cython/cython

Coercing a sliced C array to Python should create a list

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#2,159 opened on Mar 17, 2018

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Cython Language FeatureType Analysisenhancementgood first issuehelp wanted

Description

Cython already copies a C array into a Python list automatically on object coercion. It should do the same for sliced C arrays, e.g.

def test():
    cdef int i
    cdef int a[100]
    for i in range(100):
        a[i] = i**2
    return a[:20]  # rejected by type analysis

Currently results in Cannot convert 'int *' to Python object.

Contributor guide

Tech stack
cpython
Domain
tooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C programmingPythonCython type system
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
Examine how Cython currently coerces full C arrays to Python lists in the type analysis and code generation stages. Look for the code that produces the 'Cannot convert int * to Python object' error, likely in Cython/Compiler/TypeSlots.py or similar. Extend the logic to handle sliced C arrays by copying the slice into a new Python list before conversion. The existing approach for full arrays can serve as a template.