Whilst developing two webapps, one at work and one for the los palmas
seven, I found that when writing the
tests for the data model (hell yes I use test driven development, it’s very productive
as it defines short and medium term goals and keeps you in focus) I
was writing the same code over and over, but was very reliant on the
structure of the data model I was testing.
Having done lots of currying in functional programming back in uni, I
could see there was an obvious pattern, and thus it needed refactoring
to reduce the amount of code I was writing and in doing so reduce the
probability of error.
So I pulled out a set of functoins from one test, and made it a base
class, and refactored it to allow the child class to specify the data;
but due to the way python’s unittest and the test runner nose
work, I had to manually write a function in each child class to call
the worker functoin in the parent.
Obviously requiring the user to type more meant that there was a risk
of error, and it was also tedious (read: very boring) to have to do
this every time. Not knowing enough about python internals, I posted
to the new slug coders
list, christening it with
the first development related post, and got back a good suggestion,
but I didn’t try it out. Michael K suggested at the slug meeting last
friday that I check out metaclasses if his previous suggestion didn’t
work.
Saturday I jumped into the deep end with metaclasses, and it turned
out they were easier than I thought, but having done so now I
wouldn’t recommend them as a hammer to bash in all those screw and
rivet problems. They are very cool though.
So I ended up with a base class that had a metaclass that would monkey
patch the child class with the correct method names just as it was
being inspected, and had a small test program written that
demonstrated my idea was sound.
However, applying this method to the actual tests running under nose
didn’t work. Lots of debugging printfs later I eventually traced this
to a peculiarity in the way nose decides on what tests to run: to
prevent test methods being run twice, it makes sure that when running
a test, the module that it is defined in is the same as the module
currently being tested, i.e. it makes sure that __module__ matches
in both the callable and the current test case.
Now, when you define your methods in a parent, the method’s module is
that of the parent, so a structure like the following:
tests/module/
tests/module/__init__.py
tests/module/test_something.py
sets __module__ in __init__.py to tests.module and in
test_something.py to tests.module.test_something. Running a
method from tests.module.test_something that’s defined in
tests.module, nose says no.
Enter Benno and his knowledge of python
internals. An impromptu 7 hackfest on
Sunday at
jdub’s house let me show him what I was
trying to do, where he suggested using python’s new package, and the
im_func attributes on callables, to build a workaround for nose’s
features, which is much better than what I was thinking of doing to
solve the problem (something about injecting docstrings into the child
class and then evaling them in the child’s context).
Some quick hacks later, we had a test program that showed it’d work,
and patched up the base test class. Appended for your enjoyment, a
base test class that one can inherit from to automatically generate
common tests for tables when using
SQLAlchemy.
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| from unittest import TestCase
import sqlalchemy
import zookeepr.models as model
class TestBase(TestCase):
def assertRaisesAny(self, callable_obj, *args, **kwargs):
"""Assert that the ``callable_obj`` raises any exception."""
try:
callable_obj(*args, **kwargs)
except:
pass
else:
self.fail("callable %s failed to raise an exception" % callable_obj)
def monkeypatch(cls, test_name, func_name):
"""Create a method on a class with a different name.
This method patches ``cls`` with a method called ``test_name``, which
is bound to the actual callable ``func_name``.
In order to make sure test cases get run in children of the assortment
of test base classes in this module, we do not name the worker methods
with the prefix 'test_'. Instead they are named otherwise, and we
alias them in the metaclass of the test class.
However, due to the behaviour of ``nose`` to not run tests that are
defined outside of the module of the current test class being run, we
need to create these test aliases with the model of the child class,
rather than simply calling ``setattr``.
(Curious readers can study ``node.selector``, in particular the
``wantMethod``, ``callableInTests``, and ``anytests`` methods (as of
this writing).)
You can't set __module__ directly because it's a r/o attribute, so we
call ``new.function`` to create a new function with the same code as
the original. The __module__ attribute is set by the new.function
method from the globals dict that it is passed, so here we make a
shallow copy of the original and override the __name__ attribute to
point to the module of the class we're actually testing.
By this stage, you may think that this is crack. You're right.
But at least I don't have to repeat the same code over and
over in the actual tests ;-)
"""
# get the code
code = getattr(cls, func_name).im_func.func_code
# get the function globals so we can overwrite the module
g = getattr(cls, func_name).im_func.func_globals.copy()
g['__name__'] = cls.__module__
# create a new function with:
# the code of the original function,
# our patched globals,
# and the new name of the function
setattr(cls, test_name, new.function(code, g, test_name))
class TableTestGenerator(type):
"""Monkeypatching metaclass for table schema test classes.
