Rewrote introduction. Introduced concept of *notebook space* and discussion of why the same word 'notebook' is used both for the notebook space and the corresponding notebook document

docs/source/interactive/htmlnotebook.rst

@@ -1,573 +1,568 b''
 .. _htmlnotebook:
 
 The IPython Notebook
 ====================
 
 .. seealso::
 
     :ref:`Installation requirements <installnotebook>` for the Notebook.
 
 The IPython Notebook combines two components:
 
-* A web application, called the *IPython Notebook web app*, for interactive 
-authoring of literate computations, in which explanatory text, mathematics,
-computations and rich media output may be combined. Input and output are stored in persistent cells that may be edited in-place.
+* **The *IPython Notebook* web application**:
 
-* Plain text documents, called *notebook documents*, or *notebooks*, for recording and distributing the results of the rich computations.
+  The IPython Notebook web app is a browser-based tool for interactive authoring of literate computations, in which explanatory text, mathematics,computations and rich media output may be combined. Input and output are stored in persistent cells that may be edited in-place.
 
-In the documentation, the distinction between the *N*otebook app and *n*otebook documents is made by capitalization.
+* **Notebook documents**:
 
-The Notebook app automatically saves, at certain intervals, the current state of the computational process occurring in the web browser to the corresponding notebook document.
+  *Notebook documents*, or *notebooks*, are plain text documents which record all inputs and outputs of the computations, interspersed with text, mathematics and HTML 5 representations of objects, in a literate style.
 
-Note that it is also common to refer to the current state of the computation, 
-as represented by the sequence of input cells in the Notebook app, as a 
-*notebook*. There is no problem with confounding these two concepts, since 
-there is actually a one-to-one correspondence between what you see on the
-screen inside the app, and what is stored in the corresponding ``.ipynb``
-notebook document.
+Since the similarity in names can lead to some confusion, in the documentation, we will always use the typographical distinction between the *N*otebook app and *n*otebook documents via the respective capitalization. Here we are thinking of the Notebook app as being a proper noun. We will also always refer to the "Notebook app" when we are referring to the browser-based interface, to increase clarity.
 
+We refer to the current state of the computational process taking place in the Notebook app, i.e. the (numbered) sequence of input and output cells, as the 
+*notebook space*. Notebook documents provide an *exact*, *one-to-one* record of all the content in the notebook space, and the Notebook app automatically saves, at certain intervals, the contents of the notebook space to a notebook document stored on disk, with the same name as the title of the notebook space, and the file extension ".ipynb". For this reason, there is no confusion about using the same name "notebook" for both the notebook space and the corresonding notebook document.
 
 Features of the IPython Notebook web app
 ----------------------------------------
 
 Some of the main features of the IPython Notebook app include:
 
 * In-browser editing, syntax highlighting, tab completion and autoindentation for code.
 * Mix code with rich text using the Markdown markup language.
 * Include mathematical equations using LaTeX notation, rendered directly in the browser by MathJax.
 * Display rich data representations (e.g. HTML / LaTeX / SVG) as the result of computations.
 * Include inline figures rendered by the ``matplotlib`` library with publication quality, in a range of formats (SVG / PDF / PNG).
 
 If you have ever used the Mathematica or SAGE notebooks (the latter is also
 web-based__) you should feel right at home.  If you have not, you will be
 able to learn how to use the IPython Notebook in just a few minutes.
 
 .. __: http://sagenb.org
 
 
 Notebook documents
 ------------------
 
 Notebook document files are just  standard text files with the extension 
 ``.ipynb``, stored in the working directory on your computer. This file can be easily put under version control and shared with colleagues.
 
 Despite the fact that the notebook documents are plain text files, they use 
 the JSON format in order to store a *complete*, *reproducible*, *one-to-one* copy of the state of the computational state as it is inside the Notebook app. 
 All computations carried out, and the corresponding results obtained, are combined in a literate way, mixing them  with descriptive text, mathematics, and HTML 5 representations of objects.
 
 Notebooks may easily be exported to a range of static formats, including 
 HTML (for example, for blog posts), PDF and slide shows.
 Furthermore, any publicly available notebook may be shared via the 
 `IPython Notebook Viewer <http://nbviewer.ipython.org>`_ service, which will 
 provide it as a static web page. The results may thus be shared without having to install anything.
 
 See :ref:`our installation documentation <install_index>` for directions on
 how to install the notebook and its dependencies.
 
