Contributing to Pretix

Recently I had the chance to contribute to Pretix (https://pretix.eu/about/en/) a Django-based event ticketing platform.

I submitted a couple of merge requests that were accepted: - https://github.com/pretix/pretix/pull/202/files https://github.com/pretix/pretix/pull/204/files

I will briefly detail some of the key learnings:

  1. Django models: Being absolutely new to Django, I had the opportunity to learn along the way about one of its key features, the Models. Models are objects that the ORM will store in the database, a quick intro here:  http://tutorial.djangogirls.org/en/django_orm/ It was quite surprising as well the ease of the development thanks to the automatic migrations in the database schema. More detailed info on Models here: http://www.djangobook.com/en/2.0/chapter05.html

  2. all keyword in Python: In order to check whether a condition was True for all the elements of a list I simply iterated over them and checked for the condition:

    for i, op in enumerate(positions):
        cancelable_products.append(op.item.allow_cancel)
    if False in cancelable_products:
        cancelable = False
    

    However a much simpler solution making use of list comprehensions and the all() keyword in Python is possible:

    cancelable = all([op.item.allow_cancel for op in positions])
    
  3. Metaclasses in Python: Although I had read about Python Metaclasses in this excellent post (https://jeffknupp.com/blog/2013/12/28/improve-your-python-metaclasses-and-dynamic-classes-with-type/), Django Models are a really good practical example of their value and use. The Metaclasses are used in the Django Models to dynamically create classes.

  4. Difference between select_related and prefetch_related in Django: prefetch_related: does a separate lookup for each relationship, and does the ‘joining’ in Python. This allows it to prefetch many-to-many and many-to-one objects.
    select_related: limited to single-valued relationships - foreign key and one-to-one. Creates an SQL join including the fields of the related object in the SELECTstatement retrieving the related objects in the same (but more complex) database query.
    By using select_related instead of prefetch_related, one query to the DB is saved.  More info on Django QuerySet: https://docs.djangoproject.com/en/1.10/ref/models/querysets/

Head over to this post for more details on Metaclasses in Django: http://reinout.vanrees.org/weblog/2013/05/16/advanced-python-metaclasses.html

See also:

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