PyDict_*() refcount assumptions

[Gabriel Genellina]

En Wed, 31 Oct 2007 22:01:53 -0300, <sndive at gmail.com> escribió:

> i checked this:  
> http://svn.python.org/projects/python/trunk/Doc/data/refcounts.dat
> and it seems that
> PyDict_SetItem incref the value being added and the key
> and PyDict_DelItem does not decrement any refcounts
> so i have to do so manually for the key and for the data( by calling
> PyDict_GetItem first).

Looking at the implementation, PyDict_DelItem decrements both the previous  
key *and* its associated value. The stored key might not be the same as  
the parameter; consider this example:

d = {}
d[1] = 'a'
del d[1.0]
assert d == {}

I think the refcounts.dat file is more intended for automatic tools than  
for humans. It cannot express the fact that PyDict_DelItem decrements both  
the stored key and its associated value, by example. This is a limitation  
of the chosen format. PyDict_Clear decrements a lot of references but  
there is no way to express that in the refcounts.dat file.

> Did i decipher the dat file correctly and is there a wrapper for
> DelItem that reverses
> the refcounts to what they were before the SetItem or do i have have
> to do 2 calls for
> deletion?

You don't need any wrapper: PyDict_SetItem and PyDict_DelItem do "the  
right thing" with their reference counts, so there is no need of  
additional increments/decrements.

-- 
Gabriel Genellina