Posts Tagged ‘Python’

AttributeError: ‘function’ object has no attribute ‘replace’

Wednesday, September 30th, 2009

While doing some coding to move a Turbogears2 application over to Pylons, I ran into an issue with Validation and ToscaWidgets.

AttributeError: ‘function’ object has no attribute ‘replace’

Validation in Pylons listed:

@validate(form=movie_form, error_handler=index)

for the decorator syntax, but, error_handler should be a name, not a function. The correct decorator should be:

@validate(form=movie_form, error_handler=’index’)

Rapid Application Development using Turbogears and Django

Saturday, August 8th, 2009

For the last 14 months we’ve been developing an application to replace 90000 lines of PHP code. Rewriting the application from scratch to support I18N and many of the enhancements it needed was deemed to be a better long term solution.

When that project was first started, I spent a month with Django and a month with Turbogears writing the same test application so that I could compare the development cycle. Both have matured and I needed to do a rapid turnaround on another project. I decided to give Django another look since it had hit version 1.0 and had added quite a few features that were missing in my preliminary evaluation. What follows is a discussion of the major points from both of the frameworks.

Turbogears has excellent form handling. Except for the Forms Wizard in Django, working with forms is much easier in Turbogears. Validation of the form and the resulting database update methods are much cleaner in Turbogears. Django, while slightly more complex in handling form input, does handle things with a single function which might enhance readability in a large project.

Database handling through SQL Alchemy in Turbogears is much better than the database methods in Django. Yes, you can use SQL Alchemy in Django now, but, their default ORM has plenty of room for improvement.

Turbogears is true MVC. Their terminology and methods are true to the paradigm set forth by Smalltalk. Django is MVC, but they call it MTV, Model, Template, View. The differences are slight and the developers of both have made good decisions. Adapting to either project’s methods is quick and not a hindrance.

Django’s definitely wins with Authentication/Authorization. Methods to handle registration, user creation, login and forgotten passwords are built in and wrapped with very nice templates that can be overridden. For Turbogears, repoze.who and repoze.what have been pulled from Plone and put into place. While Turbogears works with repoze, the decisions made and the lack of full support behind it make it difficult to implement.

Django feels faster. Comparing 14 months of development in Turbogears on an application to an application written in Django this week, the template engine, database access and pageload time seemed faster. Django is a lighter weight framework and you are closer to the data. Turbogears puts a little more insulation in which makes some coding easier at the expense of performance.

Maintainability of code would have to go to Turbogears. IBM once stated that the maximum number of bugfree lines of code that could be written was 23. With Turbogears, much of the heavy lifting is handled by widgets and decorators and your model resulting in a smaller codebase. Django requires more code to do the same task unless you utilize some of the snippets. Turbogears makes certain assumptions and has wrapped many of the libraries that make development easy in the default installation. Django’s default installation lacks those decisions, but, you are not prevented from establishing your own middleware. If you were developing a number of Django projects, you would pick and choose snippets that would replicate the decisions that Turbogears has already made.

URL Mapping is much easier to implement with Django. While routes support in Turbogears is present, Django’s regexp mapping is much easier to manipulate.

Community, hands down, Django wins. With a much larger installed base, bugs are found and fixed much more quickly. While Turbogears was founded on loftier principles, execution and follow through are lacking. Development is done when it is needed by a client project in the core group of developers. There is a definite air of condescension when the project developers handle questions from potential developers. With Django, there are people of all experience levels willing to help on, IRC, and thorough documentation that far exceeds most of the open source documentation out there.

Documentation, again, Django. Well organized, well thought out and well documented examples on Django’s part show dedication to having a framework that is well understood and well used. Turbogears recently moved to Sphinx, but, documentation generated from poorly documented code still means poor documentation. The tutorials and examples have been improving, but, they have a long way to go.

Genshi and Mako are supported fairly well with Turbogears and are both very good engines. Jinja is also supported which is a bit faster than Genshi and is powerful and very easy to work with. Django’s template language is also very flexible, powerful and easy to work with. Django had some definite advantages with a simpler language, but, neither Django or Turbogears stood out as a clear winner.

