1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
// Copyright (c) 2017-present PyO3 Project and Contributors
use crate::conversion::{PyTryFrom, ToBorrowedObject};
use crate::err::{self, PyDowncastError, PyErr, PyResult};
use crate::gil;
use crate::pycell::{PyBorrowError, PyBorrowMutError, PyCell};
use crate::types::{PyDict, PyTuple};
use crate::{
    ffi, AsPyPointer, FromPyObject, IntoPy, IntoPyPointer, PyAny, PyClass, PyClassInitializer,
    PyRef, PyRefMut, PyTypeInfo, Python, ToPyObject,
};
use std::marker::PhantomData;
use std::mem;
use std::ptr::NonNull;

/// Types that are built into the Python interpreter.
///
/// PyO3 is designed in a way that all references to those types are bound
/// to the GIL, which is why you can get a token from all references of those
/// types.
///
/// # Safety
///
/// This trait must only be implemented for types which cannot be accessed without the GIL.
pub unsafe trait PyNativeType: Sized {
    /// Returns a GIL marker constrained to the lifetime of this type.
    #[inline]
    fn py(&self) -> Python {
        unsafe { Python::assume_gil_acquired() }
    }
    /// Cast `&PyAny` to `&Self` without no type checking.
    ///
    /// # Safety
    ///
    /// `obj` must have the same layout as `*const ffi::PyObject` and must be
    /// an instance of a type corresponding to `Self`.
    unsafe fn unchecked_downcast(obj: &PyAny) -> &Self {
        &*(obj.as_ptr() as *const Self)
    }
}

