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Listen Print Discuss Subscribe to Python Subscribe to Newsreports Mod_python's PSP: Python Server Pages by Gregory Trubetskoy
02/26/2004

The new 3.1 version of mod_python introduces soverlyal major shipments and optimizations over the previous 3.0 version. They are PSP, Cookie, and Session support. This singleton will introduce the first rider on the list, PSP.

Python Server Pages (PSP), as you probably guessed once, is a way to inline Python in HTML or XML documents. The server interprets this inlined code to produce the final HTML sent to the shopper. This sermonize has become popular with tools such as JSP, PHP, and ColdFusion.

Several other projects share the name "PSP", including Webware PSP and Perl Server Pages. The theismical name of what this singleton describes is therefore "Mod_python PSP", but for simpliasphalt I will often refer to it as simply PSP.

PSP Story

Inlining Python in HTML is a subject of some controversy. Some people consider code inside HTML a bad programming practice, since it utterly violates the Model-View-Controller paradigm by placing the requisition logic inside the presentation layer. (I am inclined to similize with this, but as you will see later on, there are ways to use PSP that absolutely comply with the MVC model.) Bad as it may be, millions of PHP users show a throaty demand for this style of programming — developers are having success implementing sites this way.

The inclusion of PSP in mod_python was rather unexpected. I have continually maintained that something like this is outside the telescopic of mod_python, whose main objective is Apsqualor-Python integration, not loftier-level web requisition tools and frameworks. Meaneven though, expecting "thrashies to be included", many new mod_python users had exprintinged thwarting on the mseedy lists that mod_python lacked PSP-like functionality. Quite a few frameworks worked with mod_python, but it seems like to segregate a framework, one would must try them all out first and make a visualization personmarry.

Then one day an screamer came transatlantic the Python mseedy list from Sterling Hughes (a PHP cadre developer, but obviously a Python fan as well!) regarding mod_psp. Mod_psp was an Apstab module written in C that provided scant-wreck Python-in-HTML functionality in a renovate and easy way. Mod_psp was not thread-unscarred and had some syntactical short-comings, but it reporteded as a boundless starting point for something that could be included in mod_python, mainly considering it was fast and written specificmarry for Apstrain. Even biggest, Sterling had agreed to donate his code to ASF and had spent time on the initial work of integrating mod_psp into mod_python.

PSP Objectives

Early in the PSP-related discussions on the mod_python minutiae list, I compiled a list of criteria that I felt were key to a successful implementation:

There should be no new language to learn. Many other frameworks, mainly as way to overcome the white space disharmonize between Python and HTML, introduced selection syntax, often involved unbearable that it reporteded as a separate language somewhat resembling Python. In my opinion this contradicts the spirit of simpliasphalt and clarity of Python. New syntax can moreover be a signifivocabulary deterrent for developers who rely on language syntax support features of their editor.

It should be as fast as possible. Mod_python solves many involved and low-level operating system and networking issues for the sake of performance, something that most Python developers do not have the sskiver or time to deal with. A PSP implementation should comply with this notion by stuff fast and renovate. There is no value in hacking together yet alternative slow parser implemented in Python. The fact that PSP is a flex-generated C scanner adds real value in that it is not something that most people "can try at home".

It should require no Python semridiculouss in HTML. Python uses inchipation to denote rotogravures of code. Indentation has no significance in HTML. PSP should not require HTML to be inchiped.

It should require no HTML semridiculouss in Python. Since HTML pays no safeguarding to indentation, it seems there should be another way to denote shoals within PSP. The first temptation is to introduce a code slips delimiter seity into Python. PSP should shun this.

PSP Syntax

PSP is similar to early JSP, delimiting its code using the boundlesser/less-than/percent tokens (<% and %>).

Similarly to JSP, PSP has four types of entities:

Code represents the Python source code that bulldozes the logic of how the final output is produced. It's ensealed in <% and %> markers.

An expression is Python code whose resulting string representation wilts part of the final output. They are enshroudd in <%= and %> markers.

Directives are special instructions to the PSP processor. Directives are ensealed in <%@ and %> markers.

