[pytest-dev] Test parameterization via Fixtures

[Daniel Neuhäuser]

Hello,

I have recently re-discovered py.test searching for testing solution for a
new project and have started using it in said project. However I have also
noticed that fixtures are rather heavily constrained, most importantly in
that a fixture cannot have multiple values.

To give an example one of the things that are part of my new project is
password hashing. What I would like to be able to do with fixtures is have
a fixture `password` that creates several random passwords and I would like
a test that uses the fixture to be called with each of these passwords.

Furthermore I would like to be able to create other fixtures such as a
`pw_hash` fixture that depends on the `password` fixture and creates hashes
for each password.

I would also like to be able to write tests that take multiple fixtures and
while being parametrized "naturally". An example would be a function
`test_pw_hash_verify(pw_hash, password)` that takes both two fixtures
`pw_hash` and `password`. In this case I would like `test_pw_hash_verify`
to be called with password and the corresponding `pw_hash`.

In more abstract terms I would like to have a fixture system for which the
following rules hold:

- A fixture `fixture()` represents set of values.
- A fixture `fixture(a)` is a n:n (n:m?) mapping of the values represented
by `a` to a new set of values.
- A fixture `fixture(*fixtures)` is a n:m mapping of the cartesian product
of the values represented by each in `fixtures` if they are all independent
from one another. If any fixture `a` in `fixtures` is dependent upon one or
more other fixtures `b_` in `fixtures` `fixture(*fixtures)` shall only be
called with the values of `a` and `b_` so that the values `a` was called
with are equal to the ones `fixtures(*fixtures)` is called with.

I have created a very simple implementation of this algorithm to play
around with as an example, which I hope will better illustrate the idea:
https://gist.github.com/4408054

What I would like to know is whether there is any convenient way to do that
now without abusing py.test significantly, whether anyone is working on
this or something equivalent already or if there is some huge flaw in this
idea, that I am missing at the moment.
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