/// A GIL-independent reference to an object allocated on the Python heap.
///
/// This type does not auto-dereference to the inner object because you must prove you hold the GIL to access it.
/// Instead, call one of its methods to access the inner object:
///  - [`Py::as_ref`], to borrow a GIL-bound reference to the contained object.
///  - [`Py::borrow`], [`Py::try_borrow`], [`Py::borrow_mut`], or [`Py::try_borrow_mut`],
/// to get a (mutable) reference to a contained pyclass, using a scheme similar to std's [`RefCell`].
/// See the [`PyCell` guide entry](https://pyo3.rs/latest/class.html#pycell-and-interior-mutability)
/// for more information.
///  - You can call methods directly on `Py` with [`Py::call`], [`Py::call_method`] and friends.
/// These require passing in the [`Python<'py>`](crate::Python) token but are otherwise similar to the corresponding
/// methods on [`PyAny`].
///
/// # Example: Storing Python objects in structs
///
/// As all the native Python objects only appear as references, storing them in structs doesn't work well.
/// For example, this won't compile:
///
/// ```compile_fail
/// # use pyo3::prelude::*;
/// # use pyo3::types::PyDict;
/// #
/// #[pyclass]
/// struct Foo<'py> {
///     inner: &'py PyDict,
/// }
///
/// impl Foo {
///     fn new() -> Foo {
///         let foo = Python::with_gil(|py| {
///             // `py` will only last for this scope.
///
///             // `&PyDict` derives its lifetime from `py` and
///             // so won't be able to outlive this closure.
///             let dict: &PyDict = PyDict::new(py);
///
///             // because `Foo` contains `dict` its lifetime
///             // is now also tied to `py`.
///             Foo { inner: dict }
///         });
///         // Foo is no longer valid.
///         // Returning it from this function is a 💥 compiler error 💥
///         foo
///     }
/// }
/// ```
///
/// [`Py`]`<T>` can be used to get around this by converting `dict` into a GIL-independent reference:
///
/// ```rust
/// use pyo3::prelude::*;
/// use pyo3::types::PyDict;
///
/// #[pyclass]
/// struct Foo {
///     inner: Py<PyDict>,
/// }
///
/// #[pymethods]
/// impl Foo {
///     #[new]
///     fn __new__() -> Foo {
///         Python::with_gil(|py| {
///             let dict: Py<PyDict> = PyDict::new(py).into();
///             Foo { inner: dict }
///         })
///     }
/// }
/// #
/// # fn main() -> PyResult<()> {
/// #     Python::with_gil(|py| {
/// #         let m = pyo3::types::PyModule::new(py, "test")?;
/// #         m.add_class::<Foo>()?;
/// #
/// #         let foo: &PyCell<Foo> = pyo3::PyTryFrom::try_from(m.getattr("Foo")?.call0()?)?;
/// #         let dict = &foo.borrow().inner;
/// #         let dict: &PyDict = dict.as_ref(py);
/// #
/// #         Ok(())
/// #     })
/// # }
/// ```
///
/// This can also be done with other pyclasses:
/// ```rust
/// use pyo3::prelude::*;
///
/// #[pyclass]
/// struct Bar {/* ... */}
///
/// #[pyclass]
/// struct Foo {
///     inner: Py<Bar>,
/// }
///
/// #[pymethods]
/// impl Foo {
///     #[new]
///     fn __new__() -> PyResult<Foo> {
///         Python::with_gil(|py| {
///             let bar: Py<Bar> = Py::new(py, Bar {})?;
///             Ok(Foo { inner: bar })
///         })
///     }
/// }
/// #
/// # fn main() -> PyResult<()> {
/// #     Python::with_gil(|py| {
/// #         let m = pyo3::types::PyModule::new(py, "test")?;
/// #         m.add_class::<Foo>()?;
/// #
/// #         let foo: &PyCell<Foo> = pyo3::PyTryFrom::try_from(m.getattr("Foo")?.call0()?)?;
/// #         let bar = &foo.borrow().inner;
/// #         let bar: &Bar = &*bar.borrow(py);
/// #
/// #         Ok(())
/// #     })
/// # }
/// ```
///
/// # Example: Shared ownership of Python objects
///
/// `Py<T>` can be used to share ownership of a Python object, similar to std's [`Rc`]`<T>`.
/// As with [`Rc`]`<T>`, cloning it increases its reference count rather than duplicating
/// the underlying object.
///
/// This can be done using either [`Py::clone_ref`] or [`Py`]`<T>`'s [`Clone`] trait implementation.
/// [`Py::clone_ref`] will be faster if you happen to be already holding the GIL.
///
/// ```rust
/// use pyo3::conversion::AsPyPointer;
/// use pyo3::prelude::*;
/// use pyo3::types::PyDict;
///
/// # fn main() {
/// Python::with_gil(|py| {
///     let first: Py<PyDict> = PyDict::new(py).into();
///
///     // All of these are valid syntax
///     let second = Py::clone_ref(&first, py);
///     let third = first.clone_ref(py);
///     let fourth = Py::clone(&first);
///     let fifth = first.clone();
///
///     // Disposing of our original `Py<PyDict>` just decrements the reference count.
///     drop(first);
///
///     // They all point to the same object
///     assert_eq!(second.as_ptr(), third.as_ptr());
///     assert_eq!(fourth.as_ptr(), fifth.as_ptr());
///     assert_eq!(second.as_ptr(), fourth.as_ptr());
/// });
/// # }
/// ```
///
/// # Preventing reference cycles
///
/// It is easy to accidentally create reference cycles using [`Py`]`<T>`.
/// The Python interpreter can break these reference cycles within pyclasses if they
/// implement the [`PyGCProtocol`](crate::class::gc::PyGCProtocol). If your pyclass
/// contains other Python objects you should implement this protocol to avoid leaking memory.
///
/// # A note on Python reference counts
///
/// Dropping a [`Py`]`<T>` will eventually decrease Python's reference count
/// of the pointed-to variable, allowing Python's garbage collector to free
/// the associated memory, but this may not happen immediately.  This is
/// because a [`Py`]`<T>` can be dropped at any time, but the Python reference
/// count can only be modified when the GIL is held.
///
/// If a [`Py`]`<T>` is dropped while its thread happens to be holding the
/// GIL then the Python reference count will be decreased immediately.
/// Otherwise, the reference count will be decreased the next time the GIL is
/// reacquired.
///
/// # A note on `Send` and `Sync`
///
/// Accessing this object is threadsafe, since any access to its API requires a [`Python<'py>`](crate::Python) token.
/// As you can only get this by acquiring the GIL, `Py<...>` "implements [`Send`] and [`Sync`].
///
/// [`Rc`]: std::rc::Rc
/// [`RefCell`]: std::cell::RefCell
#[repr(transparent)]
pub struct Py<T>(NonNull<ffi::PyObject>, PhantomData<T>);

