Empty Thoughts

Elegant Testing Decorators

I’ve been working on fleshing out the tests for Django-SQLAlchemy and right away I discovered a problem. Incidentally, this is why I love unit testing. The problems was that a very simple test of the query contains filter syntax was failing. I was expecting four items but was getting five back. After digging into it I discovered the problem was with SQLite. SQLite doesn’t seem to respect case when using the LIKE syntax.

Let us say we have the following setup:

from apps.blog.models import Category

class TestContains(object):
    def setup(self):
        Category.__table__.insert().execute({'name': 'Python'}, 
            {'name': 'PHP'}, {'name': 'Ruby'}, {'name': 'Smalltalk'}, 
            {'name': 'CSharp'}, {'name': 'Modula'}, {'name': 'Algol'},
            {'name': 'Forth'}, {'name': 'Pascal'})

a query with contains in Django on SQLite, like the following, will return five results, instead of the expected four results:

>>> Category.objects.filter(name__contains='a').count()
2008-03-28 20:41:43,228 INFO sqlalchemy.engine.base.Engine.0x..f0 BEGIN
2008-03-28 20:41:43,229 INFO sqlalchemy.engine.base.Engine.0x..f0 SELECT count(foo_category.id) AS count_1 
FROM foo_category 
WHERE foo_category.name LIKE ?
2008-03-28 20:41:43,229 INFO sqlalchemy.engine.base.Engine.0x..f0 ['%a%']
5

(Incidentally that’s actually going through the Django-SQLAlchemy backend and not Django’s ORM.)

So I kind of hemmed and hawed for a bit trying to figure out how I could make this work under different databases, at least from the testing standpoint. It finally occurred to me that SQLAlchemy has to be able to test their ORM against a lot of different backends so they must have a nice solution to this.

Decorators to the Rescue

It turns out that SQLAlchemy has implemented an elegant set of decorators for just this problem. They were also written in such a way that it was quite easy for me to extract them and modify them slightly to work with Django-SQLAlchemy tests. So what’s in this package?

• fails_if(callable_) – Mark a test as expected to fail if callable_ returns True.
• future – Mark a test as expected to unconditionally fail.
• fails_on(dbs) – Mark a test as expected to fail on one or more database implementations.
• fails_on_everything_except(dbs) – Mark a test as expected to fail on most database implementations.
• unsupported(dbs) – Mark a test as unsupported by one or more database implementations.
• exclude(db, op, spec) – Mark a test as unsupported by specific database server versions. This decorator allows an impressive list of options, for example @exclude('mydb', '<', (1,0))

There’s a lot more than that, but I will not detail them all here. If you want to dig through it all check out test/testlib/testing.py module.

The Implementation

So once I was able to extract and modify these decorators I ended up with very elegant syntax for my tests. Here is a sample:

class TestContains(object):
    def setup(self):
        Category.__table__.insert().execute({'name': 'Python'}, 
            {'name': 'PHP'}, {'name': 'Ruby'}, {'name': 'Smalltalk'}, 
            {'name': 'CSharp'}, {'name': 'Modula'}, {'name': 'Algol'},
            {'name': 'Forth'}, {'name': 'Pascal'})

    @fails_on('sqlite')
    def test_should_contain_string_in_name(self):
        assert 4 == Category.objects.filter(name__contains='a').count()
        assert 1 == Category.objects.filter(name__contains='A').count()

    @fails_on_everything_except('sqlite')
    def test_should_contain_string_in_name_on_sqlite(self):
        assert 5 == Category.objects.filter(name__contains='a').count()
        assert 5 == Category.objects.filter(name__contains='A').count()

    def test_should_contain_string_in_name_regardless_of_case(self):
        assert 5 == Category.objects.filter(name__icontains='a').count()
        assert 5 == Category.objects.filter(name__icontains='A').count()

Special thanks goes to Mike Bayer and the rest of the contributors to SQLAlchemy for providing such a great solution. I am constantly amazed by their code.

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