.. _Tutorial:

########################
Full Tutorial
########################

***************************************
Overview
***************************************


    The ``ruffus`` module is a lightweight way to add support 
    for running computational pipelines.
    
    Computational pipelines are often conceptually quite simple, especially
    if we breakdown the process into simple stages, or separate **tasks**.
    
    Each **task** is represented by a python function


.. index:: @follows

.. ???

.. _follows:

   
***************************************
**@follows**
***************************************

.. index:: 
    single: Example Programme; 1. Simple

=====================
A Simple example
=====================

    Use the **@follows(...)** python decorator before the function definitions::
    
        from ruffus import *
        import sys
        
        def first_task():
            print "First task"
    
        @follows(first_task)
        def second_task():
            print "Second task"
    
        @follows(second_task)
        def final_task():
            print "Final task"

.. Comment. End of literal block???
    
    
    
    the ``@follows`` decorator indicate that the ``first_task`` function precedes ``second_task`` in 
    the pipeline.


=====================
Running
=====================

    Now we can run the pipeline by::
        
        pipeline_run([final_task])
    
    
    Because ``final_task`` depends on ``second_task`` which depends on ``first_task`` , all 
    three functions will be executed in order.
    
=====================
Displaying
=====================

    We can see a flowchart of our fledgling pipeline by executing::
    
        pipeline_printout_graph ( open("flowchart.svg", "w"),
                                 "svg",
                                 [final_task])
    
.. ???

    or in text format with::
    
        pipeline_printout(sys.stdout, [final_task])
    
.. ???

.. _follows-out-of-order:

=====================
More to **@follows**
=====================

    All this assumes that all your pipelined tasks are defined in order.
    (``first_task`` before ``second_task`` before ``final_task``)
    
    This is usually the most sensible way to arrange your code.
    If you wish to refer to tasks which are not yet defined, you can do so by quoting the
    function name. This is also a way of referring to tasks in other modules::
    
    
        @follows(first_task, "fifth_task", "another_module.useful_task")
        def second_task():
            print "Second task"

.. ???
            
    
    Note that the ``@follows`` decorator can refer to multiple antecedent tasks.
    Alternatively, the same code can be written as::
    
        @follows(first_task)
        @follows("fifth_task")
        @follows("another_module.useful_task")
        def second_task():
            print "Second task"

.. ???

.. _follow-mkdir:

.. index:: 
    pair: @follows; mkdir

.. ???

=================================
**@follows** and **mkdir**
=================================


    A common prerequisite for any computational task, is making sure that the destination
    directories exist. As a shortcut, we can define a special ``mkdir`` dependency. For example::
    
        @follows(first_task, mkdir("output/results/here"))
        def second_task():
            print "Second task"
            
    will make sure that ``output/results/here`` exists before `second_task` is run.
    
    In other words, it will make the ``output/results/here`` directory if it does not exist.

.. index:: @parallel

.. ???

.. _parallel:

***************************************
**@parallel** 
***************************************

    Often each task consists of multiple **jobs** (in GNU make terminology) which can be
    run concurrently. 
    
    Each **job** is a separate call to the same task function but with different parameters.
    Let us try to add up (1+2), (3+4) and (5+6) in parallel::
    
        from ruffus import *
        parameters = [
                         ['A', 1, 2], # 1st job
                         ['B', 3, 4], # 2nd job
                         ['C', 5, 6], # 3rd job
                     ]
        @parallel(parameters)                                                     
        def parallel_task(name, param1, param2):                                  
            sys.stderr.write("    Parallel task %s: " % name)                     
            sys.stderr.write("%d + %d = %d\n" % (param1, param2, param1 + param2))
        
        pipeline_run([parallel_task])
        
    .. ???

    Produces the following::
    
        Task = parallel_task
            Parallel task A: 1 + 2 = 3
            Job = ["A", 1, 2] completed
            Parallel task B: 3 + 4 = 7
            Job = ["B", 3, 4] completed
            Parallel task C: 5 + 6 = 11
            Job = ["C", 5, 6] completed
        
.. ???
.. _multi_processing:

=====================
Multi Processing
=====================

    *Ruffus* uses python `multiprocessing <http://docs.python.org/library/multiprocessing.html>`_ to run
    each job in a separate process.
    