unsafe impl<T> Send for Py<T> {}
unsafe impl<T> Sync for Py<T> {}

impl<T> Py<T>
where
    T: PyClass,
{
    /// Creates a new instance `Py<T>` of a `#[pyclass]` on the Python heap.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use pyo3::prelude::*;
    ///
    /// #[pyclass]
    /// struct Foo {/* fields omitted */}
    ///
    /// # fn main() -> PyResult<()> {
    /// Python::with_gil(|py| -> PyResult<Py<Foo>> {
    ///     let foo: Py<Foo> = Py::new(py, Foo {})?;
    ///     Ok(foo)
    /// })?;
    /// # Ok(())
    /// # }
    /// ```
    pub fn new(py: Python, value: impl Into<PyClassInitializer<T>>) -> PyResult<Py<T>> {
        let initializer = value.into();
        let obj = initializer.create_cell(py)?;
        let ob = unsafe { Py::from_owned_ptr(py, obj as _) };
        Ok(ob)
    }
}

impl<T> Py<T>
where
    T: PyTypeInfo,
{
    /// Borrows a GIL-bound reference to the contained `T`.
    ///
    /// By binding to the GIL lifetime, this allows the GIL-bound reference to not require
    /// [`Python<'py>`](crate::Python) for any of its methods, which makes calling methods
    /// on it more ergonomic.
    ///
    /// For native types, this reference is `&T`. For pyclasses, this is `&PyCell<T>`.
    ///
    /// Note that the lifetime of the returned reference is the shortest of `&self` and
    /// [`Python<'py>`](crate::Python).
    /// Consider using [`Py::into_ref`] instead if this poses a problem.
    ///
    /// # Examples
    ///
    /// Get access to `&PyList` from `Py<PyList>`:
    ///
    /// ```
    /// # use pyo3::prelude::*;
    /// # use pyo3::types::PyList;
    /// #
    /// Python::with_gil(|py| {
    ///     let list: Py<PyList> = PyList::empty(py).into();
    ///     let list: &PyList = list.as_ref(py);
    ///     assert_eq!(list.len(), 0);
    /// });
    /// ```
    ///
    /// Get access to `&PyCell<MyClass>` from `Py<MyClass>`:
    ///
    /// ```
    /// # use pyo3::prelude::*;
    /// #
    /// #[pyclass]
    /// struct MyClass {}
    ///
    /// Python::with_gil(|py| {
    ///     let my_class: Py<MyClass> = Py::new(py, MyClass {}).unwrap();
    ///     let my_class_cell: &PyCell<MyClass> = my_class.as_ref(py);
    ///     assert!(my_class_cell.try_borrow().is_ok());
    /// });
    /// ```
    pub fn as_ref<'py>(&'py self, _py: Python<'py>) -> &'py T::AsRefTarget {
        let any = self.as_ptr() as *const PyAny;
        unsafe { PyNativeType::unchecked_downcast(&*any) }
    }

    /// Borrows a GIL-bound reference to the contained `T` independently of the lifetime of `T`.
    ///
    /// This method is similar to [`as_ref`](#method.as_ref) but consumes `self` and registers the
    /// Python object reference in PyO3's object storage. The reference count for the Python
    /// object will not be decreased until the GIL lifetime ends.
    ///
    /// You should prefer using [`as_ref`](#method.as_ref) if you can as it'll have less overhead.
    ///
    /// # Examples
    ///
    /// [`Py::as_ref`]'s lifetime limitation forbids creating a function that references a
    /// variable created inside the function.
    ///
    /// ```rust,compile_fail
    /// # use pyo3::prelude::*;
    /// #
    /// fn new_py_any<'py>(py: Python<'py>, value: impl IntoPy<Py<PyAny>>) -> &'py PyAny {
    ///     let obj: Py<PyAny> = value.into_py(py);
    ///
    ///     // The lifetime of the return value of this function is the shortest
    ///     // of `obj` and `py`. As `obj` is owned by the current function,
    ///     // Rust won't let the return value escape this function!
    ///     obj.as_ref(py)
    /// }
    /// ```
    ///
    /// This can be solved by using [`Py::into_ref`] instead, which does not suffer from this issue.
    /// Note that the lifetime of the [`Python<'py>`](crate::Python) token is transferred to
    /// the returned reference.
    ///
    /// ```rust
    /// # use pyo3::prelude::*;
    /// # #[allow(dead_code)] // This is just to show it compiles.
    /// fn new_py_any<'py>(py: Python<'py>, value: impl IntoPy<Py<PyAny>>) -> &'py PyAny {
    ///     let obj: Py<PyAny> = value.into_py(py);
    ///
    ///     // This reference's lifetime is determined by `py`'s lifetime.
    ///     // Because that originates from outside this function,
    ///     // this return value is allowed.
    ///     obj.into_ref(py)
    /// }
    /// ```
    pub fn into_ref(self, py: Python) -> &T::AsRefTarget {
        unsafe { py.from_owned_ptr(self.into_ptr()) }
    }
}

