It would be useful to have an efficient way to construct a fixed sized list using Python-like code in Cython.
Is your feature request related to a problem? Please describe.
For example, one might have the following code in Python...
def new_list(n: "Py_ssize_t") -> list:
l: list = n * [None]
return lThough once this is compiled, it appears to coerce n to a Python int, create a singleton list with None, and use Number based multiplication between n and the singleton list. Preferably this would allocate a list of size n and then fill it with None in a C for-loop.
Describe the solution you'd like
Instead of generating code that relies on append or handling things at the Python level, it would be helpful to have some code roughly like this (error handling and refcounting skipped for simplicity).
{
PyObject* l = PyList_New(n);
Py_ssize_t i;
for (i = 0; i < n; i++) {
PyList_SET_ITEM(l, i, Py_None);
}
// ...
]Describe alternatives you've considered
One could try to use generators, though they don't look at the size hint as noted in issue ( #1311 ) and issue ( #2844 ). Plus these wind up being less performant than this implementation is in pure Python. Though Cython performance is likely fine with the other approach. Perhaps if Cython reinterpreted n * [None] as [None for i in n], this could benefit from whatever solution is determined for the generator issues noted before.
Additional context
This comes up periodically when one wants to quickly preallocate a fixed sized Python list and some other alternative like an array wouldn't otherwise work (for example needing to hold arbitrary Python objects and/or user expecting a list to be returned).
It would be useful to have an efficient way to construct a fixed sized
listusing Python-like code in Cython.Is your feature request related to a problem? Please describe.
For example, one might have the following code in Python...
Though once this is compiled, it appears to coerce
nto a Pythonint, create a singletonlistwithNone, and useNumberbased multiplication betweennand the singletonlist. Preferably this would allocate alistof sizenand then fill it withNonein a Cfor-loop.Describe the solution you'd like
Instead of generating code that relies on
appendor handling things at the Python level, it would be helpful to have some code roughly like this (error handling and refcounting skipped for simplicity).{ PyObject* l = PyList_New(n); Py_ssize_t i; for (i = 0; i < n; i++) { PyList_SET_ITEM(l, i, Py_None); } // ... ]Describe alternatives you've considered
One could try to use generators, though they don't look at the size hint as noted in issue ( #1311 ) and issue ( #2844 ). Plus these wind up being less performant than this implementation is in pure Python. Though Cython performance is likely fine with the other approach. Perhaps if Cython reinterpreted
n * [None]as[None for i in n], this could benefit from whatever solution is determined for the generator issues noted before.Additional context
This comes up periodically when one wants to quickly preallocate a fixed sized Python
listand some other alternative like an array wouldn't otherwise work (for example needing to hold arbitrary Pythonobjects and/or user expecting alistto bereturned).