Add magics screenshots.

docs/source/whatsnew/version0.13.txt

@@ -1,583 +1,636 b''
 =============
  0.13 Series
 =============
 
 Release 0.13
 ============
 
 IPython 0.13 contains several major new features, as well as a large amount of
 bug and regression fixes.  The previous version (0.12) was released on December
 19 2011, so this release cycle was roughly 6 months long, during which we
 closed a total of 373 pull requests and 742 issues, with contributions from 62
 authors comprising over 1740 commits.
 
 The amount of work included in this release is so large, that we can only cover
 here the main highlights; please see our :ref:`detailed release statistics
 <issues_list_013>` for links to every issue and pull request closed on GitHub.
 
 
 Major Notebook improvements: new user interface and more
 --------------------------------------------------------
 
 The IPython Notebook, which has proven since its release to be wildly popular,
 has seen a massive amount of work in this release cycle, leading to a
 significantly improved user experience as well as many new features.
 
 The first user-visible change is a reorganization of the user interface; the
 left panel has been removed and was replaced by a real menu system and a
 toolbar with icons.  Both the toolbar and the header above the menu can be
 collapsed to leave an unobstructed working area:
 
 .. image:: ../_static/ipy_013_notebook_spectrogram.png
     :width: 460px
     :alt: New user interface for Notebook
     :align: center
     :target: ../_static/ipy_013_notebook_spectrogram.png
 
 The notebook handles very long outputs much better than before (this was a
 serious usability issue when running processes that generated massive amounts
 of output).  Now, in the presence of outputs longer than ~100 lines, the
 notebook will automatically collapse to a scrollable area and the entire left
 part of this area controls the display: one click in this area will expand the
 output region completely, and a double-click will hide it completely.  This
 figure shows both the scrolled and hidden modes:
 
 .. image:: ../_static/ipy_013_notebook_long_out.png
     :width: 460px
     :alt: Scrolling and hiding of long output in the notebook.
     :align: center
     :target: ../_static/ipy_013_notebook_long_out.png
 
 .. note::
 
    The auto-folding of long outputs is disabled in Firefox due to bugs in its
    scrolling behavior.  See :ghpull:`2047` for details.
 
 Uploading notebooks to the dashboard is now easier: in addition to drag and
 drop (which can be finicky sometimes), you can now click on the upload text and
 use a regular file dialog box to select notebooks to upload. Furthermore, the
 notebook dashboard now auto-refreshes its contents and offers buttons to shut
 down any running kernels (:ghpull:`1739`):
 
 .. image:: ../_static/ipy_013_dashboard.png
     :width: 460px
     :alt: Improved dashboard
     :align: center
     :target: ../_static/ipy_013_dashboard.png
 
 
 Cluster management
 ~~~~~~~~~~~~~~~~~~
 
 The notebook dashboard can now also start and stop clusters, and you can
 override the number of engines started.  There is a new tab in the dashboard
 user interface:
 
 .. image:: ../_static/ipy_013_dashboard_cluster.png
     :width: 460px
     :alt: Cluster management from the notebook dashboard
     :align: center
     :target: ../_static/ipy_013_dashboard_cluster.png
 
 This tab allows you, for each profile you have configured, to start and stop a
 cluster (and optionally override the default number of engines corresponding to
 that configuration).  While this hides all error reporting, once you have a
 configuration that you know works smoothly, it is a very convenient interface
 for controlling your parallel resources.
 
 
 New notebook format
 ~~~~~~~~~~~~~~~~~~~
 
 The notebooks saved now use version 3 of our format, which supports heading
 levels as well as the concept of 'raw' text cells that are not rendered as
 Markdown.  These will be useful with converters_ we are developing, to pass raw
 markup (say LaTeX).  That conversion code is still under heavy development and
 not quite ready for prime time, but we welcome help on this front so that we
 can merge it for full production use as soon as possible.
 
 .. _converters: https://github.com/ipython/nbconvert
 
 .. note::
 
    v3 notebooks can *not* be read by older versions of IPython, but we provide
    a `simple script`_ that you can use in case you need to export a v3
    notebook to share with a v2 user.
 