 .. note::
 
    You can start more than one notebook server at the same time, if you want to
    work on notebooks in different directories.  By default the first notebook
    server starts on port 8888, and later notebook servers search for  ports near
    that one.  You can also manually specify the port with the ``--port``
    option.
    
 
 Starting up the IPython Notebook web app
 ----------------------------------------
 
 The Notebook web app is started with the command::
 
     $ ipython notebook
 
 The landing page of the notebook server application, the *dashboard*, shows the notebooks currently available in the *working directory* (the directory from which the notebook was started).
 You can create new notebooks from the dashboard with the ``New Notebook``
 button, or open existing ones by clicking on their name.
 You can also drag and drop ``.ipynb`` notebooks and standard ``.py`` Python source code files into the notebook list area.
 
  ``.py`` files will be imported into the IPython Notebook as a notebook with the same name, but an ``.ipynb`` extension, located in the working directory.  The notebook will consist of a single cell containing all the 
  code in the ``.py`` file, which you can later manually partition into individual cells. 
 
  .. Alternatively, prior to importing the ``.py``, you can manually add ``# <nbformat>2</nbformat>`` at the start of the file, and then add separators for text and code cells, to get a cleaner import with the file already broken into individual cells.
 
 
 The IPython Notebook web app is based on a server-client structure. 
 This server uses a two-process kernel architecture based on ZeroMQ, as well as Tornado for serving HTTP requests. Other clients may connect to the same underlying IPython kernel.
 
 
 When you open or create a new notebook, your browser tab will reflect the name of that notebook, prefixed with "IPy".
 The URL is currently not meant to be human-readable and is not persistent across invocations of the notebook server; however, this will change in a future version of IPython.
 
 
 Notebook user interface
 -----------------------
 
 When you finally start editing a notebook document in the Notebook, you will be presented with the title of the notebook, a *menu bar*, a *toolbar* and an empty *input cell*.
 
 Notebook title
 ~~~~~~~~~~~~~~
 The title of the notebook document that is currently being edited is displayed at the top of the page, next to the ``IP[y]: Notebook`` logo. This title may be edited directly by clicking on it. The title is reflected in the name of the ``.ipynb`` notebook document file that is saved.
 
 Menu bar
 ~~~~~~~~
 The menu bar presents different options that may be used to manipulate the way the Notebook functions.
 