    This means that jobs do *not* necessarily complete in the order of the defined parameters.
    Task hierachies are, of course, inviolate: upstream tasks run before downstream, dependent tasks.
    
    The number of concurrent jobs can be set in ``pipeline_run``::
    
        pipeline_run([parallel_task], multiprocess = 5)
        
        
    .. ???

    if ``multiprocess`` is set to 1, then jobs will be run on a single process.

.. index:: errors, exceptions

.. ???

.. _exceptions:

=====================
Exceptions
=====================
    Python exceptions or syntax errors are gathered from all the parallel jobs before
    being reraised as an aggregate Exception. A full stack trace is provided so that you can
    see where errors occurred.
    
    In the previous example, if the number of parameters is incorrect::
    
        from ruffus import *
        @parallel([['A', 1], ['B',3]])
        def parallel_task(name, param1, param2):
            sys.stderr.write("    Parallel task %s: " % name)
            sys.stderr.write("%d + %d = %d\n" % (param1, param2, param1 + param2))
        
        pipeline_run([parallel_task])
    
        
    .. ???

    This would have produced these detailed error messages for each of the two jobs::
    
        task.RethrownJobError:
        
            Exceptions running jobs for
            'def parallel_task(...):'
        
            Original exceptions:
        
            Exception #1
            exceptions.TypeError: parallel_task() takes exactly 3 arguments (2 given)
            for Job = ["A", 1]
        
            Traceback (most recent call last):
              File "task.py", line 1022 [...]
            TypeError: parallel_task() takes exactly 3 arguments (2 given)
        
        
            Exception #2
            exceptions.TypeError: parallel_task() takes exactly 3 arguments (2 given)
            for Job = ["B", 3]
        
            Traceback (most recent call last):
              File "task.py", line 1022 [...]
            TypeError: parallel_task() takes exactly 3 arguments (2 given)
    
        
    .. ???

    (Parts of the traceback have been removed for brevity)

.. index:: signalling, interrupts, break

.. ???

    
.. _interrupting:

=================================
Interrupting the pipeline
=================================

    If your task function returns false, this will halt the pipeline at that point.::
    
        from ruffus import *
        @parallel([['A', 1], ['B',3]])
        def parallel_task(name, param1):
            if name == 'A': return False
        
        pipeline_run([parallel_task])
    
        
    .. ???

    produces the following (abbreviated)::
    
        task.RethrownJobError:
        
            Exceptions running jobs for
            'def parallel_task(...):'
        
            Original exception:
        
            Exception #1
            task.JobSignalledBreak: Job = ["A", 1] returned False
            for Job = ["A", 1]
        
.. ???

    
=====================
Multiple Errors
=====================
    For any task where exceptions are thrown, *Ruffus* will continue executing until
    the number of exceptions is equal to the number of concurrent jobs (``multiprocess``) set in
    ``pipeline_run``. This seems a fair tradeoff between being able to gather detailed
    error information for running jobs, and not wasting too much time for a task
    that is going to fail anyway.
    
    *Ruffus* always exits concurrent task operations as soon as possible if the
    pipeline is interrupted by a job returning false (see :ref:`previous section <interrupting>`).


.. index:: 
    pair: dynamic;  parameters
    pair: on the fly; parameters

.. ???

.. _on_the_fly:
    
=======================================
Generating parameters on the fly
=======================================

    The above examples assume you know the parameters each job takes beforehand.
    Sometimes, it is necessary, or perhaps more convenient, to generate parameters on the fly or
    at runtime.
    
    All this requires is a function which generate one list (or any sequence) of
    parameters per job. For example::
    
        from ruffus import *
        def generate_parameters_on_the_fly():
            """
            returns one list of parameters per job
            """
            parameters = [
                                ['A', 1, 2], # 1st job
                                ['B', 3, 4], # 2nd job
                                ['C', 5, 6], # 3rd job
                            ]
            for job_parameters in parameters:
                yield job_parameters
        
        @parallel(generate_parameters_on_the_fly)
        def parallel_task(name, param1, param2):
            sys.stderr.write("    Parallel task %s: " % name)
            sys.stderr.write("%d + %d = %d\n" % (param1, param2, param1 + param2))
        
        pipeline_run([parallel_task])
        
        
    .. ???