If you were looking to write an extremely heavy database or form involved site, I think Turbogears would be a good solution. If you choose Turbogears, be prepared to delve into the code when you are faced with problems. Bugs are not dealt with very promptly even though upgrades are pushed out. Be prepared to patch upgraded packages or hold certain packages once you move into production.

On the other hand, if you are writing a less complicated application, Django would be a good choice.

All told, the application developed this week in Django took about 12 hours and had I been working with Django for the last 14 months, I would estimate the project to have taken roughly 8 hours. Developed in Turbogears, I probably could have written it in 6 hours. PHP, to mimic all of the functionality would have taken 14-16 hours even using one of the numerous frameworks.

There is a definite advantage to using a framework and Python frameworks do appear to offer Rapid Application Development even over most of the PHP frameworks. For me, new development will probably be done in Django unless there is a strong case to use Turbogears.

* Turbogears 2.0
* Django
* Django Snippets

User Interface Design

Wednesday, June 24th, 2009

Programmers are not designers. Technical people should not design User Interfaces.

* 810 source files
* 90658 lines of code
* 10213 lines of html

For an internal project tasked to a series of programmers throughout the years without enough oversight, it is a mass of undocumented code with multiple programming styles. PHP allowed lazy programming, Smarty didn’t have some of the finesse required, so, the User Interface suffered. Functional but confusing to anyone that hadn’t worked intimately with the interface or been walked through it.

The truest statement is that it is easier for me to do things through the MySQL command line than through the application. While this does have a tendency to introduce possible typos, it has altered SQL practices here.

update table set value=123 where othervalue=246;

could have an accidental typo of

update table set value=123 where othervalue-=246;

which would have completely unintended consequences. One typo altered the DNS entries for 48000 records. Shortly after that typo, ingrained in company policy was that I never wanted to ever see a query like that executed in the command line regardless of how simple the command.

Even within code, the above command would be entered as:

update table set value=123 where othervalue in (246);

This prevented a number of potential typos. Even limit clauses with deletions were enforced to make sure things didn’t go too haywire in an update.

With Python, indenting is mandatory which results in multiple programmer’s code looking similar and easier to troubleshoot. Utilizing SQLAlchemy which enforces bind variables when talking with the database engine, we’ve eliminated the potential for a typo updating too many records. Even cascade deletes are enforced in SQLAlchemy even when running on top of MyISAM. With MVC, our data model is much better defined and we’re not tied down to remembering the relationship between two tables and possible dependencies. Conversion from the existing MySQL database to a DeclarativeBase model hasn’t been without issues, but, a simple python program allowed the generation of a simple model that took care of most of the issues. Hand tweaking the database model while developing the application has allowed for quite a bit of insight into issues that had been worked around rather than making adjustments to the database.

Fundamental design issues in the database structure were worked around with code rather than fixed. Data that should have been retained was not, relationships between tables was defined in code rather than in the database leading to a painful conversion.

When it was decided to rewrite the application in Python using TurboGears, I wasn’t that familiar with the codebase nor the user interface. Initially it was envisioned that the templates would be copied and the backend engine would be written to power those templates. After a few hours running through the application, and attempting the conversion on a number of templates, I realized the application was functional but it was extremely difficult to use in its current state. So much for having a programmer design an interface.

Some functionality from the existing system was needed so I peered into the codebase and was unprepared for that surprise. At this point it became evident that a non-programmer had designed the interface. While Smarty was a decent template language, it was not a formtool, so, methods were designed to give a consistent user experience when dealing with error handling. A single php file was responsible for display, form submission and validation and writing to the database for each ‘page’ in the application. The code inside should have been straightforward.

* Set up default CSS classes for each form field for an ‘ok’ result
* Validate any passed values and set the CSS class as ‘error’ for any value that fails validation
* Insert/Update the record if the validation passes
* Display the page

Some validation takes place numerous times throughout the application, and, for some reason one of the ‘coders’ decided that copy and paste of another function that used that same validation code was better than writing a function to do the validation. Of course when that validation method needed to be changed, it needed to be changed in eight places.

