Advanced Patterns
=================

.. currentmodule:: click

In addition to common functionality that is implemented in the library
itself, there are countless patterns that can be implemented by extending
Click.  This page should give some insight into what can be accomplished.

.. _aliases:

Command Aliases
---------------

Many tools support aliases for commands (see `Command alias example
<https://github.com/pallets/click/tree/master/examples/aliases>`_).
For instance, you can configure ``git`` to accept ``git ci`` as alias for
``git commit``.  Other tools also support auto-discovery for aliases by
automatically shortening them.

Click does not support this out of the box, but it's very easy to customize
the :class:`Group` or any other :class:`MultiCommand` to provide this
functionality.

As explained in :ref:`custom-multi-commands`, a multi command can provide
two methods: :meth:`~MultiCommand.list_commands` and
:meth:`~MultiCommand.get_command`.  In this particular case, you only need
to override the latter as you generally don't want to enumerate the
aliases on the help page in order to avoid confusion.

This following example implements a subclass of :class:`Group` that
accepts a prefix for a command.  If there were a command called ``push``,
it would accept ``pus`` as an alias (so long as it was unique):

.. click:example::

    class AliasedGroup(click.Group):

        def get_command(self, ctx, cmd_name):
            rv = click.Group.get_command(self, ctx, cmd_name)
            if rv is not None:
                return rv
            matches = [x for x in self.list_commands(ctx)
                       if x.startswith(cmd_name)]
            if not matches:
                return None
            elif len(matches) == 1:
                return click.Group.get_command(self, ctx, matches[0])
            ctx.fail('Too many matches: %s' % ', '.join(sorted(matches)))

And it can then be used like this:

.. click:example::

    @click.command(cls=AliasedGroup)
    def cli():
        pass

    @cli.command()
    def push():
        pass

    @cli.command()
    def pop():
        pass

Parameter Modifications
-----------------------

Parameters (options and arguments) are forwarded to the command callbacks
as you have seen.  One common way to prevent a parameter from being passed
to the callback is the `expose_value` argument to a parameter which hides
the parameter entirely.  The way this works is that the :class:`Context`
object has a :attr:`~Context.params` attribute which is a dictionary of
all parameters.  Whatever is in that dictionary is being passed to the
callbacks.

This can be used to make up addition parameters.  Generally this pattern
is not recommended but in some cases it can be useful.  At the very least
it's good to know that the system works this way.

.. click:example::

    import urllib

    def open_url(ctx, param, value):
        if value is not None:
            ctx.params['fp'] = urllib.urlopen(value)
            return value

    @click.command()
    @click.option('--url', callback=open_url)
    def cli(url, fp=None):
        if fp is not None:
            click.echo('%s: %s' % (url, fp.code))

In this case the callback returns the URL unchanged but also passes a
second ``fp`` value to the callback.  What's more recommended is to pass
the information in a wrapper however:

.. click:example::

    import urllib

    class URL(object):

        def __init__(self, url, fp):
            self.url = url
            self.fp = fp

    def open_url(ctx, param, value):
        if value is not None:
            return URL(value, urllib.urlopen(value))

    @click.command()
    @click.option('--url', callback=open_url)
    def cli(url):
        if url is not None:
            click.echo('%s: %s' % (url.url, url.fp.code))


Token Normalization
-------------------

.. versionadded:: 2.0

Starting with Click 2.0, it's possible to provide a function that is used
for normalizing tokens.  Tokens are option names, choice values, or command
values.  This can be used to implement case insensitive options, for
instance.

In order to use this feature, the context needs to be passed a function that
performs the normalization of the token.  For instance, you could have a
function that converts the token to lowercase:

.. click:example::

    CONTEXT_SETTINGS = dict(token_normalize_func=lambda x: x.lower())

    @click.command(context_settings=CONTEXT_SETTINGS)
    @click.option('--name', default='Pete')
    def cli(name):
        click.echo('Name: %s' % name)

And how it works on the command line:

.. click:run::

    invoke(cli, prog_name='cli', args=['--NAME=Pete'])

Invoking Other Commands
-----------------------

Sometimes, it might be interesting to invoke one command from another
command.  This is a pattern that is generally discouraged with Click, but
possible nonetheless.  For this, you can use the :func:`Context.invoke`
or :func:`Context.forward` methods.