    Similarly produces::
   
        Task = parallel_task
            Parallel task A: 1 + 2 = 3
            Job = ["A", 1, 2] completed
            Parallel task B: 3 + 4 = 7
            Job = ["B", 3, 4] completed
            Parallel task C: 5 + 6 = 11
            Job = ["C", 5, 6] completed
    
        
    .. ???

    
    The parameters often need to be generated more than once (see 
    :ref:`below <checking-multiple-times>`).


.. index:: @files

.. ???

.. _files:

***************************************
**@files**
***************************************

=======================================
Skip jobs which are up to date
=======================================

    Usually it will not be necessary to run all the tasks in a pipeline but only where
    the input data has changed or the task is no longer up to date.
    
    One easy way to do this is to check the modification times for the input
    and output files of a job. The job will only be rerun if the input file has changed
    since the output file was produced.
    
    *Ruffus* treats the first two parameters of each job as the input and output files
    and checks timestamps for you.
    
    From the command prompt, make our starting files::
    
        > echo "start 1" > a.1
        > echo "start 2" > a.2
    
        
        
    .. ???

    Then run the following python code::
    
        from ruffus import *
        parameters = [
                            [ 'a.1', 'a.2', 'A file'], # 1st job
                            [ 'b.1', 'b.2', 'B file'], # 2nd job
                      ]
        
        @files(parameters)
        def parallel_io_task(infile, outfile, text):
            infile_text = open(infile).read()
            f = open(outfile, "w").write(infile_text + "\n" + text)
        
        pipeline_run([parallel_io_task])
       
        
    .. ???

    Gives::
        
        Task = parallel_io_task
            Job = ["a.1" -> "a.2", "A file"] completed
            Job = ["b.1" -> "b.2", "B file"] completed
    
        
    .. ???

    If you ran the same code a second time, nothing would happen because 
    ``a.2`` is more recent than ``a.1`` and
    ``b.2`` is more recent than ``b.1`` .
    
    However, if you subsequently modified ``a.1`` again::
    
        > echo touch a.1
        
        
    .. ???

    You would see the following::
    
        >>> pipeline_run([parallel_io_task])
        Task = parallel_io_task
            Job = ["a.1" -> "a.2", "A file"] completed
            Job = ["b.1" -> "b.2", "B file"] unnecessary: already up to date
    
        
    .. ???

    The 2nd job is up to date and will be skipped.

.. index:: timestamp, resolution, precision

.. ???

=======================================
Caveats: Timestamp resolution
=======================================

    Note that modification times have one second precision under certain versions of Linux and
    Windows, especially over the network. This may result in some jobs running even when
    they are up-to-date because the modification times appear to be identical.

=======================================
Input/Output **@files**
=======================================
    The input and output files for each job can be 
        * A single file name
        * A list of files
        * ``None``
    
    If the input file is ``None``, the job will run if any output file is missing.
    
    If the output file is ``None``, the job will always run.
    
    If any of the output files is missing, the job will run.
    
    If any of the input files is missing when the job is run, a
    ``MissingInputFileError`` exception will be raised, For example,
    ::
    
        task.MissingInputFileError: No way to run job: Input file ['a.1'] does not exist
        for Job = ["a.1" -> "a.2", "A file"]
    
        
.. ???

    

=======================================
Short cut for single jobs
=======================================

    If you are specifying the parameters for only one job, you can leave off the brackets,
    greatly improving clarity::
    
        from ruffus import *
        @files('a.1', ['a.2', 'b.2'], 'A file')
        def single_job_io_task(infile, outfile, text):
            infile_text = open(infile).read()
            f = open(outfile, "w").write(infile_text + "\n" + text)
        
        pipeline_run([parallel_io_task])
        
        
    .. ???