So, what should have been somewhat simple has changed considerably:

* Evaluate each page
* Redesign each page to make the process understandable
* Adjust terminology to make it understandable to the application’s users
* modify the database model
* rewrite the form and validation

A process that should have been simple has turned into quite a bit more work than anticipated. Basically, development boils down to looking at the page, figuring out what it should be, pushing the buttons to see what they do and rewriting from scratch.

TurboGears has added a considerable amount of efficiency to the process. One page that dealt with editing a page of information was reduced from 117 lines of code to 12 lines of code. Since TurboGears uses ToscaWidgets and Formencode, validation and form presentation is removed from the code resulting in a controller that contains the code that modifies the tables in the database with validated input. Since Formencode already has 95% of the validators that are needed for this project, we can rest assured that someone else has done the work to make sure that field will be properly validated. Other validation methods can be maintained and self-tested locally, but, defined in such a manner that they are reused throughout the application rather than being cut and pasted into each model that is validating data. In addition, bugs should be much less frequent as a result of a much-reduced codebase.

Due to the MVC framework and the libraries selected by the developers at TurboGears, I wouldn’t be surprised if the new codebase is 10%-15% the size of the existing application with greater functionality. The code should be more maintainable as python enforces some structure which will increase readability.

While I am not a designer, even using ToscaWidgets and makeform, the interface is much more consistent. Picking the right words, adding the appropriate help text to the fields and making sure things work as expected has resulted in a much cleaner, understandable interface.

While there are some aspects of ToscaWidgets that are a little too structured for some pages, our current strategy is to develop the pages using ToscaWidgets or makeform to make things as clear as possible making notes to overload the Widget class for our special forms at a later date.

While it hasn’t been a seamless transition, it did provide a good opportunity to rework the site and see a number of the problems that the application has had for a long time.

IPhone SDK Emulator

Monday, June 15th, 2009

In the last few days, I’ve had a few projects come up that could have been interesting to develop on the IPhone. When the IPhone first came out, I signed up as a developer but never downloaded the SDK. Today, I took the time to download the SDK to test the emulator for the recent template design for this blog to see how it displayed on the IPhone.

The framework appears to support Python, Perl, Cocoa and Ruby as well as native C.

Here are two screenshots of this blog on the IPhone emulator.

The quick way to start up the emulator after you’ve gotten the SDK from Apple’s Developer Connection is the following:

Open up Xcode, File, New Project, Select Application underneath the IPhone Application, click Utility Application, Choose, enter a project name. After a few seconds a project window will show up with the default of Simulator – 2.2.1 | Debug. Click Build and Go and it will compile the test project and open the IPhone emulator. Click the power button at the bottom, to bring up the main menu which will show Photos, Settings, and your test application with Contacts and Safari icons at the bottom. If you open Safari, you can key in a url or use one of the existing bookmarks.

Command-left arrow or Command-right arrow will rotate the screen from Portrait to Landscape and back. You can also use the Hardware Menu and Rotate Left/Rotate Right.

RSA with Perl, PHP and Python

Tuesday, April 28th, 2009

Ages ago we had a system that used MySQL’s built in DES3 encryption. It made coding applications in multiple languages easy because we could send it a string with a key and it would be encoded in the database. It wasn’t secure if someone got hold of the code and the database, but, rather than use a hash, we could store things and get the plaintext back if we needed it. Due to some policy issues with Debian, DES3 encryption was removed from MySQL and we were faced with converting that data to another format. We chose RSA for the fact that it was supported in every language we were currently developing in — Perl, PHP and C, and we knew it was supported in Python even though we hadn’t started development with Python at the time.

However, due to issues with PHP and long key lengths (at the time the code was written), our payloads had to be broken into packets smaller than the smallest key length which was 256 bytes. Since we didn’t know if we would run into similar issues using a longer packet length even if we had a longer key, we opted to use a packet size smaller than the smallest key we could generate. Our code initially converted data using PHP, stored the data in MySQL, and allowed a Perl script to access the data. Getting Perl and PHP to cooperate was somewhat difficult until we dug into PHP’s source code to see just how they were handling RSA.

