********************************************************************
  Cogs - A toolkit for developing command-line utilities in Python
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Overview
========

Cogs is a toolkit for developing command-line utilities in Python.  It
handles common operations such as parsing command-line parameters,
dispatching commands, loading configuration files and is targeted
to developers, sysadmins, testers, or anybody else who needs to script
their routine tasks.

Cogs is a free software licensed under MIT license.  Cogs is written by
Kirill Simonov from Prometheus Research, LLC.


Getting Started
===============

You can install Cogs using `PIP package manager`_::

    # pip install Cogs

.. _PIP package manager: http://www.pip-installer.org/

This operation installs a command-line utility ``cogs`` and a Python
package of the same name.  The ``cogs`` utility is a dispatcher which
which lets you select and execute your scripts (called *tasks*).  Let us
show how to do it with a simple example.

In the current directory, make a subdirectory ``cogs.local`` and create a
file ``__init__.py`` with the following content::

    from cogs import task, log
    import os

    @task
    def Hello(name=None):
        """greet the given entity (if not specified, the current user)"""
        if name is None:
            name = os.getlogin()
        log("Hello, %s!" % name.capitalize())

Now run::

    $ cogs hello world
    Hello, World!

    $ cogs hello
    Hello, Xi!

    $ cogs help hello
    HELLO - greet the given entity (if not specified, the current user)
    Usage: cogs hello [<name>]

Cogs converts function ``Hello()`` into a command-line script with
parameters inferred from the function signature, so then when you
execute a command ``cogs hello world``, you call a function
``Hello('world')``.


Loading Extensions
==================

In this section, we describe how Cogs finds and loads extensions.

Cogs loads extensions from two places:

* ``cogs.local`` subdirectory in the current directory;
* all Python packages under ``cogs.extensions`` entry point.

The easiest way to add an extension is to create a ``cogs.local``
subdirectory and add your scripts there.  The subdirectory must contain
an ``__init__.py`` script, which is executed by Cogs on startup.  The
``cogs.local`` subdirectory must be owned by the same user who runs the
``cogs`` script, or by ``root``; otherwise it is ignored.

If you need to package and distribute your Cogs extensions, using
``cogs.local`` may be inconvenient.  In this case, you may package your
Cogs extensions as a regular Python distribution.

Suppose we want to pack the ``hello`` task as a separate package.
Create a directory tree with the following structure::

    Cogs-Hello/
        src/
            cogs_hello/
                __init__.py
        setup.py

The file ``cogs_hello/__init__.py`` contains the definition of the
``hello`` task and has the same content as ``cogs.local/__init__.py`` in
our previous example.

The file ``setup.py`` contains the meta-data of the package and may have
the following content::

    from setuptools import setup

    setup(
        name='Cogs-Hello',
        version='0.0.1',
        description="""A Cogs task to greet somebody""",
        packages=['cogs_hello'],
        package_dir={'': 'src'},
        install_requires=['Cogs'],
        entry_points={ 'cogs.extensions': ['Hello = cogs_hello'] },
    )

Note the parameter ``entry_points`` in ``setup()`` invocation; it adds
an entry point ``cogs.extensions`` named ``Hello`` that refers to package
``cogs_hello``.  On startup, Cogs finds and loads all packages defined for
the entry point ``cogs.extensions``.


Defining Tasks
==============

A task can be created from a function or a class by augmenting it with
the ``task`` decorator::

    from cogs import task, argument, log, fail

    @task
    def Factorial(n):
        """calculate n!

        This task calculates the value of the factorial of the given
        positive number `n`.  Factorial of n, also known as n!, is
        defined by the formula:

            n! = 1*2*...*(n-1)*n
        """
        try:
            n = int(n)
        except ValueError:
            raise fail("n must be an integer")
        if n < 1:
            raise fail("n must be positive")
        f = 1
        for k in range(2, n+1):
            f *= k
        log("%s! = `%s`" % (n, f))

    @task
    class Fibonacci:
        """calculate the n-th Fibonacci number

        The n-th Fibonacci number `F_n` is defined by:

            F_0 = 0
            F_1 = 1
            F_n = F_{n-1}+F_{n-2} (n>1)
        """

        n = argument(int)

        def __init__(self, n):
            if n < 0:
                raise ValueError("n must be non-negative")
            self.n = n

        def __call__(self):
            p, q = 0, 1
            for k in range(self.n):
                p, q = p+q, p
            log("F_%s = `%s`" % (self.n, p))

You can now execute the tasks by running::

    $ cogs factorial 10
    10! = 3628800

    $ cogs fibonacci 10
    F_10 = 55

Cogs uses the name of the function or the class as the task identifier.
The name is normalized: it is converted to lower case and has all
underscore characters converted to the dash symbol.

