Store values for callbacks on insert/update/delete operations

[gi0baro]

Ok everybody, this replace the current behaviour of Table._listify:

• all the values computed before callbacks are passed to callbacks
• removed duplicated logic for default fields: removed _defaults method
• all the fields that have callable values are computed in the same flow (defaults, updates, computations)

Rationale:

• Fixing computed fields flow (see fix issue where Set.delete_uploaded_files removes files it should not… #429 )
• Allow to use computed fields inside callbacks (see Combining compute with callbaks emmett-framework/emmett#151 and fix issue where Set.delete_uploaded_files removes files it should not… #429)
• Have a unique flow for callable values

Breaking change:

• If you accessed a callable default or update value in callbacks you had to invoke that within the callback to get the real value. Now is just the real value.

Even if this is a breaking change, I consider the old behaviour wrong, since, for example, if you had a created_at field with default=lambda: datetime.utcnow(), and in an after_insert callback you wanted to access that value in the fields, you simply couldn't, since the real value inserted calling the method is not stored in the flow.

I would like to hear @mdipierro, @niphlod, @leonelcamara on this.

This changed also the flow regarding Table._attempt_upload, since now is a callback method in the before_insert and before_update. So while the old flow was:
_attempt_upload, callable_values, before_callback, operation

the new one is:
callable_values, before_callback, operation

[codecov-io]

Current coverage is 68.68% (diff: 66.49%)

Merging #431 into master will increase coverage by 0.18%

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[gi0baro]

@mdipierro @niphlod @leonelcamara ..comments?

[niphlod]

bummer, I reeeeaally don't have time to scour the code. IMHO both the rationale and the breaking change is fine.
I don't really have the time to assert where _attempt_upload should be.
Breaking something that theoretically should be atomic (the CRUD operation in the db AND the CRUD operation in the file) in all code-paths (stored to disk, stored in a virtual FS, converted to base64 and injected in another column) is something that, no matter how, is really error-prone given the flexibility uploads have (don't even get me started on storing the upload in a real binary field, which is a huuuuuge miss on dal/pydal since its conception).
The only thing I'm midly sure about is that NOT needs to happen after the _before_callback (or, in reverse words, it needs to happen either before or in _before_callback)

[gi0baro]

@niphlod I've just moved the attempt_upload into the before callbacks.

@mdipierro @leonelcamara I consider this ready to be merged. Still waiting for your comments.

[gi0baro]

@mdipierro I will merge this by Thu 22/12 unless you or @leonelcamara have any comments.

[mdipierro]

LGTM