Python is not a Panacea ...

Published at June 11, 2012 | Tagged with: , , , ,

... neither is any other language or framework

This post was inspired by the serial discussion on the topic "Python vs other language"(in the specific case the other one was PHP, and the question was asked in a Python group so you may guess whether there are any answers in favor of PHP). It is very simple, I believe that every Python developer will tell you that Python is the greatest language ever build, how easy is to learn it, how readable and flexible it is, how much fun it is to work with it and so on. They will tell you that you can do everything with it: web and desktop development, testing, automation, scientific simulations etc. But what most of them will forgot to tell you is that it is not a Panacea.

In the matter of fact you can also build "ugly" and unstable applications in Python too. Most problems come not from the language or framework used, but from bad coding practices and bad understanding of the environment. Python will force you to write readable code but it wont solve all your problems. It is hard to make a complete list of what exactly you must know before starting to build application, big part of the knowledge comes with the experience but here is a small list of some essential things.

  • Write clean with meaningful variable/class names.
  • Exceptions are raised, learn how to handle them.
  • Learn OOP(Object Oriented Programming)
  • Use functions to granulate functionality and make code reusable
  • DRY(Don't Repeat Youtself)
  • If you are going do develop web application learn about the Client-Server relation
  • Use "layers" to seprate the different parts of your application - database methods, business logic, output etc. MVC is a nice example of such separation
  • Never store passwords in plain text. Even hashed password are not completely safe, check what Rainbow Tables are.
  • Comment/Document your code
  • Write unit test and learn TDD.
  • Learn how to use version control.
  • There is a client waiting on the other side - don't make him wait too long.
  • Learn functional programming.

I hope the above does not sounds as an anti Python talk. This is not its idea. Firstly because there are things that are more important than the language itself(the list above) and secondly because... Python is awesome )))
There are languages that will help you learn the things above faster, Python is one of them - built in documentation features, easy to learn and try and extremely useful. My advice is not to start with PHP as your first programming language it will make you think that mixing variables with different types is OK. It may be fast for some things but most of the times it is not safe so you should better start with more type strict language where you can learn casting, escaping user output etc.

Probably I have missed a few(or more) pointa but I hope I've covered the basics. If you think that anything important is missing, just add it in the comments and I will update the post.

Fabric & Django

Published at May 28, 2012 | Tagged with: , , ,

Or how automate the creation of new projects with simple script

Preface: Do you remember all this tiny little steps that you have to perform every time when you start new project - create virtual environment, install packages, start and setup Django project? Kind of annoying repetition, isn't it? How about to automate it a bit.

Solution: Recently I started learning Fabric and thought "What better way to test it in practice than automating a simple, repetitive task?". So, lets mark the tasks that I want the script to perform:

  1. Create virtual environment with the project name
  2. Activate the virtual environment
  3. Download list of packages and install them
  4. Make 'src' directory where the project source will reside
  5. Create new Django project in source directory
  6. Update the settings

Thanks to the local command the first one was easy. The problem was with the second one. Obviously each local command is run autonomously so I had to find some way have activated virtual environment for each task after this. Fortunately the prefix context manager works like a charm. I had some issues making it read and write in the paths I wants and voilà it was working exactly as I want.

The script is too long to place it here but is publicly available at https://gist.github.com/2818562
It is quite simple to use, you only need python, fabric and virtual environment. Then just use the following code.

fab start_project:my_mew_project

To Do: Here are few things that can be improved:

  • Read the packages from a file
  • Update urls.py to enable admin
  • Generate Nginx server block file

So this is my first try with Fabric, I hope that you will like it and find it useful. As always any comments, questions and/or improvement ideas are welcome.

HTTP Status Codes Site

Published at Feb. 1, 2012 | Tagged with: , , , , , ,

During the development of Simple Site Checker I realised that it would be useful for test purposes if there is a website returning all possible HTTP status codes. Thanks to Google App Engine and webapp2 framework building such website was a piece of cake.

The site can be found at http://httpstatuscodes.appspot.com.

The home page provides a list of all HTTP status codes and their names and if you want to get an HTTP response with a specific status code just add the code after the slash, example:
http://httpstatuscodes.appspot.com/200 - returns 200 OK
http://httpstatuscodes.appspot.com/500 - returns 500 Internal Server Error
Also at the end of each page is located the URL of the HTTP protocol Status Codes Definitions with detailed explanation for each one of them.

The website code is publicly available in github at HTTP Status Codes Site.

If you find it useful feel free to comment and/or share it.

Simple Site Checker

Published at Jan. 30, 2012 | Tagged with: , , , , , ,

... a command line tool to monitor your sitemap links

I was thinking to make such tool for a while and fortunately I found some time so here it is.

Simple Site Checker is a command line tool that allows you to run a check over the links in you XML sitemap.

