            if name == 'cairo':
                rcParams['cairo.format'] = validate_cairo_format(be_parts[1])
            else:
                raise ValueError('Only cairo backend has a format option')
    rcParams['backend'] = name