impl<T> Py<T>
where
    T: PyClass,
{
    /// Immutably borrows the value `T`.
    ///
    /// This borrow lasts while the returned [`PyRef`] exists.
    /// Multiple immutable borrows can be taken out at the same time.
    ///
    /// Equivalent to `self.as_ref(py).borrow()` -
    /// see [`PyCell::borrow`](crate::pycell::PyCell::borrow).
    ///
    /// # Examples
    ///
    /// ```rust
    /// # use pyo3::prelude::*;
    /// #
    /// #[pyclass]
    /// struct Foo {
    ///     inner: u8,
    /// }
    ///
    /// # fn main() -> PyResult<()> {
    /// Python::with_gil(|py| -> PyResult<()> {
    ///     let foo: Py<Foo> = Py::new(py, Foo { inner: 73 })?;
    ///     let inner: &u8 = &foo.borrow(py).inner;
    ///
    ///     assert_eq!(*inner, 73);
    ///     Ok(())
    /// })?;
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow`](#method.try_borrow).
    pub fn borrow<'py>(&'py self, py: Python<'py>) -> PyRef<'py, T> {
        self.as_ref(py).borrow()
    }

    /// Mutably borrows the value `T`.
    ///
    /// This borrow lasts while the returned [`PyRefMut`] exists.
    ///
    /// Equivalent to `self.as_ref(py).borrow_mut()` -
    /// see [`PyCell::borrow_mut`](crate::pycell::PyCell::borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// # use pyo3::prelude::*;
    /// #
    /// #[pyclass]
    /// struct Foo {
    ///     inner: u8,
    /// }
    ///
    /// # fn main() -> PyResult<()> {
    /// Python::with_gil(|py| -> PyResult<()> {
    ///     let foo: Py<Foo> = Py::new(py, Foo { inner: 73 })?;
    ///     foo.borrow_mut(py).inner = 35;
    ///
    ///     assert_eq!(foo.borrow(py).inner, 35);
    ///     Ok(())
    /// })?;
    /// # Ok(())
    /// # }
    ///  ```
    ///
    /// # Panics
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow_mut`](#method.try_borrow_mut).
    pub fn borrow_mut<'py>(&'py self, py: Python<'py>) -> PyRefMut<'py, T> {
        self.as_ref(py).borrow_mut()
    }

    /// Attempts to immutably borrow the value `T`, returning an error if the value is currently mutably borrowed.
    ///
    /// The borrow lasts while the returned [`PyRef`] exists.
    ///
    /// This is the non-panicking variant of [`borrow`](#method.borrow).
    ///
    /// Equivalent to `self.as_ref(py).borrow_mut()` -
    /// see [`PyCell::try_borrow`](crate::pycell::PyCell::try_borrow).
    pub fn try_borrow<'py>(&'py self, py: Python<'py>) -> Result<PyRef<'py, T>, PyBorrowError> {
        self.as_ref(py).try_borrow()
    }

    /// Attempts to mutably borrow the value `T`, returning an error if the value is currently borrowed.
    ///
    /// The borrow lasts while the returned [`PyRefMut`] exists.
    ///
    /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
    ///
    /// Equivalent to `self.as_ref(py).try_borrow_mut()` -
    /// see [`PyCell::try_borrow_mut`](crate::pycell::PyCell::try_borrow_mut).
    pub fn try_borrow_mut<'py>(
        &'py self,
        py: Python<'py>,
    ) -> Result<PyRefMut<'py, T>, PyBorrowMutError> {
        self.as_ref(py).try_borrow_mut()
    }
}

impl<T> Py<T> {
    /// Gets the reference count of the `ffi::PyObject` pointer.
    #[inline]
    pub fn get_refcnt(&self, _py: Python) -> isize {
        unsafe { ffi::Py_REFCNT(self.0.as_ptr()) }
    }

