[ZCM] [ZC] 1873/ 4 Comment "PersistentMapping can't be used to update/construct a BTree"

[Collector: Zope Bugs, Features, and Patches ...]

Issue #1873 Update (Comment) "PersistentMapping can't be used to update/construct a BTree"
 Status Pending, Database/bug medium
To followup, visit:
  http://www.zope.org/Collectors/Zope/1873

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= Comment - Entry #4 by tim_one on Aug 22, 2005 10:45 am

More discussion starting here:

http://mail.zope.org/pipermail/zodb-dev/2005-August/009126.html

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= Edit - Entry #3 by tim_one on Aug 22, 2005 10:43 am

 Changes: submitter email, edited transcript, revised title, new comment

Changed Title; note that PersistentMapping not playing nicely with the iteration prototol is also part of this.
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= Comment - Entry #2 by tim_one on Aug 22, 2005 10:09 am

This is so messy it may not get changed.  A BTree constructor or update() takes a sequence as argument (in which case it must be a sequence of 2-tuples), or an object with an .items() method.  Changing any of that would have a major chance of being backward-incompatible.

PersistentMapping "looks like a sequence", so that's the path that's taken.  _Any_ way of using a PersistentMapping as a sequence is likely to blow up the same way.  Examples:

>>> from ZODB.PersistentMapping import PersistentMapping
>>> pm = PersistentMapping({1: 2})
>>> for x in pm:
...   print x
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\python23\lib\UserDict.py", line 19, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0

>>> list(pm)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\python23\lib\UserDict.py", line 19, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0

>>> i = iter(pm)
>>> i.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\python23\lib\UserDict.py", line 19, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0

The same thing happens if you try to use a PersistentMapping as an argument to a BTree update method too.

Until (if ever) this changes, the way to get what you want is to invoke pm.items().  Like

>>> from BTrees.OOBTree import OOBTree
>>> b = OOBTree()
>>> b.update(pm.items()) # no problem
>>> list(b.items())
[(1, 2)]


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= Request - Entry #1 by suvit on Aug 22, 2005 5:49 am

>>> import ZODB
>>> from ZODB.PersistentMapping import PersistentMapping
>>> from BTrees.OOBTree import OOBTree

works
>>> d1 = OOBTree()
>>> d1[1] = 1
>>> pm = PersistentMapping({2:2})
>>> d = {1:1}
>>> bt = OOBTree({3:3})
>>> bt.update(d)
>>> pm.update(bt)
>>> dict(bt)
{1: 1, 3: 3}

But not works
>>> OOBTree(pm)
Traceback (most recent call last):
  File "<pyshell#12>", line 1, in ?
    OOBTree(pm)
  File "C:\Python23\Lib\UserDict.py", line 14, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0
>>> bt.update(pm)
Traceback (most recent call last):
  File "<pyshell#9>", line 1, in ?
    bt.update(pm)
  File "C:\Python23\Lib\UserDict.py", line 14, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 0
>>> 

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