    Produces::
    
        Task = single_job_io_task
            Job = ["a.1" -> ["a.2", "b.2"], "A file"] completed
            
.. ???



.. _automatic-dependency-checking:

***************************************
Automatic dependency checking
***************************************

=============================================
Running all out-of-date tasks and dependents
=============================================

    By default, *ruffus* will 
    
        * build a flow chart,
        * look upstream (among the antecedents) of the specified target(s),
        * find all the most upstream out-of-date tasks,
        * start running from there.
    
        .. _checking-multiple-times:
    
        This means that *ruffus* *may* ask any task if their jobs are out of date more than once:
    
        * once when deciding whether/how to run the pipeline
        * once when actually executing the task.
        
    *Ruffus* tries to be clever / efficient, and does the minimal amount of querying.
    
    
.. _simple-example:
    
    
=======================================
A simple example
=======================================

-------------------------------------
    Python code
-------------------------------------    
    The full code is available :ref:`here <code-for-simpler-example>`.

-------------------------------------
    Four successive tasks to run:
-------------------------------------    
        The pipeline in ``example_scripts/simpler.py`` has four successive tasks::
        
            python simpler.py -F "jpg" -d ../images/four_stage_pipeline.jpg -t task4  -K -H
        
        .. ???

        producing the following flowchart
        
        .. image:: images/four_stage_pipeline.jpg
        
        

        Flow Chart Key:
        
        .. image:: images/key.jpg
        

        
        
        We can see that all four tasks need to run reach the target task4.
   
.. ???
    

----------------------------------------
    Pipeline tasks are up-to-date:
----------------------------------------


        After the pipeline runs (``python simpler.py -d ""``), all tasks are up to date and the flowchart shows::
        
            python simpler.py -F "jpg" -d ../images/complete.jpg -t task4 -K -H
        
        
        .. ???

        .. image:: images/complete.jpg
    
        
.. ???

    

-------------------------------------
    Some tasks out of date:
-------------------------------------

        If we then made task2 and task4 out of date by modifying their input files::
        
            > touch a.1
            > touch a.3
            
        
        .. ???

        the flowchart would show::
        
            python simpler.py -F "jpg" -d ../images/maximal_mode.jpg -t task4  -K -H
        
        
        .. ???

        .. image:: images/maximal_mode.jpg
            
        

        Showing that:
        
            #. the pipeline only has to rerun from ``task2``.
            #. ``task1`` is not out of date
            #. ``task3`` will have to be re-run because it follows (depends on) ``task2``.

.. ???

=======================================
Minimal Reruns
=======================================

    In fact, you could point out that ``task3`` is not out of date. And if we were only interested
    in the immediate dependencies of ``task4``, we might not need task2 to rerun at all, only ``task4``.
    
    .. image:: images/minimal_mode.jpg
    
        

    
    In which case, we can rerun the pipeline with a different option::
    
        pipeline_run([task4], gnu_make_maximal_rebuild_mode = False)
        
        
    .. ???

    and only ``task4`` will rerun.
    
    This rather dangerous option is useful if you don't want to keep all the intermediate 
    files/results from upstream tasks. The pipeline code will iterate up the flowchart and 
    stop at the first up to date task. 
        

=======================================
Forced Reruns
=======================================
    In any case, you can always force the pipeline to run from one or more tasks, whether they
    are up to date or not. This is particularly useful, for example, if the pipeline code 
    changes (rather than the data).
    ::
    
        pipeline_run([task4], [task1])
        
        
    .. ???

    will run all tasks from ``task1`` to ``task4``
    
    .. image:: images/force_from_task1.jpg
    
        

    Both the "target" and the "forced" lists can include as many tasks as you wish. All dependencies
    are still carried out and out-of-date jobs rerun.

.. index:: @files_re

.. ???

.. _files_re:

***************************************
**@files_re**
***************************************

=======================================
i/o files using regular expressions
=======================================

    It is often not possible to come up with a predetermined list of input
    and output files for a each job in a pipeline task. Instead, you would
    like to apply an operation to whatever files that are present of the right 
    type, however many there are.
    
