Pyparsing Question

[Ant]

Hi all,

I have a question on PyParsing. I am trying to create a parser for a
hierarchical todo list format, but have hit a stumbling block. I have
parsers for the header of the list (title and description), and the body
(recursive descent on todo items).

Individually they are working fine, combined they throw an exception.
The code follows:

#!/usr/bin/python
# parser.py
import pyparsing as pp

def grammar():
underline = pp.Word("=").su ppress()
dotnum = pp.Combine(pp.W ord(pp.nums) + ".")
textline = pp.Combine(pp.G roup(pp.Word(pp .alphas, pp.printables) +
pp.restOfLine))
number = pp.Group(pp.One OrMore(dotnum))

headtitle = textline
headdescription = pp.ZeroOrMore(t extline)
head = pp.Group(headti tle + underline + headdescription )

taskname = pp.OneOrMore(do tnum) + textline
task = pp.Forward()
subtask = pp.Group(dotnum + task)
task << (taskname + pp.ZeroOrMore(s ubtask))
maintask = pp.Group(pp.Lin eStart() + task)

parser = pp.OneOrMore(ma intask)

return head, parser

text = """
My Title
========

Text on a longer line of several words.
More test
and more.

"""

text2 = """

1. Task 1
1.1. Subtask
1.1.1. More tasks.
1.2. Another subtask
2. Task 2
2.1. Subtask again"""

head, parser = grammar()

print head.parseStrin g(text)
print parser.parseStr ing(text2)

comb = head + pp.OneOrMore(pp .LineStart() + pp.restOfLine) + parser
print comb.parseStrin g(text + text2)

#============== =============== =============== =============== ========

Now the first two print statements output the parse tree as I would
expect, but the combined parser fails with an exception:

Traceback (most recent call last):
File "parser.py" , line 50, in ?
print comb.parseStrin g(text + text2)
..
.. [Stacktrace snipped]
..
raise exc
pyparsing.Parse Exception: Expected start of line (at char 81), (line:9,
col:1)

Any help appreciated!

Cheers,

--
Ant.

[Ben Finney]

James A. Donald <ja****@echeque .comwrites:

I am just getting into python, and know little about it

Welcome to Python, and this forum.

and am posting to ask on what beaches the salt water crocodiles hang
out.

Heh. You want to avoid them, or hang out with them? :-)

1. Looks to me that python will not scale to very large programs,
partly because of the lack of static typing, but mostly because there
is no distinction between creating a new variable and utilizing an
existing variable,

This seems quite a non sequitur. How do you see a connection between
these properties and "will not scale to large programs"?

so the interpreter fails to catch typos and name collisions.

These errors are a small subset of possible errors. If writing a large
program, an automated testing suite is essential, and can catch far
more errors than the compiler can hope to catch. If you run a static
code analyser, you'll be notified of unused names and other simple
errors that are often caught by static-declaration compilers.

I am inclined to suspect that when a successful small python program
turns into a large python program, it rapidly reaches ninety percent
complete, and remains ninety percent complete forever.

You may want to look at the Python success stories before suspecting
that, <URL:http://www.python.org/about/success/>.

2. It is not clear to me how a python web application scales.

I'll leave this one for others to speak to; I don't have experience
with large web applications.

--
\ "I was gratified to be able to answer promptly and I did. I |
`\ said I didn't know." -- Mark Twain, _Life on the Mississippi_ |
_o__) |
Ben Finney

[James A. Donald]

2. It is not clear to me how a python web application scales. Python

is inherently single threaded, so one will need lots of python
processes on lots of computers, with the database software handling
parallel accesses to the same or related data. One could organize it
as one python program for each url, and one python process for each
http request, but that involves a lot of overhead starting up and
shutting down python processes. Or one could organize it as one
python program for each url, but if one gets a lot of http requests
for one url, a small number of python processes will each sequentially
handle a large number of those requests. What I am really asking is:
Are there python web frameworks that scale with hardware and how do
they handle scaling?