 .. _simple script: https://gist.github.com/1935808
 
 
 JavaScript refactoring
 ~~~~~~~~~~~~~~~~~~~~~~
   
 All the client-side JavaScript has been decoupled to ease reuse of parts of the
 machinery without having to build a full-blown notebook. This will make it much
 easier to communicate with an IPython kernel from existing web pages and to
 integrate single cells into other sites, without loading the full notebook
 document-like UI. :ghpull:`1711`.
     
 This refactoring also enables the possibility of writing dynamic javascript
 widgets that are returned from Python code and that present an interactive view
 to the user, with callbacks in Javascript executing calls to the Kernel.  This
 will enable many interactive elements to be added by users in notebooks.
 
 An example of this capability has been provided as a proof of concept in
 :file:`docs/examples/widgets` that lets you directly communicate with one or more
 parallel engines, acting as a mini-console for parallel debugging and
 introspection.
 
     
 Improved tooltips
 ~~~~~~~~~~~~~~~~~
 
 The object tooltips have gained some new functionality. By pressing tab several
 times, you can expand them to see more of a docstring, keep them visible as you
 fill in a function's parameters, or transfer the information to the pager at the
 bottom of the screen. For the details, look at the example notebook
 :file:`01_notebook_introduction.ipynb`.
 
 .. figure:: ../_static/ipy_013_notebook_tooltip.png
     :width: 460px
     :alt: Improved tooltips in the notebook.
     :align: center
     :target: ../_static/ipy_013_notebook_tooltip.png
 
     The new notebook tooltips.
 
 Other improvements to the Notebook
 ----------------------------------
    
 * The notebook pager (the area at the bottom) is now resizeable by dragging its
   divider handle, a feature that had been requested many times by just about
   anyone who had used the notebook system.  :ghpull:`1705`.
 
 * If a notebook directory is specified with ``--notebook-dir`` (or with the
   corresponding configuration flag ``NotebookManager.notebook_dir``), all
   kernels start in this directory.
 
 * It is now possible to open notebooks directly from the command line; for
   example: ``ipython notebook path/`` will automatically set ``path/`` as the
   notebook directory, and ``ipython notebook path/foo.ipynb`` will further
   start with the ``foo.ipynb`` notebook opened.  :ghpull:`1686`.
   
 * Fix codemirror clearing of cells with ``Ctrl-Z``; :ghpull:`1965`.
   
 * Text (markdown) cells now line wrap correctly in the notebook, making them
   much easier to edit :ghpull:`1330`.
   
 * PNG and JPEG figures returned from plots can be interactively resized in the
   notebook, by dragging them from their lower left corner. :ghpull:`1832`.
 
 * Clear In[] prompt numbers on "Clear All Output".  For more
   version-control-friendly `.ipynb` files, this strips the `In[]` prompt
   numbers when doing a "Clear all output".  This reduces the amount of noise in
   commit-to-commit diffs that would otherwise show the (highly variable) prompt
   number changes. :ghpull:`1621`.
 
 * The notebook server now requires *two* consecutive ``Ctrl-C`` to stop within 5
   seconds (or an interactive confirmation).  This makes it less likely that you
   will accidentally kill a long-running server by typing ``Ctrl-C`` in the
   wrong terminal.  :ghpull:`1609`.
 
 * Using ``Ctrl-S`` (or ``Cmd-S`` on a Mac) actually saves the notebook rather
   than providing the fairly useless browser html save dialog.  :ghpull:`1334`.
   
 * Allow accessing local files from the notebook (in urls), by serving any local
   file as the url ``files/<relativepath>``.  This makes it possible to, for
   example, embed local images in a notebook.  :ghpull:`1211`.
 
       
 Cell magics
 -----------
 
 We have completely refactored the magic system, finally moving the magic
 objects to standalone, independent objects instead of being the mixin class
 we'd had since the beginning of IPython (:ghpull:`1732`).  Now, a separate base
 class is provided in :class:`IPython.core.magic.Magics` that users can subclass
 to create their own magics.  Decorators are also provided to create magics from
 simple functions without the need for object orientation.
     
 All builtin magics now exist in a few subclasses that group together related
 functionality, and the new :mod:`IPython.core.magics` package has been created
 to organize this into smaller files.
     
 This cleanup was the last major piece of deep refactoring needed from the
 original 2001 codebase.
     