This metaclass does some funky class method rewriting to generate
test methods so that one doesn't actually need to do any work to get
table tests written. How awesome is that for TDD? :-)
"""
def __init__(mcs, name, bases, classdict):
type.__init__(mcs, name, bases, classdict)
if 'table' in classdict:
monkeypatch(mcs, 'test_insert', 'insert')
for k in ['not_nullable', 'unique']:
if k + 's' in classdict:
monkeypatch(mcs, 'test_' + k, k)
class TableTest(TestBase):
"""Base class for testing the database schema.
Derived classes should set the following attributes:
``table`` is a string containing the name of the table being tested,
scoped relative to the module ``zookeepr.models``.
``samples`` is a list of dictionaries of columns and their values to use
when inserting a row into the table.
``not_nullables`` is a list of column names that must not be undefined
in the table.
``uniques`` is a list of column names that must uniquely identify
the object.
An example using this base class:
class TestSomeTable(TestTable):
table = 'module.SomeTable'
samples = [dict(name='testguy', email_address='test@example.org')]
not_nullables = ['name']
uniques = ['name', 'email_address']
"""
__metaclass__ = TableTestGenerator
def get_table(self):
"""Return the table, coping with scoping.
Set the ``table`` class variable to the name of the table variable
relative to anchor.model.
"""
module = model
# cope with classes in sub-models
for submodule in self.table.split('.'):
module = getattr(module, submodule)
return module
def check_empty_table(self):
"""Check that the database was left empty after the test"""
query = sqlalchemy.select([sqlalchemy.func.count(self.get_table().c.id)])
result = query.execute()
self.assertEqual(0, result.fetchone()[0])
def insert(self):
"""Test insertion of sample data
Insert a row into the table, check that it was
inserted into the database, and then delete it.
Set the attributes for this model object in the ``attrs`` class
variable.
"""
self.failIf(len(self.samples) < 1, "not enough sample data, stranger")
for sample in self.samples:
print "testing insert of s %s" % sample
query = self.get_table().insert()
print query
query.execute(sample)
for key in sample.keys():
col = getattr(self.get_table().c, key)
query = sqlalchemy.select([col])
print "query", query
result = query.execute()
print result
row = result.fetchone()
print "row", row
self.assertEqual(sample[key], row[0])
self.get_table().delete().execute()
# do this again to make sure the test data is all able to go into
# the db, so that we know it's good to do uniqueness tests, for example
for sample in self.samples:
query = self.get_table().insert()
query.execute(sample)
# get the count of rows
query = sqlalchemy.select([sqlalchemy.func.count(self.get_table().c.id)])
result = query.execute()
# check that it's the same length as the sample data
self.assertEqual(len(self.samples), result.fetchone()[0])
# ok, delete it
self.get_table().delete().execute()
self.check_empty_table()
def not_nullable(self):
"""Check that certain columns of a table are not nullable.
Specify the ``not_nullables`` class variable with a list of column names
that must not be null, and this method will insert into the table rows
with each set to null and test for an exception from the database layer.
"""
self.failIf(len(self.samples) < 1, "not enough sample data, stranger")
for col in self.not_nullables:
print "testing that %s is not nullable" % col
# construct an attribute dictionary without the 'not null' attribute
coldata = {}
coldata.update(self.samples[0])
coldata[col] = None
# create the model object
print coldata
query = self.get_table().insert()
self.assertRaisesAny(query.execute, coldata)
self.get_table().delete().execute()
self.check_empty_table()
def unique(self):
"""Check that certain attributes of a model object are unique.
Specify the ``uniques`` class variable with a list of attributes
that must be unique, and this method will create two copies of the
model object with that attribute the same and test for an exception
from the database layer.
"""
self.failIf(len(self.samples) < 2, "not enough sample data, stranger")
for col in self.uniques:
self.get_table().insert().execute(self.samples[0])
attr = {}
attr.update(self.samples[1])
attr[col] = self.samples[0][col]
query = self.get_table().insert()
self.assertRaisesAny(query.execute, attr)
self.get_table().delete().execute()
self.check_empty_table()
|
Feel free to use this in your own code, I’m placing it in the public
domain.