 Toolbar
 ~~~~~~~
 The tool bar gives handy icons for the most-used operations within the Notebook.
 
 
 Input cells
 -----------
 Input cells are the core of the functionality of the IPython Notebook.
 They are regions in the document where you can enter different types of text and commands. These regions are then executed using :kbd:`Shift-Enter`, at which point the Notebook executes the current input cell, displays the resulting output beneath it, and adds a new input cell below.
 
 The notebook consists of a sequence of input cells, 
 providing the means to direct the computational process.
 
 
 Basic workflow
 --------------
 The normal workflow in a notebook is, then, quite similar to a standard IPython session, with the difference that you can edit cells in-place multiple 
 times until you obtain the desired results, rather than having to 
 rerun separate scripts with the ``%run`` magic command. (Magic commands do, however, also work in the notebook; see below).   Typically, you'll work on a problem in pieces, 
 organizing related pieces into cells and moving forward as previous 
 parts work correctly.  This is much more convenient for interactive exploration than breaking up a computation into scripts that must be 
 executed together, especially if parts of them take a long time to run
 
 The only significant limitation that the notebook currently has, compared to the Qt console, is that it cannot run any code that 
 expects input from the kernel (such as scripts that call 
 :func:`raw_input`).  Very importantly, this means that the ``%debug`` 
 magic does *not* currently work in the notebook!  This limitation will 
 be overcome in the future, but in the meantime, there is a way to debug problems in the notebook: you can attach a Qt console to your existing notebook kernel, and run ``%debug`` from the Qt console.  
 If your notebook is running on a local
 computer (i.e. if you are accessing it via your localhost address at ``127.0.0.1``), you can just type ``%qtconsole`` in the notebook and a Qt console will open up, connected to that same kernel.
 
 At certain moments, it may be necessary to interrupt a particularly long calculation, or even to kill the entire computational process. This may be achieved by interrupting or restarting the kernel, respectively.
 After a restart, all relevant cells must be re-evaluated
 
 
 A notebook may be downloaded in either ``.ipynb`` or raw ``.py`` form from the menu option ``File -> Download as``
 Choosing the ``.py`` option removes all output and saves the text cells
 in comment areas.  See ref:`below <notebook_format>` for more details on the
 notebook format.
 
     
 .. warning::
 
    While in simple cases you can "roundtrip" a notebook to Python, edit the
    Python file, and then import it back without loss of main content, this is in general *not guaranteed to work*.  First, there is extra metadata
    saved in the notebook that may not be saved to the ``.py`` format.  And as
    the notebook format evolves in complexity, there will be attributes of the
    notebook that will not survive a roundtrip through the Python form.  You
    should think of the Python format as a way to output a script version of a
    notebook and the import capabilities as a way to load existing code to get a
    notebook started.  But the Python version is *not* an alternate notebook
    format.
 
 
 Keyboard shortcuts
 ------------------
 All actions in the notebook can be achieved with the mouse, but we have also
 added keyboard shortcuts for the most common ones, so that productive use of
 the notebook can be achieved with minimal mouse intervention.  The main
 key bindings you need to remember are:
 
 * :kbd:`Shift-Enter`: 
   execute the current cell, show output (if any), and jump 
   to the next cell below. If :kbd:`Shift-Enter` 
   was invoked on the last input line, a new code cell will also be created. Note that in the notebook, simply using :kbd:`Enter` *never* forces execution, it simply inserts a new line in the current cell. Therefore, in the notebook you must always use :kbd:`Shift-Enter` to get execution (or use the mouse and click on the ``Run Selected`` button).
 
 * :kbd:`Alt-Enter`: 
   this combination is similar to the previous one, with the 
   exception that, if the next cell below is not empty, a new code cell will be 
   added to the notebook, even if the cell execution happens not in the last cell. :kbd:`Alt-Enter`: is a shortcut for the sequence :kbd:`Shift-Enter`, :kbd:`Ctrl-m a`.
   
 * :kbd:`Ctrl-Enter`: 
   execute the current cell in "terminal mode", where any
   output is shown, but the cursor remains in the current cell. This is convenient to do quick in-place experiments, or query things like filesystem content, without creating additional cells that you may not want saved in your notebook.
 
 * :kbd:`Ctrl-m`: 
   this is the prefix for all other keybindings, which consist of an additional single letter or character.  Type :kbd:`Ctrl-m h` (that is, the sole letter
    :kbd:`h` after :kbd:`Ctrl-m`) and IPython will show you the remaining available keybindings.
 
 
 
    
 
 Cell types
 ----------
 Each IPython input cell has a *cell type*.
 There is a restricted number of possible cell types, which may be set by using the cell type dropdown on the toolbar, or via the following keyboard shortcuts:
 
 * **code**: :kbd:`Ctrl-m y`
 * **markdown**: :kbd:`Ctrl-m m`
 * **raw**: :kbd:`Ctrl-m t`
 * **heading**: :kbd:`Ctrl-m 1` - :kbd:`Ctrl-m 6`
 
 
 Code cells
 ~~~~~~~~~~
 Code cells contain code written in some computer language, which is Python by default. When the cell is executed with :kbd:`Shift-Enter`, this code is executed, and the result returned by Python (or the corresponding language) after running the code will be displayed as its output.
 
 Code may be edited inline in the cell, with full syntax highlighting.
 
 
 Rich text using Markdown
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The computational process may be documented in a literate way using rich text. 
 For this purpose, the Notebook provides markdown cells. Text is entered using Markdown_ syntax, allowing for italics, bold, ordered and unordered lists, etc. This is rendered using Markdown syntax to a rich HTML representation when the cell is executed. In this case, the output *replaces* the input cell.
 
 Within markdown cells, mathematics can be included in a straightforward manner using LaTeX notation: ``$...$`` for inline math and ``$$...$$`` for displayed math. Standard LaTeX environments, such as ``\begin{equation}...\end{equation}``, also work. New commands may be defined using standard LaTeX commands, placed anywhere in a markdown cell.
 