If the task is derived from a function, the task arguments are inferred
from the function signature.  Cogs executes such a task by calling the
function with the parsed command-line parameters.

If the task is derived from a class, the task arguments and options must
be defined using ``argument()`` and ``option()`` descriptors.  To
execute a task, Cogs creates an instance of the class passing the task
parameters as the constructor arguments.  Then Cogs invokes the
``__call__`` method of the instance.  Thus the call of::

    $ cogs factorial 10

is translated to::

    Factorial('10')

and the call of::

    $ cogs fibonacci 10

is translated to::

    t = Fibonacci(10)
    t()

The docstring of the function or the class becomes the task
description::

    $ cogs help factorial
    FACTORIAL - calculate n!
    Usage: cogs factorial <n>

    This task calculates the value of the factorial of the given
    positive number n.  Factorial of n, also known as n!, is
    defined by the formula:

        n! = 1*2*...*(n-1)*n

    $ cogs help fibonacci
    Usage: cogs fibonacci <n>

    The n-th Fibonacci number F_n is defined by:

        F_0 = 0
        F_1 = 1
        F_n = F_{n-1}+F_{n-2} (n>1)

A task derived from a function cannot have options.  To add an option to
a task derived from a class, use the ``option()`` descriptor.  For
example::

    from cogs import task, argument, option
    import sys, os

    @task
    class Write_Hello:

        name = argument(default=None)
        output = option(key='o', default=None)

        def __init__(self, name, output):
            if name is None:
                name = os.getlogin()
            self.name = name
            if output is None:
                self.file = sys.stdout
            else:
                self.file = open(output, 'w')

        def __call__(self):
            log("Hello, %s!" % self.name.capitalize(),
                file=self.file)

You can execute this task with option ``--output`` or ``-o`` to redirect
the output to a file::

    $ cogs write-hello world -o hello.txt


Configuration and Environment
=============================

Cogs allows you to define custom configuration parameters.  For example::

    from cogs import env, task, setting, log
    import os

    @setting
    def Default_Name(name=None):
        """the name to use for greetings (if not set: login name)"""
        if name is None or name == '':
            name = os.getlogin()
        if not isinstance(name, str):
            raise ValueError("a string value is expected")
        env.add(default_name=name)

    @task
    def Hello_With_Configuration(name=None):
        if name is None:
            name = env.default_name
        log("Hello, %s!" % name.capitalize())

Now you could specify the name as a configuration parameter
``default-name``.  One way to do it is to use global option
``--default-name``::

    $ cogs --default-name=world hello-with-configuration

You could also pass a configuration parameter using an environment
variable::

    $ COGS_DEFAULT_NAME=world cogs hello-with-configuration

Alternatively, you can put parameters to a configuration file.  In the
current directory, create a file ``cogs.conf`` with the following
content::

    default-name: world

Now run::

    $ cogs hello-with-configuration

Cogs reads configuration from the following locations:

* ``/etc/cogs.conf``
* ``$PREFIX/etc/cogs.conf``
* ``$HOME/.cogs/cogs.conf``
* ``./cogs.conf``
* program environment
* command-line parameters

To create a new configuration parameter, wrap a function named after the
parameter with the ``@setting`` decorator.  The function must accept
zero or one argument: the function is called without arguments if the
parameter is not specified explicitly, and is called with the value of
the parameter is it was set using one of the methods described above.

Cogs does not impose any rules on what to do with the parameter value,
but we recommend to store the value in the global ``env`` variable.  The
call of ``env.add(default_name=name)`` adds a new parameter
``default_name`` which could then be accessed as ``env.default_name``.


API Reference
=============

The following functions and classes are defined in the package ``cogs``:

``@task``

    The ``@task`` decorator converts the wrapped function or class into
    a task.  Task properties are inferred from the wrapped object as
    follows:

    *name*
        Generated from the function or the class name.  The name is
        converted to lower case and all underscores are replaced with
        dashes.

    *documentation*
        Generated from the docstring.  The first line of the docstring
        produces a one-line *hint* string, the rest of the docstring
        produces a multi-line *help* string.

