[openstack-dev] Proposal for including fake implementations in python-client packages

Alex Gaynor alex.gaynor at gmail.com
Mon Jul 1 21:08:51 UTC 2013

Hi all,

I suspect many of you don't know me as I've only started to get involved in
OpenStack recently, I work at Rackspace and I'm pretty involved in other
open source stuff, notably Django and PyPy, I also serve on the board of the
PSF. So hi<wave />!

I'd like to propose an addition to all of the python-client libraries going
forwards (and perhaps a requirement for future ones).

What I'd like is for each client library, in addition to the actual
implementation, is that they ship a fake, in-memory, version of the API. The
fake implementations should take the same arguments, have the same return
values, raise the same exceptions, and otherwise be identical, besides the
that they are entirely in memory and never make network requests.

Why not ``mock.Mock(spec=...)``:

First, for those not familiar with the distinction between fakes and mocks
doubles and stubs and ...): http://mumak.net/test-doubles/ is a great
https://www.youtube.com/watch?v=Xu5EhKVZdV8 is also a great resource which
explains much of what I'm about to say, but better.

Fakes are better than Mocks, for this because:

* Mocks tend to be brittle because they're testing for the implementation,
  not the interface.
* Each mock tends to grow its own divergent behaviors, which tend to not be

  explains how to avoid this with fakes
* Mocks tend to encourage monkey patching, instead of "just passing objects

Again: https://www.youtube.com/watch?v=Xu5EhKVZdV8 is an amazing resource.

This obviously adds a bit of a burden to development of client libraries, so
there needs to be a good justification. Here are the advantages I see:

* Helps flesh out the API: by having a simple implementation it helps in
  designing a good API.
* Helps other projects tests: right now any project which uses an openstack
  client library has to do something manual in their tests, either add their
  own abstraction layer where they hand write an in-memory implementation,
  they just monkey patch the socket, http, or client library to not make
  request. Either direction requires a bunch of work from each and every
  project using an openstack client. Having these in the core client
  would allow downstream authors to simply swap out Connection classes.

I think these benefits out weigh the disadvantages. I'm not sure what the
procedure for this is going forward. I think to demonstrate this concept it
should start with a few (or even just one) client libraries, particularly
which completely own the resources they serve (e.g. swift, marconi,
trove), as compared to ones that interact more (e.g. neutrino, cinder, and
nova). This is absolutely something I'm volunteering to work on, but I want
ensure this is an idea that has general buy in from the community and
maintainers, so it doesn't wither.


"I disapprove of what you say, but I will defend to the death your right to
say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
"The people's good is the highest law." -- Cicero
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