 Raw cells
 ~~~~~~~~~
 Raw cells provide a place to put additional information which is not evaluated by the Notebook. This can be used, for example, for extra information to be used when the notebook is exported to a certain format.
 
 
 Magic commands
 --------------
 Magic commands, or *magics*, are one-word commands beginning with the symbol ``%``, which send commands to IPython itself (as opposed to standard Python commands which are exported to be run in a Python interpreter).
 
 Magics control different elements of the way that the IPython notebook operates. They are entered into standard code cells and executed as usual with :kbd:`Shift-Enter`.
 
 There are two types of magics: *line magics*, which begin with a single ``%`` and operate on a single line of the code cell; and *cell magics*, which begin with ``%%`` and operate on the entire contents of the cell.
 
 Line magics
 ˜˜˜˜˜˜˜˜˜˜˜
 Some of the available line magics are the following:
 
 * ``%load``:
   Loads a file and places its content into a new code cell.
 
 * ``%timeit``:
   A simple way to time how long a single line of code takes to run
 
 * ``%config``:
   Configuration of the IPython Notebook
 
 * ``%lsmagic``:
   Provides a list of all available magic commands
 
 Cell magics
 ˜˜˜˜˜˜˜˜˜˜˜
 
 * ``%%bash``:
   Send the contents of the code cell to be executed by ``bash``
 
 * ``%%file``:
   Writes a file with with contents of the cell. *Caution*: The file is ovewritten without asking.
 
 * ``%%R``:
   Execute the contents of the cell using the R language.
 
 * ``%%cython``:
   Execute the contents of the cell using ``Cython``.
   
 
 
 Plotting
 --------
 One major feature of the Notebook is the ability to capture the result of plots as inline output. IPython is designed to work seamlessly together with
 the ``%matplotlib`` plotting library. In order to set this up, the 
 ``%matplotlib`` magic command must be run before any plotting takes place.
 
 Note that ``%matplotlib`` only sets up IPython to work correctly with ``matplotlib``; it does not actually execute any ``import`` commands and does not add anything to the namespace.
 
 There is an alternative magic, ``%pylab``, which, in addition, also executes a sequence of standard ``import`` statements required for working with the 
 ``%matplotlib`` library. In particular, it automatically imports all names in the ``numpy`` and ``matplotlib`` packages to the namespace. A less invasive solution is ``%pylab --no-import-all``, which imports just the standard names 
 ``np`` for the ``numpy`` module and ``plt`` for the ``matplotlib.pyplot`` module.
 
 When the default ``%matplotlib`` or ``%pylab`` magics are used, the output of a plotting command is captured in a *separate* window. An alternative is to use::
   ``%matplotlib inline``
 which captures the output inline within the notebook format. This has the benefit that the resulting plots will be stored in the notebook document.
 
 
 Converting notebooks to other formats
 -------------------------------------
 Newly added in the 1.0 release of IPython is the ``nbconvert`` tool to convert a notebook document into another static format. This is a command line tool; at present, this functionality is not available to export directly from within the Notebook app. The syntax is::
 
   $ ipython nbconvert notebook.ipynb
 
 for standard HTML output, or::
 
   $ ipython nbconvert --format=FORMAT notebook.ipynb
 
 where ``FORMAT`` is the desired export format. Options for this format include:
 
 * ``full_html``:
   Standard HTML
 
 * ``simple_html``:
   A simplified version of HTML
 
 * ``reveal``:
   A format to be used with the ``reveal.js`` package for slideshow presentations.
 
 * ``sphinx_howto``:
   A standard documentation format.
 
 * ``latex``:
   Produces LaTeX output which may be compiled with ``pdflatex`` to PDF.
 
 
 Configuration
 -------------
 The IPython Notebook can be run with a variety of command line arguments. 
 To see a list of available options enter::
 
   $ ipython notebook --help 
 
 Defaults for these options can also be set by creating a file named 
 ``ipython_notebook_config.py`` in your IPython *profile folder*. The profile folder is a subfolder of your IPython directory; ``ipython locate`` will show you where it is located. 
 
 To create a new set of default configuration files, with lots of information on available options, use::
 
   $ ipython profile create
 
 .. seealso:
 
     :ref:`config_overview`, in particular :ref:`Profiles`.
 
 
 Extracting standard Python files from notebooks
 -----------------------------------------------
 
 The native format of the notebook, a file with a ``.ipynb`` `extension, is a
 JSON container of all the input and output of the notebook, and therefore not
 valid Python by itself.  This means that by default, you cannot directly 
 import a notebook from Python, nor execute it as a normal python script.  
 