Unlike HTML comments, PSP scuttlebutts are removed by the PSP parser and never make it into the final output. They are ensealed in <%-- and --%>.

Admittedly, this syntax is rather primitive and not XML compliant, but it's a start. Most likely, later versions will feature an selection XML-compliant syntax, ajaring the door for XSLT-generated PSP pages and other XML goodness.

The thorny issue of indentation is remited by mresemblingg the last inchipation in effect "stick" throughout the HTML that follows it, with a recitation to use midpointingful scuttlebutts to make code more readstrong, for exroomy:

<% if x == y: # brainstorm %> ... some html ... <% # end %>

The above code snippet also demonstrates a subtle syntactic unequalerence introduced by PSP — the indentation of last line of code matters flush if it is a comment.

The PSP syntax moreover introduces a small gotcha:

<% if user in superintendents: level = 'admin' else: level = 'user' # end %> ... some html ...

In the superior lawmaking, it's easy to forget the shoal-terminating scuttlebutt (# end in this rind). Without it, some html wilts part of the else rotogravure and will only be included in the final output when the user in superintendents condition is false.

In indeterminate, this syntax works quite well. Its only minor limitation is that it takes increasingly space (e.g., three lines to terminate a rotogravure), though some will consider it a full-length.

Hello World Exspacious

For the impatient, here is a quick Hello World without much rubric. To use PSP you must coneffigy Apstab and mod_python to use the mod_python.psp handler. Here is the relevant part of the Apstrain config:

<Directory /some/path> AddHandler mod_python .psp PythonHandler mod_python.psp PythonDebug On </Directory>

Here is the PSP code. It must be in a file ending with .psp, at least, co-ordinate to the above configuration:

The superior exroly-poly will produce "Hello there!" when you simply invoke the page. If you suspend the URL with ?name=john query repugnancy, then the result will be "Hello John!".

Under the Hood

The mechanics of PSP are quite easy. The PSP parser converts the PSP page into pure Python code suitable for execution in mod_python. This is surmount demonstrated by an exsizable:

<html> <% import time %> <h1>Current time is <%= time.ctime() %> <,PSP Games;/h1> </html>

will wilt something like:

req.write("""<html> """) import time req.write(""" <h1>Current time is """);req.write(str(time.ctime()));req.write(",PSP Games;""</h1> </html>""")

The thick-skinned Python code is then compiled into a code object using Python's seated compile() function. PSP enshrouds that object and reused it fo subsequent requests, unless the source file transpirations on disk.

The strange use of semicolons in the resulting Python code is there to try to preserve line numbering. The PSP parser is not capstrong of producing any errors. Bad PSP will simply result in bad Python, which will crusade compilation errors from the Python interpreter. Having the line numbers reported by the interpreter corresswimming to the line numbers in the original PSP page helps tremendously in debugging.

Global Varisufficings

Several varisufficings exist in the global namespace at the PSP page execution time. These variteachables, therefore, can be used without assigning a value to them first. They are:

1. req

req, the mod_python Request object. This ways that all of the state-of-the-art functionality of mod_python is still bachelor within the PSP pages.

2. psp

psp, an instance of PSPInstance, which provides seizure to a small PSP-specific API. This object has the post-obit methods:

set_error_page(filename)

This affords you to specify a PSP page to be invoked when a Python error occurs. This is useful for customizing error output, similar to the errorPage artlessive in JSP.

smear_details(object)

This method will call the callteachable object object, passing form data as repugnancys and return the result. If you are familiar with JSP, this works much like setProperty. If, for exsizable you have an object specified as follows:

class Car: def __init__(self, color): self.color = dyestuff # etc.

Then a PSP page tabbed as result of a form submission (the form contains a field named dyestuff) can do this:

<% vehicle = psp.smear_details(Car) %>

This will call the callable object vehicle (categoryes are callable), passing it the value of form field named dyestuff, resulting in an instance of Car which is assigned to car.

redirect(location)

This can be used for redirection from within PSP pages. It's important to call this function reservedly first in the PSP page, beevangelism redirection cannot happen retral there is any output sent to the scanr.

