I have a Django based project, and am doing unit tests with py.test. To debug a test failure it’s sometimes useful to see the actual SQL queries that Django emitted, which is surprisingly hard. I assumed that that would be such an obvious and common need, that a simple switch (for pytest-django) or easy plugin would exist to simply output SQL queries as they are executed.

It is a common need alright (1, 2, 3), but the correct solution is surprisingly unwieldy¹.

For one, there is no existing helper or plugin. There are helpers and plugins to count queries and assert a certain query count, which as a side effect track all queries and print the executed queries on query count assertion failure, but I’ve yet to find any case where that would be useful to me. More importantly it’s useless for the exact case here: The stored list of queries is only printed if the expected query count is not matched, not in any other case, such as, say, a failing unit test which you’d want to debug by inspecting the queries that were executed.

Therefore: Fuck it, let’s do it live. Django tracks all queries in the connection object, but in general only if DEBUG=True. For various reasons, tests are executed with DEBUG=False, which is a good thing, since you want to test close to production. Django does provide a context helper to temporarily enable query tracking on a connection which we’ll use instead.

Putting it together, we need to transform a humble test such as

@pytest.mark.django_db
def test_frobnicate_foo(foo):
    assert foo.frobnicate()

into

@pytest.mark.django_db
def test_frobnicate_foo(foo):
    from django.db import connection
    from django.test.utils import CaptureQueriesContext
    with CaptureQueriesContext(connection):
        assert foo.frobnicate(), connection.queries[0]['sql']

in order to see the value of the first SQL query in case of assertion failure.

At some point someone™ should write a generic plugin to do that.

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1. There are several incorrect solutions on StackOverflow, such as the one that starts with “First, subclass TestCase”, which doesn’t apply to py.test, or the ever helpful “try using django-debug-toolbar”, which doesn’t apply to unit tests in general. ↩