On 2/26/2016 6:07 PM, eryk sun wrote:
On Fri, Feb 26, 2016 at 4:08 PM, Sven R. Kunze <srku...@mail.de> wrote:
Python sometimes seems not to hop back and forth between C and Python code.
Can somebody explain this?
Normally a C extension would call PySequence_SetItem, which would call
the type's sq_ass_item, which for MyList is slot_sq_ass_item. The
latter function bridges the CPython and Python sides by binding and
calling the overridden __setitem__ method. However, the _heapq
extension module uses `PyList_SET_ITEM(heap, 0, lastelt)`. This macro
expands to `((PyListObject *)(heap))->ob_item[0] = lastelt`. This
directly modifies the internal ob_item array of the list, so the
overridden __setitem__ method is never called. I presume it was
implemented like this with performance in mind, but I don't know
whether or not that justifies the loss of generality.
In other words, when that doc says *list*, it means a *list*.
"To create a heap, use a list initialized to [], or you can transform a
populated list into a heap via function heapify()."
Note: when the doc says 'dict' rather than 'dictionary' or 'mapping', it
is pretty obvious it means builtin dict. It may sometimes say 'dict or
dict subclass'. Ditto for 'str' versus 'string' or 'text'. However,
'list' (Python builtin) and generic 'list are easy to confuse. In this
case, 'initialized to []' is a hint. However, the doc could be made
clearer. How about
"A heap must be an instance of *list* (and not a subclass thereof). To
create a heap, start with [] or transform an existing list into a heap
via function heapify()."
*list* means to display it blue, linked to the list class.
--
Terry Jan Reedy
--
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