How it works: The script requires a single attribute - a URL or relative/absolute path to xml-sitemap. It loads the XML, reads all loc-tags in it and start checking the links in them one by one.
By default you will see no output unless there is an error - the script is unable to load the sitemap or any link check fails.
Using the verbosity argument you can control the output, if you need more detailed information like elapsed time, checked links etc.
You can run this script through a cron-like tool and get an e-mail in case of error.

I will appreciate any user input and ideas so feel free to comment.

Faking attributes in Python classes...

Published at Jan. 30, 2012 | Tagged with: , ,

... or how to imitate dynamic properties in a class object

Preface: When you have connections between your application and other systems frequently the data is not in the most useful form for your needs. If you have an API it is awesome but sometimes it just does not act the way you want and your code quickly becomes a series of repeating API calls like api.get_product_property(product_id, property).
Of course it will be easier if you can use objects to represent the data in you code so you can create something like a proxy class to this API:

class Product(object):
    def __init__(self, product_id):
        self.id = product_id

    @property
    def name(self):
        return api_obj.get_product_property(self.id, 'name')

    @property
    def price(self):
        return api_obj.get_product_property(self.id, 'price')

#usage
product = Product(product_id)
print product.name
In my opinion it is cleaner, more pretty and more useful than the direct API calls. But still there is something not quite right. Problem: Your model have not two but twenty properties. Defining 20 method makes the code look not that good. Not to mention that amending the code every time when you need a new property is quite boring. So is there a better way? As I mention at the end of Connecting Django Models with outer applications if you have a class that plays the role of a proxy to another API or other data it may be easier to overwrite the __getattr__ method. Solution:
class Product(object):
    def __init__(self, product_id):
        self.id = product_id

    def __getattr__(self, key):
        return api_obj.get_product_property(self.id, key)

#usage
product = Product(product_id)
print product.name

Now you can directly use the product properties as attribute names of the Product class. Depending from the way that the API works it would be good to raise AttributeError if there is no such property for the product.

Software for business

Published at Jan. 24, 2012 | Tagged with:

I am starting a new blog. The reason is that I want to keep this one more technically oriented while the other will be more business and customers oriented. Its name is Software for business and the idea is to show to the business in less technical details how the modern IT technologies like CRM & ERP systems, web sites, e-commerce solutions, online marketing and so on can help their business.
If you find the topic interesting feel free to join.

Connecting Django Models with outer applications

Published at Jan. 23, 2012 | Tagged with: , , , ,

Preface: Sometimes, parts of the data that you have to display in your application reside out of the Django models. Simple example for this is the following case - the client requires that you build them a webshop but they already have CRM solution that holds their products info. Of course they provide you with a mechanism to read this data from their CRM.

Specialty: The problem is that the data in their CRM does not hold some of the product information that you need. For instance it misses SEO-friendly description and product image. So you will have to set up a model at your side and store these images there. It is easy to join them, the only thing that you will need is a simple unique key for every product.

Solution: Here we use the product_id field to make the connection between the CRM data and the Django model.

# in models.py
class Product(models.Model):
    product_id = models.IntegerField(_('Original Product'),
                                     unique=True)
    description = models.TextField(_('SEO-friendly Description'),
                                   blank=True)
    pod_image = FilerImageField(verbose_name=_('Product Image'),
                                blank=True, null=True)

   @property
   def name(self):
       return crm_api.get_product_name(self.product_id)

# in forms.py
class ProductForm(forms.ModelForm):
    name = forms.CharField(required=False,
                           widget=forms.TextInput(attrs={
                               'readonly': True,
                               'style': 'border: none'}))

    class Meta:
        model = Product
        widgets = {
            'product_id': forms.Select(),
        }
    def __init__(self, *args, **kwargs):
        super(ProductForm, self).__init__(*args, **kwargs)
        self.fields['product_id'].widget.choices = crm_api.get_product_choices()
        if self.instance.id:
            self.fields['name'].initial = self.instance.name

The form here should be used in the admin(add/edit) page of the model. We define that the product_id field will use the select widget and we use a method that connect to the CRM and returns the product choices list.
The "self.instance.id" check is used to fill the name field for product that are already saved.

Final words: This is a very simple example but its idea is to show the basic way to connect your models with another app. I strongly recommend you to use caching if your CRM data is not modified very often in order to save some bandwidth and to speed up your application.
Also if you have multiple field it may be better to overwrite the __getattr__ method instead of defining separate one for each property that you need to pull from the outer application.

P.S. Thanks to Miga for the code error report.

Python os.stat times are OS specific ...

