Make PyTuple constructors return &PyTuple

[birkenfeld]

This has a test failure in test_gc:

---- create_pointers_in_drop stdout ----
thread 'create_pointers_in_drop' panicked at 'assertion failed: `(left == right)`
  left: `2127`,
 right: `2126`', tests/test_gc.rs:123:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Also I'm not sure about the FromPy impl...

[konstin]

I assume the test calls PyTuple::empty which had returned an owned reference (Py<PyTuble) while &PyTuple isn't owned. Creating an owned reference increases the reference count by 1, which that test expects to happen.

pyo3/tests/test_gc.rs

Lines 96 to 98 in 874d8a0

	let _empty1 = PyTuple::empty(py);
	let _empty2: PyObject = PyTuple::empty(py).into();
	let _empty3: &PyAny = py.from_owned_ptr(ffi::PyTuple_New(0));

[birkenfeld]

Tried to make this equivalent to before using from_py, but still failing...

[Alexander-N]

Added some comments to clear up why the test fails. I'm not sure what this test is trying to assert so I can't say if everything is ok.

[Alexander-N]

After calling drop(inst) the refcount of empty is increased by 4. Only 2 of those references survive the end of the scope.

[Alexander-N]

PyTuple::empty(py) increases the refcount of the empty tuple by 1 and FromPy::from_py(PyTuple::empty(py), py) by 2. So if Py<PyTuple> is used here for empty (not necessary) then cnt has to be corrected with

cnt = empty.get_refcnt() - 2;

[codecov]

Codecov Report

Merging #447 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #447   +/-   ##
======================================
  Coverage    87.8%   87.8%           
======================================
  Files          65      65           
  Lines        3435    3435           
======================================
  Hits         3016    3016           
  Misses        419     419

Impacted Files	Coverage Δ
src/types/tuple.rs	90% <100%> (+0.44%)	⬆️
src/conversion.rs	97.34% <100%> (ø)	⬆️
src/instance.rs	96.66% <0%> (-0.07%)	⬇️

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[codecov]

Codecov Report

Merging #447 into master will decrease coverage by 0.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
- Coverage    87.8%   87.44%   -0.36%     
==========================================
  Files          65       65              
  Lines        3435     3410      -25     
==========================================
- Hits         3016     2982      -34     
- Misses        419      428       +9

Impacted Files	Coverage Δ
src/types/tuple.rs	90% <100%> (+0.44%)	⬆️
src/conversion.rs	95.57% <100%> (-1.77%)	⬇️
src/types/datetime.rs	94.44% <0%> (-5.56%)	⬇️
src/class/iter.rs	77.77% <0%> (-5.56%)	⬇️
src/types/string.rs	88.13% <0%> (-1.7%)	⬇️
src/class/macros.rs	86.8% <0%> (-1.56%)	⬇️
src/err.rs	61.11% <0%> (-1.32%)	⬇️
src/buffer.rs	69.9% <0%> (-0.49%)	⬇️
src/freelist.rs	82.22% <0%> (-0.39%)	⬇️
... and 3 more

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[kngwyu]

I fixed the tests and examples.
@birkenfeld Thanks for your PR!
@Alexander-N Thanks for your advice on the failing test!