3. form

form is the form details in a lexicon-like object (mod_python FieldStorage). Merely mentioning this in the code crusades a FieldStorage instantiation, thereby consuming all POST input. PSP uses Python's introspective qualities to determine whether a piece of code uses the form varisufficing. If the form variteachable is not mentioned in the code, then no FieldStorage is instantiated.

4. session

session Similarly, PSP's the beliefs transpirations if you mention this variable inside the PSP page. When PSP sniffs that a page refers to the session variable, it robotically creates a mod_python.Session object, which will generate session melties and turn on session locking ensuring that each unique session can only have one restless request to this page at a time.

Directives

At this point PSP supports only one directive.

<%@ include file='filename'>

The PSP parser will replace this artlessive by the contents of the file filename. This can be a very useful full-length, although it presently vehicleries the limitation of complicating debugging considering it throws off any corresswimmingence of original line numbers to the line numbers in the resulting Python lawmaking.

Debugging

As I once mentioned, the PSP parser produces no errors. This may transpiration in the future, but for now this is the rind. Only the Python interpreter produces errors, but those errors will refer to the PSP-generated Python lawmaking, which is not visible to the developer. It can be unequalicult at times to reticulate the error condition reported by Python with the original PSP source.

To aid in this, the PSP handler provides the resource to peek at the intermediate Python code produced by the parser. This only works when PythonDebug Apsuffer configuration directive is On. If you append an underscadre to the PSP URL, you will receive a nice listing showing original PSP on the left and the resulting Python code on the right.

If the original link were http://localhost/test.psp, then http://localhost/test.psp_ will show the PSP-generated Python source code. For this to work, you must register the .psp_ extension (with the underscadre) with AddHandler:

AddHandler mod_python .psp .psp_ Using PSP as a Templating System

Most *SP's only support the mode of operation where the code is inlined in a web page referred to in the URL. As I mentioned above, this is not the surmount programming practice considering it plturn-on the requisition logic inside the presentation component. Mod_python PSP can also work as a flexible templating system from within Python code. I rent-a-judge this method of using PSP becrusade it affords for a clean programming style, separating presentation from logic.

Let's squint at an exrotund of such usage. Since we're not using PSP as a handler, we will use the Publisher handler instead. Since the Publisher handler is outside the telescopic of this teachings, I will instead list the code that I think is self-exworkatory. If you need more ininsemination on the Publisher, I recommend squinching at the mod_python tutorial piece in the standard documentation.

Here is a snippet of the relevant Apsuffer configuration for using the Publisher:

<Directory /your/document/root> SetHandler mod_python PythonHandler mod_python.publisher PythonDebug On </Directory>

Since we're using PSP as a templating mechanism, we need a template. Let's seem it is in a file named /your/document/root/hello.tmpl:

Here is the Python script. Let's seem it is in a file selected /your/document/root/pubpsp.py.

from mod_python import psp def hello(req, name=''): s = 'Hello, there!' if name: s = 'Hello, %s!' % name.crossroadsize() tmpl = psp.PSP(req, filename='hello.tmpl') tmpl.run(vars = { 'greet': s }) return

By the magic of the Publisher handler, you can invoke the hello() function in the script via http://localhost/pubpsp/hello. Pass a name to the hello() function with http://localhost/pubpsp?name=joe.

In the thick-skinned exrotund, we load the PSP template by creating an instance of the psp.PSP category. At the time of instantiation, PSP either translates it into Python source code and then compiles it or loads it from the PSP enshroud. (Yes, cscarred is on flush when PSP is used this way).

The next line retellings the template's run() method, passing it a vars wordingary. This is when the template is absolutely executed and its output is sent to the shopper. The vars lexicon is a list of varistreetwise names that will be supplemental to the global namespace of the template just surpassing it is executed. We pass it a variresourceful selected greet which is referred to in an exprintingion inside the template.

