Undo deprecation warning for len(batch)

[mauritsvanrees]

In Plone 5.2 you get lots of deprecation warnings like this, in real-world use, and for easy of reproduction also when running for example this test (edited to filter out other warnings):

$ bin/test -s Products.CMFPlone -t pwreset_browser.rst
...
  Running:
    1/1 (100.0%)/Users/maurits/community/plone-coredev/py3/var/cache/f8b4063628323927100f9f247fe9c029.py:460: DeprecationWarning: Using len() is deprecated. Use the `length` attribute for the size of the current page, which is what we return now. Use the `sequence_length` attribute for the size of the entire sequence. 
  if __condition:
/Users/maurits/community/plone-coredev/py3/src/Zope/src/Products/PageTemplates/engine.py:74: DeprecationWarning: Using len() is deprecated. Use the `length` attribute for the size of the current page, which is what we return now. Use the `sequence_length` attribute for the size of the entire sequence. 
  iterable = list(iterable) if iterable is not None else ()
/Users/maurits/community/plone-coredev/py3/var/cache/0a229eac7e57a317bdd047ef739d4daf.py:137: DeprecationWarning: Using len() is deprecated. Use the `length` attribute for the size of the current page, which is what we return now. Use the `sequence_length` attribute for the size of the entire sequence. 
  if __condition:
...
/Users/maurits/community/plone-coredev/py3/src/Zope/src/Products/PageTemplates/Expressions.py:253: DeprecationWarning: Using len() is deprecated. Use the `length` attribute for the size of the current page, which is what we return now. Use the `sequence_length` attribute for the size of the entire sequence. 
  return bool(value)
/Users/maurits/shared-eggs/cp37m/zope.deprecation-4.4.0-py3.7.egg/zope/deprecation/deprecation.py:88: DeprecationWarning: isDefaultPage is deprecated. Import from Products.CMFPlone.defaultpage instead
  name)
/Users/maurits/community/plone-coredev/py3/var/cache/16a60114d1d42d8280752740f6a1c9e7.py:1871: DeprecationWarning: Using len() is deprecated. Use the `length` attribute for the size of the current page, which is what we return now. Use the `sequence_length` attribute for the size of the entire sequence. 
  if __condition:

Those are actually two different warnings. The second one is due to a missing __bool__ and will be fixed with my PR #3175. The first one is from code in Products.PageTemplates that handle iteration over a repeat. If you have a batch and in a template you do tal:repeat="item batch", then this code calls len(batch) to get the length of repeat. Actually, list(batch) already does this, when I look at the traceback.

The call to len(batch) is deprecated in CMFPlone. This was first done in 2011, for Plone 4.2, in commit 84bdce6. It advocated using the pagesize attribute at first. In 2015, for Plone 5.0, in commit a66ed54 I changed this to recommend length or sequence_length, depending on what you wanted.

But as you can see, Products.PageTemplates happily calls len(batch), and this happens everytime someone uses a batch in a template. This makes the deprecation useless by now, I think: people cannot avoid this warning.

An option could be to change this in Products.PageTemplates (and possibly chameleon as well, but I don't want to think about that just now), something like:

if hasattr(iterable, "length"):
    length = iterable.length
    if callable(length):
        length = length()
else:
    length = len(iterable)

But I don't think it makes sense to have special code like this for PloneBatch in Products.PageTemplates.

So a proposal: let's undo this deprecation. In other words: remove these lines. (But keep them as a comment probably.) This would be for Plone 5.2.3.

@plone/framework-team Thoughts?

[ale-rt]

I agree with undoing the deprecation. It makes sense that the length of the batch is the number of batched element.