Steven D'Aprano wrote: > "is" is a comparison operator: it compares identity, not equality. It is > more or less equivalent to the expression id(x) == id(y).
Yes, the implementation of the is operator comes down to the comparison of PyObject* pointer addresses and in CPython id() returns the address of the PyObject*. But it's very important to understand that "a is b" can have a different result than "id(a) == id(b)". CPython uses all sorts of tricks to speed up common operations. Memory allocation/de-allocation and object creation can have a huge speed impact. Therefor Python uses free lists to keep some empty objects around for recycling. Small integers are cached, too. Compare >>> id(0) 135689168 >>> id(1) 135689184 >>> id(2) 135689200 with >>> id(2000) 3084440752 >>> id(2001) 3084440752 >>> id(2002) 3084440752 to see the effect of small int cache vs. int free list. Christian -- http://mail.python.org/mailman/listinfo/python-list
