On Sat, Mar 5, 2022 at 11:22 PM Marco Sulla <marco.sulla.pyt...@gmail.com> wrote: > > I noticed that some functions inside dictobject.c that call insertdict > or PyDict_SetItem do an incref of key and value before the call, and a > decref after it. An example is dict_merge.
First of all, insertdict and PyDict is totally different about reference ownership handling. * PyDict_SetItem borrows reference of key and value from the caller as usual Python/C APIs. And it INCREF them before calling the insertdict(). * insertdict() takes the reference from its caller. In other words, insertdict() moves the owner of reference from its caller to the dict. merge_dict is very special and complex case. I assume you are talking about this part. https://github.com/python/cpython/blob/6927632492cbad86a250aa006c1847e03b03e70b/Objects/dictobject.c#L2885-L2912 In general, when reference is borrowed from a caller, the reference is available during the API. But merge_dict borrows reference of key/value from other dict, not caller. So dict_merge must have strong reference of key/value by INCREF before calling any APIs (e.g. _PyDict_Contains_KnownHash). That's why dict_merge calls INCREF key/value **twice** before calling insertdict, and DECREF key/value **once** after it. > Other functions, such as > _PyDict_FromKeys, don't do an incref before. > Again, insertdict takes the reference. So _PyDict_FromKeys() **does** INCREF before calling insertdict, when key/value is borrowed reference. https://github.com/python/cpython/blob/6927632492cbad86a250aa006c1847e03b03e70b/Objects/dictobject.c#L2287-L2290 https://github.com/python/cpython/blob/6927632492cbad86a250aa006c1847e03b03e70b/Objects/dictobject.c#L2309-L2311 On the other hand, slow path uses PyIter_Next() which returns strong reference. So no need to INCREF it. Additionally, the slow path uses PyDict_SetItem(), not insertdict(). PyDict_SetItem() does INCREF key/value for insertdict. So the slow path need to DECREF(key). https://github.com/python/cpython/blob/6927632492cbad86a250aa006c1847e03b03e70b/Objects/dictobject.c#L2327-L2329 This is complete guide why/when INCREF/DECREF key/value. -- Inada Naoki <songofaca...@gmail.com> -- https://mail.python.org/mailman/listinfo/python-list