They work similarly, but the difference is that :func:`Context.invoke` merely
invokes another command with the arguments you provide as a caller,
whereas :func:`Context.forward` fills in the arguments from the current
command.  Both accept the command as the first argument and everything else
is passed onwards as you would expect.

Example:

.. click:example::

    cli = click.Group()

    @cli.command()
    @click.option('--count', default=1)
    def test(count):
        click.echo('Count: %d' % count)

    @cli.command()
    @click.option('--count', default=1)
    @click.pass_context
    def dist(ctx, count):
        ctx.forward(test)
        ctx.invoke(test, count=42)

And what it looks like:

.. click:run::

    invoke(cli, prog_name='cli', args=['dist'])


.. _callback-evaluation-order:

Callback Evaluation Order
-------------------------

Click works a bit differently than some other command line parsers in that
it attempts to reconcile the order of arguments as defined by the
programmer with the order of arguments as defined by the user before
invoking any callbacks.

This is an important concept to understand when porting complex
patterns to Click from optparse or other systems.  A parameter
callback invocation in optparse happens as part of the parsing step,
whereas a callback invocation in Click happens after the parsing.

The main difference is that in optparse, callbacks are invoked with the raw
value as it happens, whereas a callback in Click is invoked after the
value has been fully converted.

Generally, the order of invocation is driven by the order in which the user
provides the arguments to the script; if there is an option called ``--foo``
and an option called ``--bar`` and the user calls it as ``--bar
--foo``, then the callback for ``bar`` will fire before the one for ``foo``.

There are three exceptions to this rule which are important to know:

Eagerness:
    An option can be set to be "eager".  All eager parameters are
    evaluated before all non-eager parameters, but again in the order as
    they were provided on the command line by the user.

    This is important for parameters that execute and exit like ``--help``
    and ``--version``.  Both are eager parameters, but whatever parameter
    comes first on the command line will win and exit the program.

Repeated parameters:
    If an option or argument is split up on the command line into multiple
    places because it is repeated -- for instance, ``--exclude foo --include
    baz --exclude bar`` -- the callback will fire based on the position of
    the first option.  In this case, the callback will fire for
    ``exclude`` and it will be passed both options (``foo`` and
    ``bar``), then the callback for ``include`` will fire with ``baz``
    only.

    Note that even if a parameter does not allow multiple versions, Click
    will still accept the position of the first, but it will ignore every
    value except the last.  The reason for this is to allow composability
    through shell aliases that set defaults.

Missing parameters:
    If a parameter is not defined on the command line, the callback will
    still fire.  This is different from how it works in optparse where
    undefined values do not fire the callback.  Missing parameters fire
    their callbacks at the very end which makes it possible for them to
    default to values from a parameter that came before.

Most of the time you do not need to be concerned about any of this,
but it is important to know how it works for some advanced cases.

.. _forwarding-unknown-options:

Forwarding Unknown Options
--------------------------

In some situations it is interesting to be able to accept all unknown
options for further manual processing.  Click can generally do that as of
Click 4.0, but it has some limitations that lie in the nature of the
problem.  The support for this is provided through a parser flag called
``ignore_unknown_options`` which will instruct the parser to collect all
unknown options and to put them to the leftover argument instead of
triggering a parsing error.

This can generally be activated in two different ways:

1.  It can be enabled on custom :class:`Command` subclasses by changing
    the :attr:`~BaseCommand.ignore_unknown_options` attribute.
2.  It can be enabled by changing the attribute of the same name on the
    context class (:attr:`Context.ignore_unknown_options`).  This is best
    changed through the ``context_settings`` dictionary on the command.