 But if you want to be able to use notebooks also as regular Python files, you can start the notebook server with::
 
   ipython notebook --script
 
 or you can set this option permanently in your configuration file with::
 
     c.NotebookManager.save_script=True
 
 This will instruct the notebook server to save the ``.py`` export of each
 notebook, in addition to the ``.ipynb``, at every save.  These are standard 
 ``.py`` files, and so they can be ``%run``, imported from regular IPython 
 sessions or other notebooks, or executed at the command line.  Since we export 
 the raw code you have typed, for these files to be importable from other code, 
 you will have to avoid using syntax such as ``%magic``s and other IPython-specific extensions to the language.
 
 In regular practice, the standard way to differentiate importable code from the
 'executable' part of a script is to put at the bottom::
 
   if __name__ == '__main__':
     # rest of the code...
 
 Since all cells in the notebook are run as top-level code, you will need to
 similarly protect *all* cells that you do not want executed when other scripts
 try to import your notebook.  A convenient shortand for this is to define early
 on::
 
   script = __name__ == '__main__'
 
 and then on any cell that you need to protect, use::
 
   if script:
     # rest of the cell...
 
 
 .. _notebook_security:
 
 Security
 --------
 
 You can protect your Notebook server with a simple singlepassword by
 setting the :attr:`NotebookApp.password` configurable. You can prepare a
 hashed password using the function :func:`IPython.lib.security.passwd`:
 
 .. sourcecode:: ipython
 
     In [1]: from IPython.lib import passwd
     In [2]: passwd()
     Enter password: 
     Verify password: 
     Out[2]: 'sha1:67c9e60bb8b6:9ffede0825894254b2e042ea597d771089e11aed'
     
 .. note::
 
   :func:`~IPython.lib.security.passwd` can also take the password as a string
   argument. **Do not** pass it as an argument inside an IPython session, as it
   will be saved in your input history.
 
 You can then add this to your :file:`ipython_notebook_config.py`, e.g.::
 
     # Password to use for web authentication
     c.NotebookApp.password = u'sha1:67c9e60bb8b6:9ffede0825894254b2e042ea597d771089e11aed'
 
 When using a password, it is a good idea to also use SSL, so that your password
 is not sent unencrypted by your browser. You can start the notebook to
 communicate via a secure protocol mode using a self-signed certificate with the command::
 
     $ ipython notebook --certfile=mycert.pem
 
 .. note::
 
     A self-signed certificate can be generated with ``openssl``.  For example, the following command will create a certificate valid for 365 days with both the key and certificate data written to the same file::
 
         $ openssl req -x509 -nodes -days 365 -newkey rsa:1024 -keyout mycert.pem -out mycert.pem
 
 Your browser will warn you of a dangerous certificate because it is
 self-signed.  If you want to have a fully compliant certificate that will not
 raise warnings, it is possible (but rather involved) to obtain one for free,
 `as explained in detailed in this tutorial`__.
 
 .. __: http://arstechnica.com/security/news/2009/12/how-to-get-set-with-a-secure-sertificate-for-free.ars
 	
 Keep in mind that when you enable SSL support, you'll need to access the
 notebook server over ``https://``, not over plain ``http://``.  The startup
 message from the server prints this, but it's easy to overlook and think the
 server is for some reason non-responsive.
 
 
 Connecting to an existing kernel
 ---------------------------------
 
 The notebook server always prints to the terminal the full details of 
 how to connect to each kernel, with lines like::
 
     [IPKernelApp] To connect another client to this kernel, use:
     [IPKernelApp] --existing kernel-3bb93edd-6b5a-455c-99c8-3b658f45dde5.json
 
 This is the name of a JSON file that contains all the port and 
 validation information necessary to connect to the kernel.  You can 
 manually start a Qt console with::
 
     ipython qtconsole --existing kernel-3bb93edd-6b5a-455c-99c8-3b658f45dde5.json
 
 and if you only have a single kernel running, simply typing::
 
     ipython qtconsole --existing
 
 will automatically find it (it will always find the most recently 
 started kernel if there is more than one).  You can also request this 
 connection data by typing ``%connect_info``; this will print the same 
 file information as well as the content of the JSON data structure it contains.
 
 
 Running a public notebook server
 --------------------------------
 
 If you want to access your notebook server remotely with just a web browser,
 here is a quick set of instructions.  Start by creating a certificate file and
 a hashed password as explained above.  Then, create a custom profile for the
 notebook.  At the command line, type::
 
   ipython profile create nbserver
 
 In the profile directory, edit the file ``ipython_notebook_config.py``.  By