Reid Priedhorsky

This sounds like a good match for Apache with mod_python.

I would hope that it is, but the question that I would like to know is
how does mod_python handle the problem - how do python programs and
processes relate to web pages and http requests when one is using
mod_python, and what happens when one has quite a lot of web pages and
a very large number of http requests?
--
----------------------
We have the right to defend ourselves and our property, because
of the kind of animals that we are. True law derives from this
right, not from the arbitrary power of the omnipotent state.

http://www.jim.com/ James A. Donald

[James A. Donald]

On Mon, 19 May 2008 21:04:28 -0400, "David Stanek"
<ds*****@dstane k.comwrote:

What is the difference if you have a process with 10 threads or 10
separate processes running in parallel? Apache is a good example of a
server that may be configured to use multiple processes to handle
requests. And from what I hear is scales just fine.

I think you are looking at the problem wrong. The fundamentals are the
same between threads and processes.

I am not planning to write a web server framework, but to use one.
Doubtless a python framework could be written to have satisfactory
scaling properties, but what are the scaling properties of the ones
that have been written?

--
----------------------
We have the right to defend ourselves and our property, because
of the kind of animals that we are. True law derives from this
right, not from the arbitrary power of the omnipotent state.

http://www.jim.com/ James A. Donald

[Arnaud Delobelle]

James A. Donald <ja****@echeque .comwrites:

Ben Finney

The larger the program, the greater the likelihood of inadvertent name
collisions creating rare and irreproducible interactions between
different and supposedly independent parts of the program that each
work fine on their own, and supposedly cannot possibly interact.

>These errors are a small subset of possible errors. If writing a large
program, an automated testing suite is essential, and can catch far
more errors than the compiler can hope to catch. If you run a static
code analyser, you'll be notified of unused names and other simple
errors that are often caught by static-declaration compilers.

That is handy, but the larger the program, the bigger the problem with
names that are over used, rather than unused.

Fortunately for each file that you group functionality in (called a
'module'), Python creates a brand new namespace where it puts all the
names defined in that file. That makes name collision unlikely,
provided that you don't write gigantic modules with plenty of globals
in them (which would be very unnatural in Python), and don't use from
mymodule import * too liberally.

Why not download a largish project in Python (a web framework for
instance, since you have a particular interest in this), study the
code and see if your concerns seem founded?

Arnaud

--
----------------------
We have the right to defend ourselves and our property, because
of the kind of animals that we are. True law derives from this
right, not from the arbitrary power of the omnipotent state.

--
La propriete, c'est le vol !
- Pierre-Joseph Proudhon

[Marc 'BlackJack' Rintsch]

On Tue, 20 May 2008 13:57:26 +1000, James A. Donald wrote:

The larger the program, the greater the likelihood of inadvertent name
collisions creating rare and irreproducible interactions between
different and supposedly independent parts of the program that each
work fine on their own, and supposedly cannot possibly interact.

How should such collisions happen? You don't throw all your names into
the same namespace!?

Ciao,
Marc 'BlackJack' Rintsch

[Marc 'BlackJack' Rintsch]

On Tue, 20 May 2008 10:47:50 +1000, James A. Donald wrote:

2. It is not clear to me how a python web application scales.

Ask YouTube. :-)

Ciao,
Marc 'BlackJack' Rintsch

[Graham Dumpleton]

On May 20, 2:00*pm, James A. Donald <jam...@echeque .comwrote:

2. *It is not clear to me how a python web application scales. *Python
is inherently single threaded, so one will need lots of python
processes on lots of computers, with the database software handling
parallel accesses to the same or related data. *One could organize it
as one python program for each url, and one python process for each
http request, but that involves a lot of overhead starting up and
shutting down python processes. *Or one could organize it as one
python program for each url, but if one gets a lot of http requests
for one url, a small number of python processes will each sequentially
handle a large number of those requests. *What I am really asking is:
Are there python web frameworks that scale with hardware and how do
they handle scaling?