For most situations the easiest solution is the second.  Once the behavior
is changed something needs to pick up those leftover options (which at
this point are considered arguments).  For this again you have two
options:

1.  You can use :func:`pass_context` to get the context passed.  This will
    only work if in addition to :attr:`~Context.ignore_unknown_options`
    you also set :attr:`~Context.allow_extra_args` as otherwise the
    command will abort with an error that there are leftover arguments.
    If you go with this solution, the extra arguments will be collected in
    :attr:`Context.args`.
2.  You can attach a :func:`argument` with ``nargs`` set to `-1` which
    will eat up all leftover arguments.  In this case it's recommended to
    set the `type` to :data:`UNPROCESSED` to avoid any string processing
    on those arguments as otherwise they are forced into unicode strings
    automatically which is often not what you want.

In the end you end up with something like this:

.. click:example::

    import sys
    from subprocess import call

    @click.command(context_settings=dict(
        ignore_unknown_options=True,
    ))
    @click.option('-v', '--verbose', is_flag=True, help='Enables verbose mode')
    @click.argument('timeit_args', nargs=-1, type=click.UNPROCESSED)
    def cli(verbose, timeit_args):
        """A fake wrapper around Python's timeit."""
        cmdline = ['echo', 'python', '-mtimeit'] + list(timeit_args)
        if verbose:
            click.echo('Invoking: %s' % ' '.join(cmdline))
        call(cmdline)

And what it looks like:

.. click:run::

    invoke(cli, prog_name='cli', args=['--help'])
    println()
    invoke(cli, prog_name='cli', args=['-n', '100', 'a = 1; b = 2; a * b'])
    println()
    invoke(cli, prog_name='cli', args=['-v', 'a = 1; b = 2; a * b'])

As you can see the verbosity flag is handled by Click, everything else
ends up in the `timeit_args` variable for further processing which then
for instance, allows invoking a subprocess.  There are a few things that
are important to know about how this ignoring of unhandled flag happens:

*   Unknown long options are generally ignored and not processed at all.
    So for instance if ``--foo=bar`` or ``--foo bar`` are passed they
    generally end up like that.  Note that because the parser cannot know
    if an option will accept an argument or not, the ``bar`` part might be
    handled as an argument.
*   Unknown short options might be partially handled and reassembled if
    necessary.  For instance in the above example there is an option
    called ``-v`` which enables verbose mode.  If the command would be
    ignored with ``-va`` then the ``-v`` part would be handled by Click
    (as it is known) and ``-a`` would end up in the leftover parameters
    for further processing.
*   Depending on what you plan on doing you might have some success by
    disabling interspersed arguments
    (:attr:`~Context.allow_interspersed_args`) which instructs the parser
    to not allow arguments and options to be mixed.  Depending on your
    situation this might improve your results.

Generally though the combinated handling of options and arguments from
your own commands and commands from another application are discouraged
and if you can avoid it, you should.  It's a much better idea to have
everything below a subcommand be forwarded to another application than to
handle some arguments yourself.


Global Context Access
---------------------

.. versionadded:: 5.0

Starting with Click 5.0 it is possible to access the current context from
anywhere within the same thread through the use of the
:func:`get_current_context` function which returns it.  This is primarily
useful for accessing the context bound object as well as some flags that
are stored on it to customize the runtime behavior.  For instance the
:func:`echo` function does this to infer the default value of the `color`
flag.

Example usage::

    def get_current_command_name():
        return click.get_current_context().info_name

It should be noted that this only works within the current thread.  If you
spawn additional threads then those threads will not have the ability to
refer to the current context.  If you want to give another thread the
ability to refer to this context you need to use the context within the
thread as a context manager::

    def spawn_thread(ctx, func):
        def wrapper():
            with ctx:
                func()
        t = threading.Thread(target=wrapper)
        t.start()
        return t

Now the thread function can access the context like the main thread would
do.  However if you do use this for threading you need to be very careful
as the vast majority of the context is not thread safe!  You are only
allowed to read from the context, but not to perform any modifications on
it.
