root/django/branches/schema-evolution-ng/docs/outputting_pdf.txt

===========================
Outputting PDFs with Django
===========================

This document explains how to output PDF files dynamically using Django views.
This is made possible by the excellent, open-source ReportLab_ Python PDF
library.

The advantage of generating PDF files dynamically is that you can create
customized PDFs for different purposes -- say, for different users or different
pieces of content.

For example, Django was used at kusports.com_ to generate customized,
printer-friendly NCAA tournament brackets, as PDF files, for people
participating in a March Madness contest.

.. _ReportLab: http://www.reportlab.org/rl_toolkit.html
.. _kusports.com: http://www.kusports.com/

Install ReportLab
=================

Download and install the ReportLab library from http://www.reportlab.org/downloads.html.
The `user guide`_ (not coincidentally, a PDF file) explains how to install it.

Test your installation by importing it in the Python interactive interpreter::

    >>> import reportlab

If that command doesn't raise any errors, the installation worked.

.. _user guide: http://www.reportlab.org/rsrc/userguide.pdf

Write your view
===============

The key to generating PDFs dynamically with Django is that the ReportLab API
acts on file-like objects, and Django's ``HttpResponse`` objects are file-like
objects.

.. admonition:: Note

    For more information on ``HttpResponse`` objects, see
    `Request and response objects`_.

    .. _Request and response objects: http://www.djangoproject.com/documentation/request_response/

Here's a "Hello World" example::

    from reportlab.pdfgen import canvas
    from django.http import HttpResponse

    def some_view(request):
        # Create the HttpResponse object with the appropriate PDF headers.
        response = HttpResponse(mimetype='application/pdf')
        response['Content-Disposition'] = 'attachment; filename=somefilename.pdf'

        # Create the PDF object, using the response object as its "file."
        p = canvas.Canvas(response)

        # Draw things on the PDF. Here's where the PDF generation happens.
        # See the ReportLab documentation for the full list of functionality.
        p.drawString(100, 100, "Hello world.")

        # Close the PDF object cleanly, and we're done.
        p.showPage()
        p.save()
        return response

The code and comments should be self-explanatory, but a few things deserve a
mention:

    * The response gets a special mimetype, ``application/pdf``. This tells
      browsers that the document is a PDF file, rather than an HTML file. If
      you leave this off, browsers will probably interpret the output as HTML,
      which would result in ugly, scary gobbledygook in the browser window.

    * The response gets an additional ``Content-Disposition`` header, which
      contains the name of the PDF file. This filename is arbitrary: Call it
      whatever you want. It'll be used by browsers in the "Save as..."
      dialogue, etc.

    * The ``Content-Disposition`` header starts with ``'attachment; '`` in this
      example. This forces Web browsers to pop-up a dialog box
      prompting/confirming how to handle the document even if a default is set
      on the machine. If you leave off ``'attachment;'``, browsers will handle
      the PDF using whatever program/plugin they've been configured to use for
      PDFs. Here's what that code would look like::

          response['Content-Disposition'] = 'filename=somefilename.pdf'

    * Hooking into the ReportLab API is easy: Just pass ``response`` as the
      first argument to ``canvas.Canvas``. The ``Canvas`` class expects a
      file-like object, and ``HttpResponse`` objects fit the bill.

    * Note that all subsequent PDF-generation methods are called on the PDF
      object (in this case, ``p``) -- not on ``response``.

    * Finally, it's important to call ``showPage()`` and ``save()`` on the PDF
      file.

Complex PDFs
============

If you're creating a complex PDF document with ReportLab, consider using the
cStringIO_ library as a temporary holding place for your PDF file. The
cStringIO library provides a file-like object interface that is particularly
efficient. Here's the above "Hello World" example rewritten to use
``cStringIO``::

    from cStringIO import StringIO
    from reportlab.pdfgen import canvas
    from django.http import HttpResponse

    def some_view(request):
        # Create the HttpResponse object with the appropriate PDF headers.
        response = HttpResponse(mimetype='application/pdf')
        response['Content-Disposition'] = 'attachment; filename=somefilename.pdf'

        buffer = StringIO()

        # Create the PDF object, using the StringIO object as its "file."
        p = canvas.Canvas(buffer)

        # Draw things on the PDF. Here's where the PDF generation happens.
        # See the ReportLab documentation for the full list of functionality.
        p.drawString(100, 100, "Hello world.")

        # Close the PDF object cleanly.
        p.showPage()
        p.save()

        # Get the value of the StringIO buffer and write it to the response.
        pdf = buffer.getvalue()
        buffer.close()
        response.write(pdf)
        return response

.. _cStringIO: http://www.python.org/doc/current/lib/module-cStringIO.html

Further resources
=================

    * PDFlib_ is another PDF-generation library that has Python bindings. To
      use it with Django, just use the same concepts explained in this article.
    * HTMLdoc_ is a command-line script that can convert HTML to PDF. It
      doesn't have a Python interface, but you can escape out to the shell
      using ``system`` or ``popen`` and retrieve the output in Python.
    * `forge_fdf in Python`_ is a library that fills in PDF forms.

.. _PDFlib: http://www.pdflib.org/
.. _HTMLdoc: http://www.htmldoc.org/
.. _forge_fdf in Python: http://www.accesspdf.com/article.php/20050421092951834