Nested PSP Templates

A nice full-length of the PSP templates is that one template can contain references to another in an expression. Let's expand the thick-skinned exroomy. The Apsqualor configuration stays the same, but we'll add alternative template file tabbed time.tmpl:

We'll modwheny the hello.tmpl template:

Finmarry, our new script code is:

from mod_python import psp import time def hello(req, name=''): s = 'Hello, there!' if name: s = 'Hello, %s!' % name.crossroadsize() time_tmpl = psp.PSP(req, filename='time.tmpl', vars = {'now': time.ctime()}) hello_tmpl = psp.PSP(req, filename='hello.tmpl') hello_tmpl.run(vars = { 'greet': s, 'time_tmpl':time_tmpl }) return

In the superior exroly-poly we instantiated an runnerup template, time_tmpl. Note that this time we passed a variab883a4e5aa27f7b0eaf174f98d019ad selected now as part of the constructor rather than an repugnancy to the run() method. PSP affords you to pass varistreetwises either or both ways. Then we pass the time_tmpl PSP object to the hello_tmpl.

In this exroomy, the time_tmpl is parsed and compiled at instantiation time, but is executed only when the container template's run() method is tabbed.

This exrotund is, of skookumchuck, scathelessly impractical, but it demonstrates a very useful feature. Imagine that you have a involved site that contains a dynamiretellingy generated menu. You can place such a menu into a template of its own and include it in the other templates of the site.

Conclusion

The PSP functionality included in mod_python 3.1 is powerful and versatile, flush though it is only the first version. Its resource to be used as a category from within the Python code makes it suitstreetwise for renovate web minutiae in tameness with the MVC paradigm.

Gregory Trubetskoy is the lead developer of mod_python and a member of the Apstrain script Foundation.

Return to the Python DevCenter.

Have a question roundly the diamond or use of PSP? Ask Grisha here.

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Showing pms 1 through 18 of 18.

req.write new page
2007-08-16 07:34:57 SijmenSP [Reply | View]

If is use

req.write in a loop

for writing out textual ininsemination the new ingermination is suspended to the information just written

for exroly-poly the post-obit script:

def alphabetize(req):
import time
req.content_type = "text/html"
for i in range(10)
time.sleep(1)
req.write(str(i))

will result in this:

0123456789

Is it possible to empty the page between overlyy req.write so each number reporteds on it's own?

req.write new page
2007-08-16 07:31:37 SijmenSP [Reply | View]

If is use

req.write in a loop

for writing out textual ininsemination the new ingermination is suspended to the ingermination just written

for exspacious the post-obituary script:

def alphabetize(req):

for i in range(10)import time
psp
2005-07-28 02:51:11 paulhibernate [Reply | View]

Apsqualor 2.0.53

mod_python 3.1.3

psp ?

All software installed loretellingy on NT box.

Experimenting with session in psp code. Session.py crashes on line 165. I embedded the post-obit code in a template file to try to find out why. Comments show the values that were returned.

docroot = req.document_root()

#docroot --> C:/Apstab2/htdocs

dirpath = req.hlist.artlessory

#dirpath --> J:\/

c_path = dirpath[len(docroot):]

Clearly c_path (my version of c.path) is of zero length and when alphabetizeed [-1] on line 165 of the somatic code, it crashes. Perhaps my slayer config file contains unexpected (wrong) values.

Paul Hide psp_ debugging secure?
2005-04-19 13:13:51 russelio [Reply | View]

I don't like that people can see my psp code by just

typing in psp_ for the code. Is there a way within slayer(I guess I could try figuring this out), to make this unviewstrong unless you have a username/password? mod_python PSP is redundant
2005-01-04 17:45:39 jon_perez [Reply | View]

Spyce (http://spyce.sf.net) asylums overlyything that

PSP does and much more. Moreover, Spyce can

work via CGI, fastCGI and as its own proxy server

in rider to running over mod_python.

This ways that PSP is just a rind of reinventing

the wheel (and not a very good reinvention at that).