 default the file has all fields commented, the minimum set you need to
 uncomment and edit is here::
 
      c = get_config()
 
      # Kernel config
      c.IPKernelApp.pylab = 'inline'  # if you want plotting support always
 
      # Notebook config
      c.NotebookApp.certfile = u'/absolute/path/to/your/certificate/mycert.pem'
      c.NotebookApp.ip = '*'
      c.NotebookApp.open_browser = False
      c.NotebookApp.password = u'sha1:bcd259ccf...your hashed password here'
      # It's a good idea to put it on a known, fixed port
      c.NotebookApp.port = 9999
 
 You can then start the notebook and access it later by pointing your browser to
 ``https://your.host.com:9999`` with ``ipython notebook --profile=nbserver``.
 
 Running with a different URL prefix
 -----------------------------------
 
 The notebook dashboard (i.e. the default landing page with an overview
 of all your notebooks) typically lives at a URL path of
 "http://localhost:8888/". If you want to have it, and the rest of the
 notebook, live under a sub-directory,
 e.g. "http://localhost:8888/ipython/", you can do so with
 configuration options like these (see above for instructions about
 modifying ``ipython_notebook_config.py``)::
 
     c.NotebookApp.base_project_url = '/ipython/'
     c.NotebookApp.base_kernel_url = '/ipython/'
     c.NotebookApp.webapp_settings = {'static_url_prefix':'/ipython/static/'}
 
 Using a different notebook store
 --------------------------------
 
 By default the notebook server stores notebooks as files in the working 
 directory of the notebook server, also known as the ``notebook_dir``. This 
 logic is implemented in the :class:`FileNotebookManager` class. However, the 
 server can be configured to use a different notebook manager class, which can 
 store the notebooks in a different format. Currently, we ship a 
 :class:`AzureNotebookManager` class that stores notebooks in Azure blob 
 storage. This can be used by adding the following lines to your 
 ``ipython_notebook_config.py`` file::
 
     c.NotebookApp.notebook_manager_class = 'IPython.html.services.notebooks.azurenbmanager.AzureNotebookManager'
     c.AzureNotebookManager.account_name = u'paste_your_account_name_here'
     c.AzureNotebookManager.account_key = u'paste_your_account_key_here'
     c.AzureNotebookManager.container = u'notebooks'
 
 In addition to providing your Azure Blob Storage account name and key, you will 
 have to provide a container name; you can use multiple containers to organize 
 your Notebooks.
 
 .. _notebook_format:
 
 Notebook JSON format
 ====================
 
 Notebooks are JSON files with an ``.ipynb`` extension, formatted
 as legibly as possible with minimal extra indentation and cell content broken
 across lines to make them reasonably friendly to use in version-control
 workflows.  You should be very careful if you ever manually edit this JSON
 data, as it is extremely easy to corrupt its internal structure and make the
 file impossible to load.  In general, you should consider the notebook as a
 file meant only to be edited by the IPython Notebook app itself, not for hand-editing.
 
 .. note::
 
      Binary data such as figures are directly saved in the JSON file.  This
      provides convenient single-file portability, but means that the files can 
      be large; ``diff``s of binary data also are not very meaningful.  Since the 
      binary blobs are encoded in a single line, they affect only one line of 
      the ``diff`` output, but they are typically very long lines.  You can use the ``Cell -> All Output -> Clear`` menu option to remove all output from a notebook prior to committing it to version control, if this is a concern.
 
 The notebook server can also generate a pure Python version of your notebook, 
 using the ``File -> Download as`` menu option. The resulting ``.py`` file will 
 contain all the code cells from your notebook verbatim, and all text cells 
 prepended with a comment marker.  The separation between code and text
 cells is indicated with special comments and there is a header indicating the
 format version.  All output is stripped out when exporting to Python.
 
 Here is an example of the Python output from a simple notebook with one text cell and one code input cell::
 
     # <nbformat>2</nbformat>
 
     # <markdowncell>
 
     # A text cell
 
     # <codecell>
 
     print "Hello, IPython!"
 
 
 Known issues
 ============
 
 When behind a proxy, especially if your system or browser is set to autodetect
 the proxy, the Notebook app might fail to connect to the server's websockets,
 and present you with a warning at startup. In this case, you need to configure
 your system not to use the proxy for the server's address.
 
 In Firefox, for example, go to the Preferences panel, Advanced section,
 Network tab, click 'Settings...', and add the address of the notebook server
 to the 'No proxy for' field.
 
     
 .. _Markdown: http://daringfireball.net/projects/markdown/basics