    Typically, in traditional make files, you would manage this via file extensions.
    For example, compiling all ".c" source files into object files with ".obj"
    extension. Because python has such good regular expression support, it is
    easy to have more sophisticated schemes to organise your files, such as putting
    them into different directories, giving them different names.
    
=======================================
A simple example
=======================================

    ::
    
        from ruffus import *
        #
        #   convert all files ending in ".1" into files ending in ".2"
        #
        @files_re('*.1', '(.*).1', r'\1.2')
        def task_re(infile, outfile):
            open(outfile, "w").write(open(infile).read() + "\nconverted\n")
        
        pipeline_run([task_re])

        
    .. ???

    This pipeline task:

        #. takes each file which has the ``.1`` suffix,
        #. adds the line ``converted`` to its contents, and
        #. outputs a corresponding file with a ``.2`` suffix.
        
    If you ran this, you may be surprised to see that nothing happens. This is because
    there are no ``*.1`` files to begin with.
    
    but if you first created ``a.1`` and ``b.1`` ::
    
        >>> open("a.1", "w")
        <open file 'a.1', mode 'w' at 0x96643e0>
        >>> open("b.1", "w")
        <open file 'b.1', mode 'w' at 0x9664e80>
        >>> pipeline_run([task_re])
    
        
    .. ???

    You would see the creation of two new files::
    
        Task = task_re
            Job = ["a.1" -> "a.2"] completed
            Job = ["b.1" -> "b.2"] completed
    
        
.. ???

    
=======================================
**@files_re** in more detail
=======================================

    Let us look at the ``@files_re`` directive again to see what it does::
    
        @files_re(
                    '*.1',              # 'glob' * .1
                    '(.*).1',           # Regular expression match
                    r'\1.2'             # add the ".2" extension
                  )
    
        
    .. ???

    There are three parameters:
        #. The first is a "glob" pattern, such as you might type in a command prompt to
           find a list of files (``ls *`` or ``dir *.*``)
           
           (See python `glob <http://docs.python.org/library/glob.html>`_ documentation.)
        #. For each file name returned by the "glob",  we make sure it has a ``.1`` extension,
           and save the "root" of each file name.
           
           (See python `regular expression (re) <http://docs.python.org/library/re.html>`_ documentation.)
        #. ``.2`` is appended to the matching file name root to give a new output file.
        
           (See the documentation for `re.sub <http://docs.python.org/library/re.html#re.sub>`_\ .)
           

=======================================
More ambitious **@files_re** 
=======================================

    Because @files_re uses regular expressions, you organise your files for each job with
    greater power and flexibility. Let us try putting files in different subdirectories.
    
    First create some files for different animals::
        
        > touch mammals.tiger.wild.animals
        > touch mammals.lion.wild.animals
        > touch mammals.lion.handreared.animals
        > touch mammals.dog.tame.animals
        > touch mammals.dog.wild.animals
        > touch reptiles.crocodile.wild.animals
    
        
    .. ???

    We are only interested in mammals, and we would like the files of each species to
    end up in its own directory after we have processed it. 

    Let us also prepare the directories. (We could also use :ref:`@follows(mk_dir(xxx)) <follow-mkdir>`  
    to do this but let us keep this example simple.)::
        
        > mkdir -p tiger lion dog
     
        
    .. ???

    Then, the following::     
    
        from ruffus import *
        @files_re('*.animals', 
                    r'mammals\.(.+)\.(.+)\.animals',    # save species and 'wild'/'tame'
                    r'\1/\1.\2.in_my_zoo')
        def capture_mammals(infile, outfile):
            open(outfile, "w").write(open(infile).read() + "\ncaptured\n")
        
        pipeline_run([capture_mammals])
    
        
    .. ???

    Will put each captured mammal in its own directory::

        Task = capture_mammals
            Job = ["mammals.dog.tame.animals" -> "dog/dog.tame.in_my_zoo"] completed
            Job = ["mammals.dog.wild.animals" -> "dog/dog.wild.in_my_zoo"] completed
            Job = ["mammals.lion.handreared.animals" -> "lion/lion.handreared.in_my_zoo"] completed
            Job = ["mammals.lion.wild.animals" -> "lion/lion.wild.in_my_zoo"] completed
            Job = ["mammals.tiger.wild.animals" -> "tiger/tiger.wild.in_my_zoo"] completed

        
    .. ???