 We have also introduced a new type of magic function, prefixed with `%%`
-instead of `%`, which operates at the cell level.  A cell magic receives two
-arguments: the line it is called on (like a line magic) and the body of the
+instead of `%`, which operates at the whole-cell level.  A cell magic receives
+two arguments: the line it is called on (like a line magic) and the body of the
 cell below it.
     
 Cell magics are most natural in the notebook, but they also work in the
 terminal and qt console, with the usual approach of using a blank line to
 signal cell termination.
     
 For example, to time the execution of several statements::
 
     %%timeit x = 0   # setup
     for i in range(100000):
         x += i**2
 
 This is particularly useful to integrate code in another language, and cell
 magics already exist for shell scripts, Cython, R and Octave. Using ``%%script
 /usr/bin/foo``, you can run a cell in any interpreter that accepts code via
 stdin.
 
 Another handy cell magic makes it easy to write short text files: ``%%file
 ~/save/to/here.txt``.
 
 The following cell magics are now included by default; all those that use
 special interpreters (Perl, Ruby, bash, etc., assume you have the requisite
 interpreter installed):
 
 * ``%%!``: run cell body with the underlying OS shell; this is similar to
   prefixing every line in the cell with ``!``.
   
 * ``%%bash``: run cell body under bash.
   
 * ``%%capture``: capture the output of the code in the cell (and stderr as
   well).  Useful to run codes that produce too much output that you don't even
   want scrolled.
   
 * ``%%file``: save cell body as a file.
   
 * ``%%perl``: run cell body using Perl.
   
 * ``%%prun``: run cell body with profiler (cell extension of ``%prun``).
   
 * ``%%python3``: run cell body using Python 3.
   
 * ``%%ruby``: run cell body using Ruby.
   
 * ``%%script``: run cell body with the script specified in the first line.
   
 * ``%%sh``: run cell body using sh.
   
 * ``%%sx``: capture cell output running the code with the system shell (cell
   extension of ``%sx``).
   
 * ``%%system``: run cell with system shell (``%%!`` is an alias to this).
   
 * ``%%timeit``: time the execution of the cell (extension of ``%timeit``).
 
 This is what some of the script-related magics look like in action:
 
 .. image:: ../_static/ipy_013_notebook_script_cells.png
     :width: 460px
     :alt: Cluster management from the notebook dashboard
     :align: center
     :target: ../_static/ipy_013_notebook_script_cells.png
   
 In addition, we have also a number of :ref:`extensions <extensions_overview>`
 that provide specialized magics.  These typically require additional software
 to run and must be manually loaded via ``%load_ext <extension name>``, but are
-extremely useful.  In particular, we want to highlight a few:
+extremely useful.  The following extensions are provided:
 
+Cython magics (extension :ref:`octavemagic <extensions_cythonmagic>`)
+    This extension provides magics to automatically build and compile Python
+    extension modules using the Cython_ language. You must install Cython
+    separately, as well as a C compiler, for this to work.  The examples
+    directory in the source distribution ships with a full notebook
+    demonstrating these capabilities:
 
+.. image:: ../_static/ipy_013_notebook_cythonmagic.png
+    :width: 460px
+    :alt: Cython magic
+    :align: center
+    :target: ../_static/ipy_013_notebook_cythonmagic.png
+
+.. _cython: http://cython.org
+
+Octave magics (extension :ref:`octavemagic <extensions_octavemagic>`)
+    This extension provides several magics that support calling code written in
+    the Octave_ language for numerical computing.  You can execute single-lines
+    or whole blocks of Octave code, capture both output and figures inline
+    (just like matplotlib plots), and have variables automatically converted
+    between the two languages.  To use this extension, you must have Octave
+    installed as well as the oct2py_ and h5py_ packages.  The examples
+    directory in the source distribution ships with a full notebook
+    demonstrating these capabilities:
+
+.. image:: ../_static/ipy_013_notebook_octavemagic.png
+    :width: 460px
+    :alt: Octave magic
+    :align: center
+    :target: ../_static/ipy_013_notebook_octavemagic.png
+
+.. _octave: http://www.gnu.org/software/octave
+.. _oct2py: http://pypi.python.org/pypi/oct2py
+.. _h5py: http://code.google.com/p/h5py
+
+
+R magics (extension :ref:`rmagic <extensions_rmagic>`)
+    This extension provides several magics that support calling code written in
+    the R_ language for statistical data analysis.  You can execute
+    single-lines or whole blocks of R code, capture both output and figures
+    inline (just like matplotlib plots), and have variables automatically
+    converted between the two languages.  To use this extension, you must have
+    R installed as well as the rpy2_ package that bridges Python and R.  The
+    examples directory in the source distribution ships with a full notebook
+    demonstrating these capabilities:
+
+.. image:: ../_static/ipy_013_notebook_rmagic.png
+    :width: 460px
+    :alt: R magic
+    :align: center
+    :target: ../_static/ipy_013_notebook_rmagic.png
+
+.. _R: http://www.r-project.org
+.. _rpy2: http://rpy.sourceforge.net/rpy2.html
 