    *arguments*
        When the task is inferred from a function, the arguments are
        generated from the function signature.  Each function parameter
        becomes a task argument, those which have default values are
        optional.

        If the task is inferred from a class, the arguments must be
        specified using the ``argument()`` descriptor.

    *options*
        A task inferred from a function has no options.  A task inferred
        from a class may have options specified using the ``option()``
        descriptor.

    When a task is executed, the wrapped object is invoked according
    to the following rules:

    * If the task is inferred from a function, parsed command-line
      parameters are passed as the function arguments.
    * If the task is inferred from a class, command-line parameters
      are passed to the class constructor, then the ``__call__``
      method is called on the instance.

``@setting``

    The ``@setting`` decorator converts the wrapped function to a
    configuration parameter, which properties are inferred from the
    function attributes.

    The setting name is generated from the function name.  The name is
    converted to lower case and has all underscores replaced with
    dashes.

    The setting documentation is generated from the function docstring.

    The function must be able to accept zero and one parameter.  The
    function is called at startup with no parameters if the setting is
    not explicitly set by the user; otherwise it is called with the
    value of the setting.  The function is responsible for storing the
    value in the ``env`` object.

``argument(check, default, plural=False)``

    Describe a task argument.

    ``check``
        A function which is called to check and/or transform the
        argument value.  The function must return the transformed value
        or raise ``ValueError`` exception on error.

    ``default``
        The default value to be used if the argument is optional and not
        specified.  If this parameter is not set, the argument is mandatory.

    ``plural``
        If set, the argument consumes all the remaining command-line
        parameters.  Must be the last argument specified.


``option(key, check, default, plural=False, value_name=None, hint=None)``

    Describe a task option.

    ``key``
        A one-character shorthand.

    ``check``
        A function called to check and transform the value of the option.
        The function must return the transformed value or raise ``ValueError``
        exception on error.

    ``default``
        The default value used when the option is not specified.  If this
        parameter is not set, the option does not accept a value.  Such an
        option is treated is a toggle and takes a value ``True`` if set
        and ``False`` if not set.

    ``plural``
        If set, indicates that the option could be specified more than once.

    ``value_name``
        The preferred name for the option value; used for the task
        description.

    ``hint``
        A one-line description of the option; used for the task
        description.

``env``

    A global object that keeps values of configuration parameters and
    other properties.

    ``env.add(**keywords)``
        Add new parameters.

    ``env.set(**keywords)``
        Set values for existing parameters.

    ``env.push(**keywords)``
        Save the current state and set new values for existing parameters.

    ``env.pop()``
        Restore a previously saved state of parameters and values.

    ``env(**keywords)``
        A context manager for ``with`` statement.  On entering, saves
        the current state and sets new parameter values.  On exiting,
        restores the saved state.

``log(*msgs, sep=' ', end='\n', file=sys.stdout)``
    Print the data to the standard output.

    ``log()`` has the interface and behavior similar to the standard
    ``print()`` function.

    ``log()`` supports styling: a substring of the form::

        `...`

    or::

        :fmt:`...`

    is colorized when displayed on a color terminal.  The supported
    formats are: *default* (white), ``footnote`` (dark grey),
    ``warning`` (red), ``success`` (green).

``debug(*msgs, sep=' ', end='\n', file=sys.stderr)``
    Print the data when the ``env.debug`` parameter is set.  We
    recommend to accompany any permanent change to the filesystem or
    other system state with a respective ``debug()`` call.

    Add command-line parameter ``--debug`` or set environment variable
    ``COGS_DEBUG=1`` to see debug output.

``warn(*msgs, sep=' ', end='\n', file=sys.stderr)``
    Display a warning.  ``warn()`` should be used for reporting error
    conditions which do not prevent the script from continuing the job.

``fail(*msgs, sep=' ', end='\n', file=sys.stderr)``
    Display an error message and return an exception object.  It should
    be used in the following manner::

        raise fail("no more beer in the refrigerator")

``cp(src, dst)``
    Copy a file or a directory tree.

``mv(src, dst)``
    Move a file or a directory tree.

``rm(path)``
    Remove a file.

``mktree(path)``
    Create all directories in the path.

``rmtree()``
    Remove a directory tree.

``exe(cmd)``
    Replace the current process with the given shell command.

``sh(cmd, data=None, cd=None)``
    Execute a shell command with the given input and working directory.

``pipe(cmd, data=None, cd=None)``
    Execute a shell command with the given input and working directory;
    return the command output.


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