"Assignment to reference" bug reintroduced in Cython 0.26.x

[dalleyg]

A bug was introduced in Cython 0.18 that caused code like this to fail

cdef void foo(double& out):
    out = 123

with an "Assignment to reference" error. See https://groups.google.com/d/msg/cython-users/j58Sp3QMrD4/y9vJy9YBi_kJ The bug was fixed and code like that worked through Cython 0.25.2. I just tried Cython 0.26.1 and the bug has reappeared.

As before, the workaround is to upgrade the reference parameter to a full pointer then dereference it:

cdef void foo(double& out):
    (&out)[0] = 123

[scoder]

Seems to be allowed in C++, but when I remove the error catcher in ExprNodes.py, it doesn't do what you would think. It simply reassigns the variable. I'll ask @robertwb what the intention of this error was.

[shaggyrogers]

This issue is still present as of version 0.27.3. Cheers for the workaround @dalleyg.

[ibarrond]

Still present as of 0.28.4

[spencer-hann]

still present in 0.29.13

[CrimsonVex]

still present in 0.29.14

[ewquon]

still present in 0.29.2

The upgrade/dereference workaround works for cython, but the resulting source code does not compile.

[scoder]

It may not be obvious to everyone, but the issue will remain present as long as no-one does something about it.

Since I don't have time to look into this, further investigations and/or a PR that resolves this are welcome.

[lixin-wei]

I've met this issue, too. Please fix this issue.

[da-woods]

Seems to be allowed in C++, but when I remove the error catcher in ExprNodes.py, it doesn't do what you would think. It simply reassigns the variable.

I think it does actually do what's intended (which is to modify the variable that the reference refers to)

#3987

[datdinhquok]

I'm converting Python types to C types and changing values of passed-by-ref params is a must coz Python is passbyref always, oh god, why is it not allowed in Cython? and my var is std::string, not even numeric values.

[da-woods]

@datdinhquok

changing values of passed-by-ref params is a must coz Python is passbyref always

But Python doesn't do pass-by-reference like C++ assignment to reference. For example

def f(x):
   x = False
a = True
print(a)  # True
f(a)
print(a)  # True

Based on Python behaviour we shouldn't support assignment to reference. Thanks for pointing that out.