 
 Tab completer improvements
 --------------------------
 
 Useful tab-completion based on live inspection of objects is one of the most
 popular features of IPython. To make this process even more user-friendly, the
 completers of both the Qt console and the Notebook have been reworked.
 
 The Qt console comes with a new ncurses-like tab completer, activated by
 default, which lets you cycle through the available completions by pressing tab,
 or select a completion with the arrow keys (:ghpull:`1851`).
 
 .. figure:: ../_static/ipy_013_qtconsole_completer.png
     :width: 460px
     :alt: ncurses-like completer, with highlighted selection.
     :align: center
     :target: ../_static/ipy_013_qtconsole_completer.png
 
     The new improved Qt console's ncurses-like completer allows to easily
     navigate thought long list of completions.
 
 In the notebook, completions are now sourced both from object introspection and
 analysis of surrounding code, so limited completions can be offered for
 variables defined in the current cell, or while the kernel is busy 
 (:ghpull:`1711`).
 
 
 We have implemented a new configurable flag to control tab completion on
 modules that provide the ``__all__`` attribute::
 
   IPCompleter.limit_to__all__= Boolean
 
 This instructs the completer to honor ``__all__`` for the completion.
 Specifically, when completing on ``object.<tab>``, if True: only those names
 in ``obj.__all__`` will be included.  When False [default]: the ``__all__``
 attribute is ignored. :ghpull:`1529`.
 
 
 Improvements to the Qt console
 ------------------------------
 
 * changes for easier integration into other projects such as Spyder_.
 
 * Improved menus with a new Magic menu that is organized by magic groups (this
   was made possible by the reorganization of the magic system
   internals). :ghpull:`1782`.
 
 * Allow for restarting kernels without clearing the qtconsole, while leaving a
   visible indication that the kernel has restarted. :ghpull:`1681`.
   
 * Allow the native display of jpeg image in the qtconsole. :ghpull:`1643`.
 
 .. _spyder: https://code.google.com/p/spyderlib
 
 
   
 Parallel
 --------
 
 The parallel tools have been improved and fine-tuned on multiple fronts.  Now,
 the creation of an :class:`IPython.parallel.Client` object automatically
 activates a line and cell magic function ``px`` that sends its code to all the
 engines. Further magics can be easily created with the :meth:`.Client.activate`
 method, to conveniently execute code on any subset of engines. :ghpull:`1893`.
 
 The ``%%px`` cell magic can also be given an optional targets argument, as well
 as a ``--out`` argument for storing its output.
 
 A new magic has also been added, ``%pxconfig``, that lets you configure various
 defaults of the parallel magics.  As usual, type  ``%pxconfig?`` for details.
 
 The exception reporting in parallel contexts has been improved to be easier to
 read.  Now, IPython directly reports the remote exceptions without showing any
 of the internal execution parts:
 
 .. image::  ../_static/ipy_013_par_tb.png
     :width: 460px
     :alt: Improved parallel exceptions.
     :align: center
     :target: ../_static/ipy_013_par_tb.png
 
 
 The parallel tools now default to using ``NoDB`` as the storage backend for
 intermediate results.  This means that the default usage case will have a
 significantly reduced memory footprint, though certain advanced features are
 not available with this backend.  For more details, see :ref:`parallel_db`.
 
 The parallel magics now display all output, so you can do parallel plotting or
 other actions with complex display.  The ``px`` magic has now both line and cell
 modes, and in cell mode finer control has been added about how to collate
 output from multiple engines. :ghpull:`1768`.
 
 Incremental improvements to SSH launchers:
     
 * add to_send/fetch steps for moving connection files around.
   
