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A command line object dispatcher

Project Description
tambo
=====
Use *any* argument parser you want for *each* sub-command. Easily manage each
command as a self-contained application.

Command Line Traversing Engine
------------------------------
``tambo`` is a Python package that helps to automatically traverse a tree of
command line options and subcommands dispatching them to mapped classes that
can use any command line parser they want.

Whenever a command line interface of a program grows beyond a few flags and
options it becomes painful to manage all the different options and calls
happening on a single place.

`tambo`'s approach
------------------
What if we could **map** the command line options to objects and just deal with
the incoming action *once*? Dealing with subcommands would not be up to
a single object that gets constructed, but rather, to a chain of events that
start at the root of an object that has the first level options mapped.

This would be an example of how ``tambo`` would a dispatch of a subcommand:

.. code-block:: python

parser = tambo.Transport(args)
parser.mapper = { 'subcommand' : MySubcommandClass }
parser.dispatch()

The dispatcher would call ``MySubcommandClass`` passing in all the arguments
that came in initially to the constructor and would then call the
``main`` method so that your class can handle the logic of what to do
with the incoming arguments and options there.

Do you need to add more commands? Just add them to this root mapper and they
will be kept self contained. No need to declare *every* single option for all
commands in one place. This is how it would look for a few more commands:

.. code-block:: python

parser = tambo.Transport(args)
parser.mapper = {'subcommand': MySubcommandClass,
'bar': BarClass,
'foo': FooClass}
parser.dispatch()

j
If the ``main`` dispatcher is doing other stuff after dispatching, like
displaying help (useful when nothing is matched when dispatching) then
a ``with_exit`` flag can be passed to get a ``SystemExit(0)`` be called::

parser.dispatch(with_exit=True)

You can still handle options, boolean flags and anything you want before
hitting ``tambo`` to dispatch to subcommands, and again, you may use *whatever
argument parser you want.*

Lets put this abundantly clear:

-------------------------------------------------
**You can use whatever argument parser you want**
-------------------------------------------------

What is wrong with current approaches
-------------------------------------
Even current, widely used command line option parsers in Python suffer from
this case: ``argparse`` and ``optparser`` both require one to explicitly
construct the objects with the parameters when the interface is called.

Not only it forces you to create them before hand, but it also forces one to
make decisions based on what the parsed objects got. For example, if you have
a ``--verbose`` flag you would first need to add it to the parsing object,
something along the lines of:

.. code-block:: python

parser = ParserObject()
options, arguments = parser.parse_args(args)

parser.add_option('--verbose', action='store_true', help='Increase
verbosity')

And then to act upon whatever the parser object got, you would do something
like:

.. code-block:: python

if parser.verbose:
# do something about verbosity here
my_program.verbose()

Again, this is all *OK* if you have just a few flags and options, but if you
have, say, 10 or 20 of those, or are combining some with subcommands, you get
highly convoluted methods or functions that are trying to deal with the high
demand for object construction.

Moreover, you are causing that method to create and evaluate *everything* all
the time.

If this was a web framework, it would be a highly inefficient one, wouldn't it?
Executing all the code all the time when a request comes in?


Command Line Class
------------------
The command line class is what ``tambo`` would look forward when dispatching to
subcommands. They need to follow a couple of constraints but will still allow
to handle the command line arguments in whatever way you want with whatever
library you want.

The most simple class you would need to have a valid dispatch call would look
like this (following the example of the verbose flag from above):

.. code-block:: python

class MySubCommand(object):

def __init__(self, argv):
self.argv = argv

def main(self):
if '--verbose' in self.argv:
my_program.verbose()

In ``tambo`` internals, the above class will get called when it matches the
mapping defined in your root dictionary, and will receive the ``argv`` argument
which is nothing else than the list of arguments (same as what you would expect
from ``sys.argv`` received on the command line.
If we are following the examples from above, the call would've been like this
on the CLI::

my_cli subcommand --verbose

Using ``tambo`` parsed args
---------------------------
Although you can use whatever argument parser you want, ``tambo`` also comes
with its own little engine that maps arguments in the command line to values,
that represents the flags and arguments that you expect:

.. code-block:: python

from tambo import Transport

class MySubCommand(object):

def __init__(self, argv):
self.argv = argv
self.parser = Transport(self.argv)

def main(self):
if self.parser.has('--verbose'):
my_program.verbose()

In the above case ``--verbose`` wasn't expecting a value assigned so later we
just verified it existed by calling ``has('--verbose')``.

The ``Transport`` object allows you to define all the flags and options you need as
a tuple or a list so that they can be taken into account when mapping the
values. If you want to define aliases, you can do so by grouping them in a list
within the main list passed in to ``Transport``::

>>> from tambo import Transport
>>> options = [['-i', '--import'], '--verbose']
>>> sys_argv = ['/bin/myapp', '-i', 'somevalue']
>>> parse = Transport(sys_argv, options=options)
>>> parse.parse_args()
>>> parse.get('-i')
'somevalue'
>>> parse.get('--import')
'somevalue'

So aliases work by grouping them together in a list, but what happens on
boolean flags? You can check them by calling the ``has`` method::


>>> from tambo import Transport
>>> options = [['-i', '--import'], '--verbose']
>>> sys_argv = ['/bin/myapp', '--verbose']
>>> parse = Transport(sys_argv, options=options)
>>> parse.parse_args()
>>> parse.has('-i')
False
>>> parse.has('--verbose')
True

If you need to check for boolean flags in batch, you can pass in a list::

>>> from tambo import Transport
>>> options = [['-i', '--import'], '--verbose']
>>> sys_argv = ['/bin/myapp', '--verbose']
>>> parse = Transport(sys_argv, options=options)
>>> parse.parse_args()
>>> parse.has('-i')
False
>>> parse.has(['-v', '--verbose'])
True



Help generation
---------------
A common problem for subcommands and command line tools that have these is
generating help in a semi-automated way. ``tambo`` has a way to do this for
subcommands that are mapped by calling the help property if there is one and
would in turn output that information when called:

.. code-block:: python

class MySubcommandClass(object):

help = 'A sub-command that does some stuff'


And then in the handler for your arguments it will automaticall check for the
presence of the help attribute to display it if needed:

.. code-block:: python

# parser is an instance of the Transport class from ``tambo``
parser.parse_args()


Which would make sure that when help is set on the command line it would output
something like this::

my_cli_tool version 0.0.1

SubCommands:

subcommand A sub-command that does some stuff

This is again, entirely optional, as you can avoid making those calls to catch
help by telling the ``Transport`` class to avoid checking for it:

.. code-block:: python

parser = Transport(sys.argv, check_help=False)

If for some reason you wanted to force printing the help menu, for example when
no options have been matched, you can also do that with ``print_help()``:

.. code-block:: python

parser = Transport(sys.argv, check_help=False)

if parser.has('--mandatory-option'):
my_program.mandatory_thing()
else:
parser.print_help()
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