    Note that we have ignored the crocodile file because it doesn't match the ``mammal`` part
    of the regular expression.

=======================================
Multiple parameters with **@files_re** 
=======================================

    So far, even with the complicated example above, we are only generating one input file name,
    and one output file name per job. Sometimes this is not sufficient. By analogy with 
    ``@files``, we can use regular expressions to create any number of parameters for each
    job, so long as 

        #. The first parameter are input file(s)
        #. The second parameter are output file(s)
        
    Regular expression substitution is carried out on all strings, or on list of strings

    `None` and all other types of objects are passed through unchanged.
    
    Let us see how this works in practice. Building on the previous example::

        > touch mammals.tiger.wild.animals
        > touch mammals.lion.wild.animals mammals.lion.handreared.animals
        > touch mammals.dog.tame.animals  mammals.dog.wild.animals
        > touch reptiles.crocodile.wild.animals
        > mkdir -p tiger lion dog

        
    .. ???

     
    Then, the following::     

        from ruffus import *
        @files_re('*.animals', r'mammals\.(.+)\.(.+)\.animals',      # save species and 'wild'/'tame'
                               r'\g<0>',                             # input:  entire match unchanged
                               [r'\1/\1.\2.in_my_zoo',               # output file names
                                r'all_species/\1.\2.in_my_zoo'],
                                r'\1' )                              # species name
        def capture_mammals(infile, outfiles, species):
            for f in outfiles:
                open(f, "w").write(open(infile).read() + "\nCaptured %s\n" % species)
        
        pipeline_run([capture_mammals])
    
        
    .. ???

    Will put each captured mammal in the ``all_species`` directory as well as its own::
    
        Task = capture_mammals
            Job = ["mammals.dog.tame.animals" -> ["dog/dog.tame.in_my_zoo", "all_species/dog.tame.in_my_zoo"], "dog"] completed
            Job = ["mammals.dog.wild.animals" -> ["dog/dog.wild.in_my_zoo", "all_species/dog.wild.in_my_zoo"], "dog"] completed
            Job = ["mammals.lion.handreared.animals" -> ["lion/lion.handreared.in_my_zoo", "all_species/lion.handreared.in_my_zoo"], "lion"] completed
            Job = ["mammals.lion.wild.animals" -> ["lion/lion.wild.in_my_zoo", "all_species/lion.wild.in_my_zoo"], "lion"] completed
            Job = ["mammals.tiger.wild.animals" -> ["tiger/tiger.wild.in_my_zoo", "all_species/tiger.wild.in_my_zoo"], "tiger"] completed

        
    .. ???

    Note the third ``species`` parameter for ``capture_mammals(...)``.

.. index:: @check_if_uptodate

.. ???
    

    
.. _check_if_uptodate:

***************************************
**@check_if_uptodate**
***************************************

=======================================
Manual dependency checking
=======================================
    tasks specified with :ref:`@files <files>` or :ref:`@files_re <files_re>` have automatic
    dependency checking based on file modification times.
    
    Sometimes, you might want to decide have more control over whether to run jobs, especially
    if a task does not rely on or produce files (i.e. with :ref:`@parallel <parallel>`)
    
    You can write your own custom function to decide whether to run a job.
    This takes as many parameters as your task function, and needs to return True if an
    update is needed.
    
    This simple example which create ``a.1`` if it does not exist::
        
        from ruffus import *
        @files(None, "a.1")
        def create_if_necessary(input_file, output_file):
            open(output_file, "w")
                    
        pipeline_run([create_if_necessary])


        
    .. ???

    Could be rewritten as::
    
        
        from ruffus import *
        import os
        def check_file_exists(input_file, output_file):
            return not os.path.exists(output_file)
        
        @parallel([[None, "a.1"]])
        @check_if_uptodate(check_file_exists)
        def create_if_necessary(input_file, output_file):
            open(output_file, "w")
        
        pipeline_run([create_if_necessary])
        
        
    .. ???