Reid Priedhorsky

This sounds like a good match for Apache withmod_python.

I would hope that it is, but the question that I would like to know is
how does mod_python handle the problem - how do python programs and
processes relate to web pages and http requests when one is using mod_python, and what happens when one has quite a lot of web pages and
a very large number of http requests?

Read:

http://blog.dscpl.com.au/2007/09/par...d-modwsgi.html
http://code.google.com/p/modwsgi/wik...esAndThreading

They talk about multi process nature of Apache and how GIL is not as
big a deal when using it.

The latter document explains the various process/threading modes when
using Apache/mod_wsgi. The embedded modes described in that
documentation also apply to mod_python.

The server is generally never the bottleneck, but if you are paranoid
about performance, then also look at relative comparison of mod_wsgi
and mod_python in:

http://code.google.com/p/modwsgi/wik...manceEstimates

Graham

[Nick Craig-Wood]

Marc 'BlackJack' Rintsch <bj****@gmx.net wrote:

On Tue, 20 May 2008 13:57:26 +1000, James A. Donald wrote:

The larger the program, the greater the likelihood of inadvertent name
collisions creating rare and irreproducible interactions between
different and supposedly independent parts of the program that each
work fine on their own, and supposedly cannot possibly interact.

How should such collisions happen? You don't throw all your names into
the same namespace!?

If you ever did a lot of programming in C with large projects you have
exactly that problem a lot - there is only one namespace for all the
external functions and variables, and macro definitions from one
include are forever messing up those from another. I suspect the OP
is coming from that background.

However python doesn't have that problem at all due to its use of
module namespaces - each name is confined to within a module (file)
unless you take specific action otherwise, and each class attribute is
confined to the class etc.

From the Zen of Python "Namespaces are one honking great idea -- let's
do more of those!" - as a battle scarred C programmer I'd agree ;-)

--
Nick Craig-Wood <ni**@craig-wood.com-- http://www.craig-wood.com/nick

[Duncan Booth]

Marc 'BlackJack' Rintsch <bj****@gmx.net wrote:

On Tue, 20 May 2008 10:47:50 +1000, James A. Donald wrote:

>2. It is not clear to me how a python web application scales.

Ask YouTube. :-)

Or look at Google appengine where unlike normal Python you really are
prevented from making good use of threading.

Google App Engine takes an interesting approach by forcing the programmer
to consider scalability right from the start: state is stored in a
distributed database which cannot do all the hard to scale things that SQL
databases do. This means that you have to work as though your application
were spread on servers all round the world from day 1 instead of waiting
until the structure that was 'good enough' is threatening to kill your
business before you address them.

It also puts strict limits on how much a single web request can do, so
again you have to work from day 1 to make sure that page requests are as
efficient as possible.

In return you get an application which should scale well. There is nothing
Python specific about the techniques, it is just that Python is the first
(and so far only) language supported on the platform.

--
Duncan Booth http://kupuguy.blogspot.com

[David Stanek]

On Tue, May 20, 2008 at 12:03 AM, James A. Donald <ja****@echeque .comwrote:

On Mon, 19 May 2008 21:04:28 -0400, "David Stanek"
<ds*****@dstane k.comwrote:

>What is the difference if you have a process with 10 threads or 10
separate processes running in parallel? Apache is a good example of a
server that may be configured to use multiple processes to handle
requests. And from what I hear is scales just fine.

I think you are looking at the problem wrong. The fundamentals are the
same between threads and processes.

I am not planning to write a web server framework, but to use one.
Doubtless a python framework could be written to have satisfactory
scaling properties, but what are the scaling properties of the ones
that have been written?

Both Django and TurborGears work well for me. When you step back and
think about it all of the popular web frameworks would do just fine.
The ones that don't do multiprocessing out of the box would be trivial
to load balance behind Apache or a real load balancer. Again the
problem here is the number of connections to the database, once you
get big enough to worry about it.

--
David
http://www.traceback.org