 * add SSHProxyEngineSetLauncher, for invoking to `ipcluster engines` on a
   remote host. This can be used to start a set of engines via PBS/SGE/MPI
   *remotely*.
     
 This makes the SSHLauncher usable on machines without shared filesystems.
     
 When sending files, the destination directory must *already exist* - that is,
 `ipython profile create` may be necessary on the remote system, before the
 security dir exists for putting the connection file the first
 time. :ghpull:`1634`.
 
 Add sugar methods/properties to AsyncResult that are generically useful
 (:ghpull:`1548`):
     
     * ``ar.wall_time`` = received - submitted
     * ``ar.serial_time`` = sum of serial computation time
     * ``ar.elapsed`` = time since submission (wall_time if done)
     * ``ar.progress`` = (int) number of sub-tasks that have completed
     * ``len(ar)`` = # of tasks
     * ``ar.wait_interactive()``: prints progress
     
 Added :meth:`.Client.spin_thread` / :meth:`~.Client.stop_spin_thread` for running
 spin in a background thread, to keep zmq queue clear.  This can be used to
 ensure that timing information is as accurate as possible (at the cost of
 having a background thread active).
 
 Set TaskScheduler.hwm default to 1 instead of 0.  1 has more
 predictable/intuitive behavior, if often slower, and thus a more logical
 default.  Users whose workloads require maximum throughput and are largely
 homogeneous in time per task can make the optimization themselves, but now the
 behavior will be less surprising to new users. :ghpull:`1294`.
 
 
 Kernel/Engine unification
 -------------------------
 
 :ghpull:`1640`
 
 Add :func:`IPython.embed_kernel()` as a public API.
 Embedding an IPython kernel in an application is useful when you want to use
 IPython.embed() but don't have a terminal attached on stdin and stdout.
 
 :func:`IPython.parallel.bind_kernel` allows you to promote Engines to listening Kernels,
 and connect QtConsoles directly to an Engine and debug it directly.
 
 This also means that Engines are now fully IPython, allowing access to magics,
 etc. in your parallel execution.
 
 
 Official Public API
 -------------------
 
 We have begun organizing our API for easier public use, with an eye towards an
 official IPython 1.0 release which will firmly maintain this API compatible for
 its entire lifecycle.  There is now an :mod:`IPython.display` module that
 aggregates all display routines, and the :mod:`IPython.config` namespace has
 all public configuration tools.  We will continue improving our public API
 layout so that users only need to import names one level deeper than the main
 ``IPython`` package to access all public namespaces.
 
 
 IPython notebook file icons
 ---------------------------
 
 The directory ``docs/resources`` in the source distribution contains SVG and
 PNG versions of our file icons, as well as an ``Info.plist.example`` file with
 instructions to install them on Mac OSX.  This is a first draft of our icons,
 and we encourage contributions from users with graphic talent to improve them
 in the future:
 
 .. image:: ../../resources/ipynb_icon_128x128.png
    :alt:  IPython notebook file icon.
 
 	  
 New top-level `locate` command
 ------------------------------
 
 Add `locate` entry points; these would be useful for quickly locating IPython
 directories and profiles from other (non-Python) applications. :ghpull:`1762`.
     
 Examples::
     
     $> ipython locate
     /Users/me/.ipython
   
     $> ipython locate profile foo
     /Users/me/.ipython/profile_foo
   
     $> ipython locate profile
     /Users/me/.ipython/profile_default
   
     $> ipython locate profile dne
     [ProfileLocate] Profile u'dne' not found.
 
 	
 Other new features and improvements
 -----------------------------------
 
 * **%install_ext**: A new magic function to install an IPython extension from
   a URL. E.g. ``%install_ext
   https://bitbucket.org/birkenfeld/ipython-physics/raw/default/physics.py``.
 
 * The ``%loadpy`` magic is no longer restricted to Python files, and has been
   renamed ``%load``. The old name remains as an alias.
 
 * New command line arguments will help external programs find IPython folders:
   ``ipython locate`` finds the user's IPython directory, and ``ipython locate
   profile foo`` finds the folder for the 'foo' profile (if it exists).
 
 * The :envvar:`IPYTHON_DIR` environment variable, introduced in the Great
   Reorganization of 0.11 and existing only in versions 0.11-0.13, has been
   deprecated. As described in :ghpull:`1167`, the complexity and confusion of
   migrating to this variable is not worth the aesthetic improvement. Please use
   the historical :envvar:`IPYTHONDIR` environment variable instead.
 