    /// Makes a clone of `self`.
    ///
    /// This creates another pointer to the same object, increasing its reference count.
    ///
    /// You should prefer using this method over [`Clone`] if you happen to be holding the GIL already.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use pyo3::conversion::AsPyPointer;
    /// use pyo3::prelude::*;
    /// use pyo3::types::PyDict;
    ///
    /// # fn main() {
    /// Python::with_gil(|py| {
    ///     let first: Py<PyDict> = PyDict::new(py).into();
    ///     let second = Py::clone_ref(&first, py);
    ///
    ///     // Both point to the same object
    ///     assert_eq!(first.as_ptr(), second.as_ptr());
    /// });
    /// # }
    /// ```
    #[inline]
    pub fn clone_ref(&self, py: Python) -> Py<T> {
        unsafe { Py::from_borrowed_ptr(py, self.0.as_ptr()) }
    }

    /// Returns whether the object is considered to be None.
    ///
    /// This is equivalent to the Python expression `self is None`.
    pub fn is_none(&self, _py: Python) -> bool {
        unsafe { ffi::Py_None() == self.as_ptr() }
    }

    /// Returns whether the object is considered to be true.
    ///
    /// This is equivalent to the Python expression `bool(self)`.
    pub fn is_true(&self, py: Python) -> PyResult<bool> {
        let v = unsafe { ffi::PyObject_IsTrue(self.as_ptr()) };
        err::error_on_minusone(py, v)?;
        Ok(v != 0)
    }

    /// Extracts some type from the Python object.
    ///
    /// This is a wrapper function around `FromPyObject::extract()`.
    pub fn extract<'p, D>(&'p self, py: Python<'p>) -> PyResult<D>
    where
        D: FromPyObject<'p>,
    {
        FromPyObject::extract(unsafe { py.from_borrowed_ptr(self.as_ptr()) })
    }

    /// Retrieves an attribute value.
    ///
    /// This is equivalent to the Python expression `self.attr_name`.
    pub fn getattr<N>(&self, py: Python, attr_name: N) -> PyResult<PyObject>
    where
        N: ToPyObject,
    {
        attr_name.with_borrowed_ptr(py, |attr_name| unsafe {
            PyObject::from_owned_ptr_or_err(py, ffi::PyObject_GetAttr(self.as_ptr(), attr_name))
        })
    }

    /// Sets an attribute value.
    ///
    /// This is equivalent to the Python expression `self.attr_name = value`.
    pub fn setattr<N, V>(&self, py: Python, attr_name: N, value: V) -> PyResult<()>
    where
        N: ToPyObject,
        V: ToPyObject,
    {
        attr_name.with_borrowed_ptr(py, move |attr_name| {
            value.with_borrowed_ptr(py, |value| unsafe {
                err::error_on_minusone(py, ffi::PyObject_SetAttr(self.as_ptr(), attr_name, value))
            })
        })
    }

    /// Calls the object.
    ///
    /// This is equivalent to the Python expression `self(*args, **kwargs)`.
    pub fn call(
        &self,
        py: Python,
        args: impl IntoPy<Py<PyTuple>>,
        kwargs: Option<&PyDict>,
    ) -> PyResult<PyObject> {
        let args = args.into_py(py).into_ptr();
        let kwargs = kwargs.into_ptr();
        let result = unsafe {
            PyObject::from_owned_ptr_or_err(py, ffi::PyObject_Call(self.as_ptr(), args, kwargs))
        };
        unsafe {
            ffi::Py_XDECREF(args);
            ffi::Py_XDECREF(kwargs);
        }
        result
    }

    /// Calls the object with only positional arguments.
    ///
    /// This is equivalent to the Python expression `self(*args)`.
    pub fn call1(&self, py: Python, args: impl IntoPy<Py<PyTuple>>) -> PyResult<PyObject> {
        self.call(py, args, None)
    }

    /// Calls the object without arguments.
    ///
    /// This is equivalent to the Python expression `self()`.
    pub fn call0(&self, py: Python) -> PyResult<PyObject> {
        cfg_if::cfg_if! {
            // TODO: Use PyObject_CallNoArgs instead after https://bugs.python.org/issue42415.
            // Once the issue is resolved, we can enable this optimization for limited API.
            if #[cfg(all(Py_3_9, not(Py_LIMITED_API)))] {
                // Optimized path on python 3.9+
                unsafe {
                    PyObject::from_owned_ptr_or_err(py, ffi::_PyObject_CallNoArg(self.as_ptr()))
                }
            } else {
                self.call(py, (), None)
            }
        }
    }

