CODEWITHSUNDEEP
pythonpython-3.xpython-2.7python-c-api
Rakesh Agarwal
Rakesh Agarwal

Reputation: 3059

Is there a way to resize python list using Python-C API?

My C-application is loading the python interpreter dll and calling the function list = PyList_New(len) to create a new python list of 'len' size.

Two questions:
1. What happens if I call PyList_Append(list, item) more than len times? I assume it should throw an exception. Is my understanding correct?

2. More importantly, my second question is whether there is a way to resize the list? I looked at the C APIs at: https://docs.python.org/2/c-api/list.html#c.PyList_SetItem, but could not find such an API. Any suggestions/comments/workaround?

The context of my question is that I need to create the list much earlier than when the list items and the count of them is known.

Upvotes: 2

Views: 311

Answers (1)

DavidW
DavidW

Reputation: 30936

PyList_New creates a list filled with NULL pointers. You're supposed to set all of those pointers using PyList_SetItem before sending the list back to Python or calling any Python functions on it.

PyList_Append (and PyList_Insert) are really just equivalent to calling somelist.append (and somelist.insert) from Python. They resize the list for you. I'm not 100% sure if they account any NULL elements, so it's probably best not to call them on a list that you've made from PyList_New and that you haven't completely filled yet. What you're doing (allocating a list with some NULL pointers then appending it) is definitely wrong since the NULL pointers are never filled in. Instead you should just be using PyList_Append and PyList_Insert on a list built from empty. You could also call list.extend to add multiple elements to the list at once - there is no direct access to that through the C API but you can call it through PyObject_CallMethodObjArgs or similar.

Internally the list allocates more space than it needs to it only has to reallocate occasionally. There's no way for you to control this.

Upvotes: 0

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