Original use case:

I want to mock something that on invocation does 2 things:

• updates database
• returns custom, unrelated value

The problem:
side_effect is a catch-all setting that is used for:

• true side-effects
• return values
• exceptions

Moreover, side_effect takes precedence over return_value, unless the earlier returns mock.DEFAULT. Very quirky.

Proposed change option 1:
Side-effect is not used as return value; explicit return value may be used for that purpose.
(probably changes a lot of tests)

Proposed change option 2:
return_value, being more specific and explicit, takes precedence over side_effect; that is side_effect is still executed but it's rv is lost if overwritten by return_value is set.
(this is unlikely to change existing tests, as it is currently very odd to use both side_effect and return_value at the same time.)

Current workaround 1:
mock.Mock(side_effect=lambda *args: [db.update(), mock.DEFAULT][-1],
return_value=42)

Current workaround 2:
mock.Mock(side_effect=lambda *args: [db.update(), 42][-1])

Can you explain why you can't use side_effect to set the return value?

I can, as shown in workaround #2.

However it's hackey when true side-effect's own return value has nothing in common with return value for the mock.

The shortest workaround I managed is this:

mock.Mock(side_effect=lambda *arg: db.update(...) or my_return_value) # relies on db.update returning None

or more explicit like this:

mock.Mock(side_effect=lamda *arg: db.update(...) or mock.DEFAULT, return_value=my_return_value) # -"-

Oh, sorry, I missed the workarounds when I read the issue. So your issue is you want to use a lambda rather than a full function for the side effect and it looks ugly.

I agree that given the names 'side_effect' and 'return_value' you'd think return_value would take precedence. On the other hand, the API is what it is, and I'm not sure we can change it for backward compatibility reasons. Option 1 is definitely not on the table, and the problem with option 2 is that it is easy to imagine a side_effect that conditionally returns mock.DEFAULT.

One of the use cases for side_effect is for dynamically changing the returned value based on input - so your option 1 just doesn't work.

Using two line functions would make your code easier to read, and then your "workarounds" would look natural instead of awkward.

Maybe return_value should take precedence over side_effect if both are set.

It would probably be more pythonic to raise an exception in that case - although that would be annoying if you're just reconfiguring a mock. I've also written side_effect functions that accessed the return_value, which is probably the genesis of the current API.

As you point out, both your suggested changes are backwards incompatible, so I'm closing this issue.