PHP code:


function cp_encrypt($hostname,$message) {
  $public_key=file_get_contents(KEY_LOCATION . $hostname . '.public.key');

  $blockct = intval(strlen($message) / 245)  + 1;
  $encpayload = "";
  for ($loop=0;$loop<$blockct;$loop++) {
    $blocktext = substr($message,$loop * 245, 245);
    $encpayload .= $encblocktext;

function cp_decrypt($hostname,$message) {
  $priv_key=file_get_contents(KEY_LOCATION . $hostname . '.private.key');
  openssl_get_privatekey ($priv_key);
  $arr = unpack('Nblockct/a*',$message);
  $blockct = $arr['blockct'];$encpayload=$arr[1];
  $decmessage = "";
  for ($loop=0;$loop<$blockct;$loop++) {
    $blocktext = substr($encpayload, $loop*256, 256);
    $finaltext .= $decblocktext;


Perl Code:

use Crypt::OpenSSL::RSA;
use constant ENCPAYLOAD_FORMAT => 'Na*';

sub cp_encrypt {
  my $hostname = shift;
  my $message = shift;

  my $keyfile = $KEY_LOCATION . $hostname . '.public.key';
  if (-e $keyfile) {
    open PUBLIC, $keyfile;
    my $public_key = do{local $/; };
    my $rsa = Crypt::OpenSSL::RSA->new_public_key($public_key);

    my $blockct = int(length($message) / 245)  + 1;
    my $encpayload = "";
      for ($loop=0;$loop<$blockct;$loop++) {
        $encpayload .= $rsa->encrypt(substr($message,$loop * 245, 245));

sub cp_decrypt {
  my $hostname = shift;
  my $message = shift;

  my $keyfile = $KEY_LOCATION . $hostname . '.private.key';
  if (-e $keyfile) {
    open PRIVATE, $keyfile;
    my $private_key = do{local $/; };
    my $rsa = Crypt::OpenSSL::RSA->new_private_key($private_key);
    my ($blockct,$encpayload) = unpack(ENCPAYLOAD_FORMAT,$message);
    my $decmessage = "";
    for ($loop=0;$loop<$blockct;$loop++) {
      $decmessage .= $rsa->decrypt(substr($encpayload, $loop*256, 256));


Python code:

import M2Crypto.RSA
from os import path
from struct import unpack, pack, calcsize


def perl_unpack (perlpack, payload):
    if (perlpack == 'Na*'):
        count = calcsize('!L')
        perlpack = '!L%ds' % (len(payload) - count)
        return unpack(perlpack,payload)

def perl_pack (perlpack, blockcount, payload):
    if (perlpack == 'Na*'):
        perlpack = '!L%ds' % len(payload)
        return pack(perlpack,blockcount,payload)

def cp_encrypt (hostname,message):
  keyfile = KEY_LOCATION + hostname + '.public.key'
  if (path.exists(keyfile)):
    public_key = M2Crypto.RSA.load_pub_key(keyfile)

    blockct = int(len(message) / 245)  + 1
    encpayload = ""
    for loop in range(0,blockct):
      encpayload += public_key.public_encrypt(message[(loop*245):(245*(loop+1))],
    return(perl_pack(ENCPAYLOAD_FORMAT, blockct, encpayload))

def cp_decrypt (hostname, message):
  keyfile = KEY_LOCATION + hostname + '.private.key';
  if (path.exists(keyfile)):
    privatekey = M2Crypto.RSA.load_key(keyfile)
    (blockct,encpayload) = perl_unpack(ENCPAYLOAD_FORMAT,message)

    decmessage = ""
    for loop in range(0,blockct):
      decmessage += privatekey.private_decrypt(encpayload[(loop*256):(256*(loop+1))], M2Crypto.RSA.pkcs1_padding)

There is nothing really that restricts you from encrypting a message longer than the key size, but, if the message contains enough data, portions of the key can be discovered. Since the key is a very large prime number, exposing a portion of that key can be vital to decrypting the data.

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