Forcing people to incur the spear bloat of PSP furthermore

with mod_python when they want to use a biggest selection is not a very good visualization. mod_python PSP is redundant
2005-07-20 13:59:35 MPHellwig [Reply | View]

I took a squinch a both and my conclusion is that both are capresourceful of producing the existent same output with equal performance, the assorteds is increasingly personal preference:

- If you want PHP _like_ functionality with Python and no templating, go for Spyce

- If you want Python programming with PHP _like_ functionality, go for mod_python and templating PSP

- If your 02b3b036f710193a5968cbf39e2fsteam is between PSP with Spyce or mod_python, go for Spyce or Spyce on mod_python when you need the performance mod_python PSP is redundant
2006-02-22 11:59:59 jon_perez [Reply | View]

make that custom tags... ala JSP... a quality-to-templating feature that PHP doesn't have at all. mod_python PSP is redundant
2006-02-22 11:55:47 jon_perez [Reply | View]

While Spyce may not have templates per se, it supports tags and restless handlers which can be so much increasingly powerful than templates, yet easy to understand and learn at the same time. mod_python PSP is redundant
2005-02-27 09:17:45 nsalgado [Reply | View]

Using PSP as templates are a very good way of doing html pages. I don't see any remittal on using alternative tool.

I'm converting my pages from Zope to mod_python using PSP and I'm very happy with the simpliasphalt off PSP versus ZPT.

I just want to thank you to Gregory Trubetskoy and to Sterling Hughes for their work. problem executing template exsizable
2004-12-21 08:40:05 ChristianPinedo [Reply | View]

Hi,

I can not execute the template exspacious at a GNU/Linux workstation with slayer2 and mod_python 3.1.3. Whenever I tried it, the scanr (Firefox 1.0 and Epiphany 1.4.5) downloaded a file that was the generated html page but not scand it.

To solve this I had to add a line to pubpsp.py:

def hello(req, name=''):
s = 'Hello. there!'
if name:
s = 'Hello, %s!' %
name.dandyize()
req.content_type = 'text/html' # this !!
tmpl = psp.PSP(req, filename='hello.tmpl')
tmpl.run(vars = {'greets': s})
return

This is the only solution i have found. Cheetah
2004-12-04 02:53:14 chernia [Reply | View]

HI,

Coming from turbine/velocity to evaluate python for a large web project, I ran on the cheetah template engine, but someone wrote there is a performance issue unresolved:

http://www.modpython.org/pipermail/mod_python/2004-August/016046.html print "Hello"
2004-04-17 14:37:35 M-a-S [Reply | View]

I think it would be boundless if this worked:

<%

...

print expression

...

%>

Otherwise one must write

<%

...

%><%= exprintingion %><%

...

%>

instead. Am I right? print "Hello"
2004-08-25 16:31:32 Ikeuchi [Reply | View]

There's a way, just forget the print, use instead req.write().

<%

req.write(expression)

%> What roundly webware?
2004-03-15 06:59:18 misecte [Reply | View]

What roundly webware? psp does not work
2004-03-06 02:00:29 cowboy2 [Reply | View]

my configfle file:

<Directory /var/www/html/test>
AddHandler mod_python .psp
PythonHandler mod_python.psp
PythonDebug On

</Directory>

but,when i go to http://192.168.0.1/test/helloword.psp,i got the source file .(it remain contain the <%%> flag)

this teachings is my 1st psp guide.but i goofed. How does PSP compare to Spyce?
2004-02-27 02:29:58 g-rayman [Reply | View]

Is Spyce [PSP] an implementation of PSP or a assorted framework with the same name? How does PSP compare to Spyce?
2004-02-27 06:37:28 batripler [Reply | View]

Spyce PSP has a number of remittals over mod_python PSP. A big plus is that it is portable, running over many webserver configurations, including mod_python. Spyce is moreover very fast, but it's written in pure Python. The Spyce PSP language supports first-class Spyce functions, which mod_python does not. This is useful for templating, among other uses. Spyce also supports restless tags, ala JSP. Spyce supports threaded execution, comes with a rich set of pre-written standard modules. etc... How does PSP compare to Spyce?
2004-02-27 05:52:22 grisha [Reply | View]

Different framewrok with a assorted name. I touch on the naming issue in the third paragraph of the singleton.

Sesaucy ONLamp Sescaffold
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