    Both produce the same output::
    
        Task = create_if_necessary
            Job = [null, "a.1"] completed
        
.. ???

    
    
.. note::
    
    The function specified by :ref:`@check_if_uptodate <check_if_uptodate>` can be called
    more than once for each job. 

    See the discussion of how *ruffus* decides which tasks
    to run in :ref:`@automatic dependency checking <automatic-dependency-checking>`
        
        
.. ???

.. index:: @posttask

.. ???

    
.. _posttask:

***************************************
**@posttask**
***************************************

=======================================
Signalling the completion of each task
=======================================
    
    It is often useful to signal the completion of each task by specifying
    one or more function(s) using ``@posttask`` ::
    
        from ruffus import *
        
        def task_finished():
            print "hooray"
            
        @posttask(task_finished)
        @files(None, "a.1")
        def create_if_necessary(input_file, output_file):
            open(output_file, "w")
                    
        pipeline_run([create_if_necessary])

        
.. ???

    
.. note::

    The function(s) provided to ``@posttask`` will be called if the pipeline passes 
    through a task, even if none of its jobs are run because they are up-to-date.
    This happens when a upstream task is out-of-date, and the execution passes through
    this point in the pipeline
    
        
.. ???

.. index:: 
    pair: @posttask; touch_file

.. ???
.. _posttask-touch-file:

=======================================
touch_file
=======================================

    One common way to note the completion of a task is to create some sort of
    "flag" file. Each stage in a traditional ``make`` pipeline would contain a 
    ``touch completed.flag``.
    
    This is such a common use that there is a special shortcut for posttask::
    
        from ruffus import *
        
        @posttask(touch_file("task_completed.flag"))
        @files(None, "a.1")
        def create_if_necessary(input_file, output_file):
            open(output_file, "w")
                    
        pipeline_run([create_if_necessary])
        
.. ???

.. index:: logging

.. ???
    

.. _logging-tasks:

***************************************
Logging
***************************************

=================================
Logging task/job completion
=================================
    *Ruffus* logs each task and each job as it is completed. The results of each
    of the examples in this tutorial were produced by default logging to stderr.
    
    You can specify your own logging by providing a python 
    `logging <http://docs.python.org/library/logging.html>`_ object to ``pipeline_run``::
    
        import logging
        import logging.handlers
        
        LOG_FILENAME = '/tmp/ruffus.log'
        
        # Set up a specific logger with our desired output level
        my_ruffus_logger = logging.getLogger('My_Ruffus_logger')
        my_ruffus_logger.setLevel(logging.DEBUG)
        
        # Add the log message handler to the logger
        handler = logging.handlers.RotatingFileHandler(
                      LOG_FILENAME, maxBytes=2000, backupCount=5)
        
        my_ruffus_logger.addHandler(handler)
        
        
        from ruffus import *
        
        @files(None, "a.1")
        def create_if_necessary(input_file, output_file):
            """Description: Create the file if it does not exists"""
            open(output_file, "w")
        
        pipeline_run([create_if_necessary], [create_if_necessary], logger=my_ruffus_logger)
        print open("/tmp/ruffus.log").read()

        
    .. ???

    The contents of ``/tmp/ruffus.log`` are, as expected::
    
        Task = create_if_necessary
            Description: Create the file if it does not exists
            Job = [null -> "a.1"] completed

=================================
Custom job progress logging
=================================

    It is often useful to log the progress of each job.
    
    Each job runs in a separate process, it is *not* possible to pass the logging object
    between jobs:
    
    #) logging is not synchronised between processes
    #) the logger object can not be "pickled" and sent across processes
        
    The best thing to do is to have a centralised log and to have each job invoke the
    logging methods (e.g. `debug` and `info`) across the process boundaries.
    
    :ref:`This example <sharing-data-across-jobs-example>` shows how this can be coded.
    

        
.. ???

.. index:: cleanup

.. ???

.. _cleanup:

***************************************
Cleaning up
***************************************

To be implemented later!!
See :ref:`todo-combining`