Published at Nov. 21, 2011 | Tagged with: , , , ,

... no problem cause we have stat_float_times

Preface: recently I downloaded PyTDDMon on my office station and what a surprise - the files changes did not affect on the monitor. So I run a quick debug to check what is going on: the monitor was working running a check on every 5 seconds as documented but it does not refresh on file changes.

Problem: it appeared that the hashing algorithm always returns the same value no matter whether the files are changed or not. The calculation is based on three factors: file path, file size and time of last modification. I was changing a single file so the problem was in the last one. Another quick check and another surprise - os.stat(filename).st_mtime returns different value after the files is changed but the total hash stays unchanged. The problem was caused from st_mtime returning floating point value instead of integer.

Solution: Unfortunately the os.stat results appeared to be OS specific. Fortunately the type of the returned result is configurable. Calling os.stat_float_times(False) in the main module force it to return integer values.

Final words: these OS/version/settings things can always surprise you. And most of the time it is not a pleasant surprise. Unfortunately testing the code on hundred of different workstations is not an option. But this is one of the good sides of the open source software. You download it, test it, debug it and have fun.
Also special thanks to Olof(the man behind the pytddmon concept) for the tool, the quick response to my messages and for putting my fix so fast in the repo. I hope that this has saved someone nerves )

P.S. If you are using Python and like TDD - PYTDDMON is the right tool for you.

Django CMS Plugins with selectable template ...

Published at Oct. 9, 2011 | Tagged with: , , , , ,

... or how to reuse your plugins inside sections with different design

Problem: Frequently on the websites I am developing I need to display same set of data in several different ways. For example if I have a news box that needs to appear in different sections of the website e.i. in sidebar, main content etc. Using Django CMS plugins make this quite easy.
For simplicity we will take the following case. An image/text tuple with two layout variations - image on left of text and image on right.

Django CMS Plugins

Same data but different layout. All you need to do is just to allow your users to change the plugin template according to their needs. If you don't have experience with Django CMS Plugins I advice you to check how to create custom Django CMS Plugins before you continue with solution.

Solution: First you will have to create a tuple holding your templates(and their human readable names) and add a field that will hold the chosen template to the plugin model.

#models.py
PLUGIN_TEMPLATES = (
  ('image_on_left.html', 'Image on left'),
  ('image_on_right.html', 'Image on right'),
)

class SamplePlugin(CMSPlugin):
    # your plugin properties here
    template = models.CharField('Template', max_length=255,
                                choices = PLUGIN_TEMPLATES)
Now it is time to tweak the template render method too:
#cms_plugins.py
class CMSSamplePlugin(CMSPluginBase):
    model = SamplePlugin
    name = 'Sample plugin'
    render_template = PLUGIN_TEMPLATES[0][0]
    
    def render(self, context, instance, placeholder):
        if instance and instance.template:
            self.render_template = instance.template
        #your stuff here
        return context

Final words: Yep, this is all. Simple isn't it? It is amazing how sometimes such small things are so useful. If you are having bigger difference in the layout of your templates you will probably have to put a little more stuff in the context that some of your templates may not need but it is OK. Feel free to comment and if you are using this "trick" please add your use case - it will be interesting to see in how many different cases this works.

Neil Gaiman's Anansi Boys...

Published at Oct. 2, 2011 | Tagged with: , ,

... an unexpected gift and unexpected dedication

YOU KNOW HOW IT IS. You pick up a book, flip to the dedication, and find that, once again, the author has dedicated a book to someone else and not you.

Not this time.

Because we haven't yet met/have only a glancing acquaintance/are just crazy about each other/haven't seen each other in much too long/are in some way related/will never meet, but will, I trust, despite that, always think fondly of each other...

This one's for you.

With you know what, and you probably know why."

This is how Neil Gaiman's Anansi Boys starts.

About a week ago I spent few days in Greece. One day we were sitting under the shadow of a near bar when something grabbed my attention. Next to the bar there was a book shelf with something special on it:
Neil Gaiman's Anansi Boys

It was really strange to go in a foreign country, in unknown place and there a few meters away from me standing out with its black cover to see a book of one of my favourite authors. So I just took my camera, go there and take a shot on it.

The bartender asked me what am I shooting so I explained to him that it was a surprise for me to see that book. He took it as he was seeing it for a first time in his life, took a brief look at it and asked me whether I have read it. I said that unfortunately I am not. Then he took another look at the book and gave it to me as a gift. I took and open it and... it was starting with the dedication from the top of this post.

Coincidence? Maybe... or maybe not.

It is always a pleasure for me to discover pieces of art, no matter whether I am travelling or not, no matter is it an ancient castle or a book, it is great.
I haven't still finished the book but it looks amazing. I really recommend it as any other Gaiman's book. Hopefully I'll meet Neil one day till than(and after) I'll keep reading.
So if you are reading this go travel, or read books, or both. Have fun and share your experience.

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