Undoc Assumption that Container Keys Cannot Be Non-String Immutables

[Jeff Rush]

The traversal code makes an undocumented assumption that keys() are strings or something that list(path) can be called on. But containers that have non-string but immutable values as keys raise the exception:

  path = list(path)
  TypeError: iteration over non-sequence

from within zope/app/traversing/adapters.py

In my case, my keys are small integers, which (correctly) fail to test as strings, and then raise the above exception when the code in adapters.py tries to coerce an integer to a list(). My workaround is to change my keys() to stringified ints. This issue should be reviewed as to whether this restriction is intentional/desirable and either removed or documented. There shouldn't be an unclear unassumption of the return type of IContainer.keys().

To duplicate the problem, just toss together a minimal IContainer, have keys() return unique integers, and traverse it, in my case, using @@singleBranchTree.xml from within the Rotterdam skin. But it fails for any traversal.

[Philipp von Weitershausen (philikon)]

The assumption is that container names are unicodes. Because of Python's (stupid) feature that strings can be compared to unicodes and are considered equal sometimes (e.g. 'hi' == u'hi'), ASCII strings are also acceptable.

If this is undocumented (I couldn't find any documetnation in IWriteContainer), then this indeed needs to be documented. Feel free to round up a patch, Jeff. :)

By the way, the traversing API is also documented by interfaces. Perhaps it should be made clear that names are always expected to be unicodes for traversal as well.

[Stephan Richter (srichter)]

Changes: submitter email, edited transcript, importance (medium => critical), revised version_info, new comment

I have fallen into this trap before as well. This really deserves a few lines of writing in the interfaces.

[Philipp von Weitershausen (philikon)]

Status: Pending => Resolved

IWriteContainer says:

    def __setitem__(name, object):
        """Add the given `object` to the container under the given name.

        Raises a ``TypeError`` if the key is not a unicode or ascii string.
        Raises a ``ValueError`` if key is empty.

So, it's pretty clear that names must be Unicode objects or ASCII strings. I have no idea how new this docstring is, but at least it's there now.

In addition to that I went through the traversal interfaces in zope.publisher and zope.traversing and added notes wherever applicable that traversed names must be ASCII or unicode strings as well. Checked in in r69595/r69596.