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<H1><A NAME="SECTION005300000000000000000"></A><A NAME="tut-what-it-do"></A>
<BR>
3.3 So what Exactly does Mod-python do?
</H1>
<P>
Let's pretend we have the following configuration:
<div class="verbatim"><pre>
<Directory /mywebdir>
AddHandler mod_python .py
PythonHandler myscript
PythonDebug On
</Directory>
</pre></div>
<P>
<strong>NB:</strong> <span class="file">/mywebdir</span> is an absolute physical path.
<P>
And let's say that we have a python program (Windows users: substitute
forward slashes for backslashes) <span class="file">/mywedir/myscript.py</span> that looks like
this:
<P>
<div class="verbatim"><pre>
from mod_python import apache
def handler(req):
req.content_type = "text/plain"
req.write("Hello World!")
return apache.OK
</pre></div>
<P>
Here is what's going to happen: The <code>AddHandler</code> directive tells
Apache that any request for any file ending with <span class="file">.py</span> in the
<span class="file">/mywebdir</span> directory or a subdirectory thereof needs to be
processed by mod_python. The "<tt class="samp">PythonHandler myscript</tt>" directive
tells mod_python to process the generic handler using the
<code>myscript</code> script. The "<tt class="samp">PythonDebug On</tt>" directive instructs
mod_python in case of an Python error to send error output to the
client (in addition to the logs), very useful during development.
<P>
When a request comes in, Apache starts stepping through its request
processing phases calling handlers in mod_python. The mod_python
handlers check whether a directive for that handler was specified in
the configuration. (Remember, it acts as a dispatcher.) In our
example, no action will be taken by mod_python for all handlers except
for the generic handler. When we get to the generic handler,
mod_python will notice "<tt class="samp">PythonHandler myscript</tt>" directive and do
the following:
<P>
<OL>
<LI>If not already done, prepend the directory in which the
<code>PythonHandler</code> directive was found to <code>sys.path</code>.
<P>
</LI>
<LI>Attempt to import a module by name <code>myscript</code>. (Note that if
<code>myscript</code> was in a subdirectory of the directory where
<code>PythonHandler</code> was specified, then the import would not work
because said subdirectory would not be in the <code>sys.path</code>. One
way around this is to use package notation, e.g. "<tt class="samp">PythonHandler
subdir.myscript</tt>".)
<P>
</LI>
<LI>Look for a function called <code>handler</code> in <code>myscript</code>.
<P>
</LI>
<LI>Call the function, passing it a request object. (More on what a
request object is later)
<P>
</LI>
<LI>At this point we're inside the script:
<P>
<UL>
<LI><div class="verbatim"><pre>
from mod_python import apache
</pre></div>
<P>
This imports the apache module which provides us the interface to
Apache. With a few rare exceptions, every mod_python program will have
this line.
<P>
</LI>
<LI><div class="verbatim"><pre>
def handler(req):
</pre></div>
<P>
<a id='l2h-23' xml:id='l2h-23'></a> This is our <i class="dfn">handler</i> function declaration. It
is called "<tt class="samp">handler</tt>" because mod_python takes the name of the
directive, converts it to lower case and removes the word
"<tt class="samp">python</tt>". Thus "<tt class="samp">PythonHandler</tt>" becomes
"<tt class="samp">handler</tt>". You could name it something else, and specify it
explicitly in the directive using "<tt class="samp">::</tt>". For example, if the
handler function was called "<tt class="samp">spam</tt>", then the directive would
be "<tt class="samp">PythonHandler myscript::spam</tt>".
<P>
Note that a handler must take one argument - the request
object. The request object is an object that provides all of the
information about this particular request - such as the IP of
client, the headers, the URI, etc. The communication back to the
client is also done via the request object, i.e. there is no
``response'' object.
<P>
</LI>
<LI><div class="verbatim"><pre>
req.content_type = "text/plain"
</pre></div>
<P>
This sets the content type to "<tt class="samp">text/plain</tt>". The default is usually
"<tt class="samp">text/html</tt>", but since our handler doesn't produce any html,
"<tt class="samp">text/plain</tt>" is more appropriate.
<strong>Important:</strong> you should <strong>always</strong> make sure this is set
<strong>before</strong> any call to "<tt class="samp">req.write</tt>". When you first call
"<tt class="samp">req.write</tt>", the response HTTP header is sent to the client and all
subsequent changes to the content type (or other HTTP headers) are simply
lost.
<P>
</LI>
<LI><div class="verbatim"><pre>
req.write("Hello World!")
</pre></div>
<P>
This writes the "<tt class="samp">Hello World!</tt>" string to the client. (Did I really
have to explain this one?)
<P>
</LI>
<LI><div class="verbatim"><pre>
return apache.OK
</pre></div>
<P>
This tells Apache that everything went OK and that the request has
been processed. If things did not go OK, that line could be return
<tt class="constant">apache.HTTP_INTERNAL_SERVER_ERROR</tt> or return
<tt class="constant">apache.HTTP_FORBIDDEN</tt>. When things do not go OK, Apache
will log the error and generate an error message for the client.
</LI>
</UL>
</LI>
</OL>
<P>
<strong>Some food for thought:</strong> If you were paying attention, you
noticed that the text above didn't specify that in order for the
handler code to be executed, the URL needs to refer to
<span class="file">myscript.py</span>. The only requirement was that it refers to a
<span class="file">.py</span> file. In fact the name of the file doesn't matter, and
the file referred to in the URL doesn't have to exist. So, given the
above configuration, "<tt class="samp">http://myserver/mywebdir/myscript.py</tt>" and
"<tt class="samp">http://myserver/mywebdir/montypython.py</tt>" would give the exact
same result. The important thing to understand here is that a handler
augments the server behaviour when processing a specific type of file,
not an individual file.
<P>
<em>At this point, if you didn't understand the above paragraph, go
back and read it again, until you do.</em>
<P>
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<span class="release-info">Release 3.3.1, documentation updated on January 29, 2007.</span>
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