    /// Calls a method on the object.
    ///
    /// This is equivalent to the Python expression `self.name(*args, **kwargs)`.
    pub fn call_method(
        &self,
        py: Python,
        name: &str,
        args: impl IntoPy<Py<PyTuple>>,
        kwargs: Option<&PyDict>,
    ) -> PyResult<PyObject> {
        name.with_borrowed_ptr(py, |name| unsafe {
            let args = args.into_py(py).into_ptr();
            let kwargs = kwargs.into_ptr();
            let ptr = ffi::PyObject_GetAttr(self.as_ptr(), name);
            if ptr.is_null() {
                return Err(PyErr::fetch(py));
            }
            let result = PyObject::from_owned_ptr_or_err(py, ffi::PyObject_Call(ptr, args, kwargs));
            ffi::Py_DECREF(ptr);
            ffi::Py_XDECREF(args);
            ffi::Py_XDECREF(kwargs);
            result
        })
    }

    /// Calls a method on the object with only positional arguments.
    ///
    /// This is equivalent to the Python expression `self.name(*args)`.
    pub fn call_method1(
        &self,
        py: Python,
        name: &str,
        args: impl IntoPy<Py<PyTuple>>,
    ) -> PyResult<PyObject> {
        self.call_method(py, name, args, None)
    }

    /// Calls a method on the object with no arguments.
    ///
    /// This is equivalent to the Python expression `self.name()`.
    pub fn call_method0(&self, py: Python, name: &str) -> PyResult<PyObject> {
        cfg_if::cfg_if! {
            if #[cfg(all(Py_3_9, not(Py_LIMITED_API)))] {
                // Optimized path on python 3.9+
                unsafe {
                    let name = name.into_py(py);
                    PyObject::from_owned_ptr_or_err(py, ffi::PyObject_CallMethodNoArgs(self.as_ptr(), name.as_ptr()))
                }
            } else {
                self.call_method(py, name, (), None)
            }
        }
    }

    /// Create a `Py<T>` instance by taking ownership of the given FFI pointer.
    ///
    /// # Safety
    /// `ptr` must be a pointer to a Python object of type T.
    ///
    /// Callers must own the object referred to by `ptr`, as this function
    /// implicitly takes ownership of that object.
    ///
    /// # Panics
    /// Panics if `ptr` is null.
    #[inline]
    pub unsafe fn from_owned_ptr(py: Python, ptr: *mut ffi::PyObject) -> Py<T> {
        match NonNull::new(ptr) {
            Some(nonnull_ptr) => Py(nonnull_ptr, PhantomData),
            None => crate::err::panic_after_error(py),
        }
    }

    /// Create a `Py<T>` instance by taking ownership of the given FFI pointer.
    ///
    /// If `ptr` is null then the current Python exception is fetched as a `PyErr`.
    ///
    /// # Safety
    /// If non-null, `ptr` must be a pointer to a Python object of type T.
    #[inline]
    pub unsafe fn from_owned_ptr_or_err(py: Python, ptr: *mut ffi::PyObject) -> PyResult<Py<T>> {
        match NonNull::new(ptr) {
            Some(nonnull_ptr) => Ok(Py(nonnull_ptr, PhantomData)),
            None => Err(PyErr::fetch(py)),
        }
    }

    /// Create a `Py<T>` instance by taking ownership of the given FFI pointer.
    ///
    /// If `ptr` is null then `None` is returned.
    ///
    /// # Safety
    /// If non-null, `ptr` must be a pointer to a Python object of type T.
    #[inline]
    pub unsafe fn from_owned_ptr_or_opt(_py: Python, ptr: *mut ffi::PyObject) -> Option<Self> {
        NonNull::new(ptr).map(|nonnull_ptr| Py(nonnull_ptr, PhantomData))
    }

    /// Create a `Py<T>` instance by creating a new reference from the given FFI pointer.
    ///
    /// # Safety
    /// `ptr` must be a pointer to a Python object of type T.
    ///
    /// # Panics
    /// Panics if `ptr` is null.
    #[inline]
    pub unsafe fn from_borrowed_ptr(py: Python, ptr: *mut ffi::PyObject) -> Py<T> {
        match Self::from_borrowed_ptr_or_opt(py, ptr) {
            Some(slf) => slf,
            None => crate::err::panic_after_error(py),
        }
    }

