tuple parameter unpacking not removed for some lambdas

[daira]

    d.addCallback(lambda (rc, out, err), fn=fn:
                  self.failUnlessEqual(out, fn * 1000))

I would have expected this to be changed to something like

    d.addCallback(lambda rc_and_out_and_err, fn=fn:
                  self.failUnlessEqual(rc_and_out_and_err[1], fn * 1000))

(moved from mitsuhiko/python-modernize#23 )

[takluyver]

Playing with it a bit, it appears to only work if the lambda only has one argument. However, the code for this is in lib2to3, so it's not easy for us to fix.

[takluyver]

Yep, this is tracked in the Python bug tracker: http://bugs.python.org/issue16094

[daira]

It looks like they're not going to fix it, though (with reasoning I consider invalid).

This is a big issue for modernizing Tahoe-LAFS, which has hundreds of these lambdas.

[takluyver]

If it's important, we can copy and modify the fixer from 2to3:
http://hg.python.org/cpython/file/4d4a9094bdb0/Lib/lib2to3/fixes/fix_tuple_params.py

That code looks pretty involved, though.

[daira]

I think I see the problem: the fixer pattern is just not matching on a case like lambda (rc, out, err), fn=fn: ... because it is expecting parens around the whole parameter list.

The discussion in http://bugs.python.org/issue16094 is confused/wrong, because they're complaining about issues for which the fixer design has already chosen the same compromises for other very similar cases.