Cython Cannot Insert Into Deque Using Iterators

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1 cython cannot insert into deque using iterators

Problem Description:

I am wondering if this is a bug or I am doing something wrong:

from libcpp.deque cimport deque as cdeque

cdef cdeque[int] dq1, dq2

dq1.push_back(0); dq1.push_back(1)
dq2.push_back(2); dq2.push_back(3)
dq1.insert(dq1.begin(), dq2.begin(), dq2.end())

The above code gives me 2 similar errors at compile time: Cannot convert ‘iterator’ to Python object. Each error pointing at dq2.begin() and dq2.end().
When inserting only an int it seems to work but not with iterators.
Any ideas?

Solution – 1

This was a bug in Cython versions prior to 3.0 (see this PR).

On an prior version, we have the following options

A: wrap insert, e.g.

....
cdef extern from *:
    """
    template <typename T, typename It>
    void insert(T& dest, const It& begin, const It& end){
        dest.insert(dest.begin(), begin, begin)
    }
    """
    void insert[T, It](T& dest, const It& begin, const It& end)  
...
insert(dq1, dq2.begin(), dq2.end())

B: changing/patching libcpp/deque.pxd in the local installation from

void insert(iterator, input_iterator, input_iterator)

to correct

void insert[InputIt](iterator, InputIt, InputIt) except +

This however, will not help if pyx-files needs to be build somewhere else until Cython 3.0 isn’t released/used.

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