Allow checks based on values

[mitchellrj]

This pull request adds the ability to perform checks based on:

• Attribute deletion as opposed to setting
• New values to be assigned to attributes if the check passes
• Operands to in-place mutation operations (__iadd__, __imul__, etc) which would normally be called with check rather than check_getattr.

It achieves this by adding a new interface with the following extra check methods which are called by the proxy objects if provided:

• check_setattr_with_value: for checks based on values to be assigned to attributes. Signature is as check_setattr except with an extra value parameter.
• check_with_value: for checks based on operands of in-place mutation operations. Signature is as check except with an extra value parameter.
• check_delattr: for checks based on attribute deletion rather than mutation.

[mitchellrj]

(I have signed the Zope contributor agreement already)

[mitchellrj]

@tseaver any input?

[tseaver]

A couple of style preferences here:

• Please enclose even single-statement suites in braces (FBO later edits by indenttion-centric Python developers).
• Can you add / fix comments for each suite? I think the one in line #187 is really for line #203.

[tseaver]

Other than the inline comment, the PR looks good to me. I don't think the performance impact is going to be important (but I guess I could be wrong).

[mitchellrj]

Thanks! Should all be sorted now. Ready for merge / release whenever.

[tseaver]

It looks good to me. I'm going to ask @jimfulton to review for performance impact, as I don't actually use the checkers in production.

[mitchellrj]

@tseaver @jimfulton bump

[mitchellrj]

I have now lost access to the repository from which this pull request was made. If it needs rebasing, this will have to be done elsewhere.

[mgedmin]

I pushed your changes to the mitchellrj-extended-setattr-behavior branch in this repo so they don't get lost.

[tseaver]

Superseded by PR #25.