Example:

    class BrokenMapping:
        def __getitem__(self, key):
            1/0
    
    # this silences the ZeroDivisionError and raises KeyError('world')
    'hello {world}'.format_map(BrokenMapping())

I have tried this on several different CPython versions on Ubuntu 14.04
(including the latest Python 3.7.0a0 from github) and they all do this.
PyPy passes the ZeroDivisionError through correctly.

Yet one consequence: this swallows KeyboardInterrupt and converts it to KeyError.

While KeyboardInterrupt and ZeroDivisionError are easy to understand, IndexError converted to KeyError doesn't look like a major bug. So I'm not sure about fixing Python 3.5 and 3.6. Maybe it's ok because this function is rarely used and exceptions are not "expected": I hardly imagine a code explicitly catching KeyError to do something on format_map() error.

In short, my vote for backport is +0 :-)

I'm -0 on a backport. The new behavior is more correct, but I don't like changing the exception type in a micro release.

What are your thoughts about backporting this Raymond?

For Python 3.5 and 3.6, would it make sense to modify the code to pass though BaseException, but replace all other exceptions with KeyError? It would be a compromise between doing nothing and fixing the bug :-)

What are your thoughts about backporting this Raymond?

+0 On the one hand, this can be considered a bug fix. On the other hand, I would suspect that no code that is currently working would benefit.

I think all exceptions should be passed through. Things like
dict.__contains__ don't selectively turn some errors to KeyError
either; if something is not hashable it's a TypeError, not a KeyError.

    >>> [] in {}
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: unhashable type: 'list'

Oops, I forgot to mention that I have no thoughts about backporting
this. It would be nice to see this fixed in 3.7 though :)

I'm -0 on the backport since as Raymond and Eric pointed out it won't benefit currently working code and I personally don't like changing what exception is raised in a bugfix release.

New changeset 5075416 by Serhiy Storchaka in branch 'master':
bpo-30978: str.format_map() now passes key lookup exceptions through. (bpo-2790)
5075416

New changeset f08b2be by Serhiy Storchaka in branch '3.6':
[3.6] bpo-30978: str.format_map() now passes key lookup exceptions through. (GH-2790) (bpo-2992)
f08b2be