 * The default value of *interactivity* passed from
   :meth:`~IPython.core.interactiveshell.InteractiveShell.run_cell` to
   :meth:`~IPython.core.interactiveshell.InteractiveShell.run_ast_nodes`
   is now configurable.
 
 * New ``%alias_magic`` function to conveniently create aliases of existing
   magics, if you prefer to have shorter names for personal use.
 
 * We ship unminified versions of the JavaScript libraries we use, to better
   comply with Debian's packaging policies.
 
 * Simplify the information presented by ``obj?/obj??`` to eliminate a few
   redundant fields when possible.  :ghpull:`2038`.
 
 * Improved continuous integration for IPython.  We now have automated test runs
   on `Shining Panda <https://jenkins.shiningpanda.com/ipython>`_ and `Travis-CI
   <http://travis-ci.org/#!/ipython/ipython>`_, as well as `Tox support
   <http://tox.testrun.org>`_.
 
 * The `vim-ipython`_ functionality (externally developed) has been updated to
   the latest version.
 
 .. _vim-ipython: https://github.com/ivanov/vim-ipython
 
 * The ``%save`` magic now has a ``-f`` flag to force overwriting, which makes
   it much more usable in the notebook where it is not possible to reply to
   interactive questions from the kernel. :ghpull:`1937`.
 
 * Use dvipng to format sympy.Matrix, enabling display of matrices in the Qt
   console with the sympy printing extension. :ghpull:`1861`.
 
 * Our messaging protocol now has a reasonable test suite, helping ensure that
   we don't accidentally deviate from the spec and possibly break third-party
   applications that may have been using it.  We encourage users to contribute
   more stringent tests to this part of the test suite.  :ghpull:`1627`.
 
 * Use LaTeX to display, on output, various built-in types with the SymPy
   printing extension. :ghpull:`1399`.
 
 * Add Gtk3 event loop integration and example. :ghpull:`1588`.
 
 * ``clear_output`` improvements, which allow things like progress bars and other
   simple animations to work well in the notebook (:ghpull:`1563`):
     
     * `clear_output()` clears the line, even in terminal IPython, the QtConsole
       and plain Python as well, by printing `\r` to streams.
     
     * `clear_output()` avoids the flicker in the notebook by adding a delay,
       and firing immediately upon the next actual display message.
     
     * `display_javascript` hides its `output_area` element, so using display to
       run a bunch of javascript doesn't result in ever-growing vertical space.
 
 * Add simple support for running inside a virtualenv.  While this doesn't
   supplant proper installation (as users should do), it helps ad-hoc calling of
   IPython from inside a virtualenv. :ghpull:`1388`.
 
   
 Major Bugs fixed
 ----------------
 
 In this cycle, we have :ref:`closed over 740 issues <issues_list_013>`, but a
 few major ones merit special mention:
 
 * The ``%pastebin`` magic has been updated to point to gist.github.com, since
   unfortunately http://paste.pocoo.org has closed down. We also added a -d flag
   for the user to provide a gist description string. :ghpull:`1670`.
 
 * Fix ``%paste`` that would reject certain valid inputs. :ghpull:`1258`.
 
 * Fix sending and receiving of Numpy structured arrays (those with composite
   dtypes, often used as recarrays). :ghpull:`2034`.
 
 * Reconnect when the websocket connection closes unexpectedly. :ghpull:`1577`.
 
 * Fix truncated representation of objects in the debugger by showing at least
   80 characters' worth of information.  :ghpull:`1793`.
 
 * Fix logger to be Unicode-aware: logging could crash ipython if there was
   unicode in the input. :ghpull:`1792`.
 
 * Fix images missing from XML/SVG export in the Qt console. :ghpull:`1449`.
 
 * Fix deepreload on Python 3. :ghpull:`1625`, as well as having a much cleaner
   and more robust implementation of deepreload in general. :ghpull:`1457`.
 
 
 Backwards incompatible changes
 ------------------------------
 
 * The exception :exc:`IPython.core.error.TryNext` previously accepted
   arguments and keyword arguments to be passed to the next implementation
   of the hook. This feature was removed as it made error message propagation
   difficult and violated the principle of loose coupling.