    /// Create a `Py<T>` instance by creating a new reference from the given FFI pointer.
    ///
    /// If `ptr` is null then the current Python exception is fetched as a `PyErr`.
    ///
    /// # Safety
    /// `ptr` must be a pointer to a Python object of type T.
    #[inline]
    pub unsafe fn from_borrowed_ptr_or_err(py: Python, ptr: *mut ffi::PyObject) -> PyResult<Self> {
        Self::from_borrowed_ptr_or_opt(py, ptr).ok_or_else(|| PyErr::fetch(py))
    }

    /// Create a `Py<T>` instance by creating a new reference from the given FFI pointer.
    ///
    /// If `ptr` is null then `None` is returned.
    ///
    /// # Safety
    /// `ptr` must be a pointer to a Python object of type T.
    #[inline]
    pub unsafe fn from_borrowed_ptr_or_opt(_py: Python, ptr: *mut ffi::PyObject) -> Option<Self> {
        NonNull::new(ptr).map(|nonnull_ptr| {
            ffi::Py_INCREF(ptr);
            Py(nonnull_ptr, PhantomData)
        })
    }

    /// For internal conversions.
    ///
    /// # Safety
    /// `ptr` must point to a Python object of type T.
    #[inline]
    unsafe fn from_non_null(ptr: NonNull<ffi::PyObject>) -> Self {
        Self(ptr, PhantomData)
    }

    /// Returns the inner pointer without decreasing the refcount.
    #[inline]
    fn into_non_null(self) -> NonNull<ffi::PyObject> {
        let pointer = self.0;
        mem::forget(self);
        pointer
    }
}

impl<T> ToPyObject for Py<T> {
    /// Converts `Py` instance -> PyObject.
    fn to_object(&self, py: Python) -> PyObject {
        unsafe { PyObject::from_borrowed_ptr(py, self.as_ptr()) }
    }
}

impl<T> IntoPy<PyObject> for Py<T> {
    /// Converts a `Py` instance to `PyObject`.
    /// Consumes `self` without calling `Py_DECREF()`.
    #[inline]
    fn into_py(self, _py: Python) -> PyObject {
        unsafe { PyObject::from_non_null(self.into_non_null()) }
    }
}

impl<T> AsPyPointer for Py<T> {
    /// Gets the underlying FFI pointer, returns a borrowed pointer.
    #[inline]
    fn as_ptr(&self) -> *mut ffi::PyObject {
        self.0.as_ptr()
    }
}

impl<T> IntoPyPointer for Py<T> {
    /// Gets the underlying FFI pointer, returns a owned pointer.
    #[inline]
    #[must_use]
    fn into_ptr(self) -> *mut ffi::PyObject {
        self.into_non_null().as_ptr()
    }
}

impl<T> std::convert::From<&'_ T> for PyObject
where
    T: AsPyPointer + PyNativeType,
{
    fn from(obj: &T) -> Self {
        unsafe { Py::from_borrowed_ptr(obj.py(), obj.as_ptr()) }
    }
}

impl<T> std::convert::From<Py<T>> for PyObject
where
    T: AsRef<PyAny>,
{
    #[inline]
    fn from(other: Py<T>) -> Self {
        unsafe { Self::from_non_null(other.into_non_null()) }
    }
}

// `&PyCell<T>` can be converted to `Py<T>`
impl<'a, T> std::convert::From<&PyCell<T>> for Py<T>
where
    T: PyClass,
{
    fn from(cell: &PyCell<T>) -> Self {
        unsafe { Py::from_borrowed_ptr(cell.py(), cell.as_ptr()) }
    }
}

impl<'a, T> std::convert::From<PyRef<'a, T>> for Py<T>
where
    T: PyClass,
{
    fn from(pyref: PyRef<'a, T>) -> Self {
        unsafe { Py::from_borrowed_ptr(pyref.py(), pyref.as_ptr()) }
    }
}

impl<'a, T> std::convert::From<PyRefMut<'a, T>> for Py<T>
where
    T: PyClass,
{
    fn from(pyref: PyRefMut<'a, T>) -> Self {
        unsafe { Py::from_borrowed_ptr(pyref.py(), pyref.as_ptr()) }
    }
}

impl<T> PartialEq for Py<T> {
    #[inline]
    fn eq(&self, o: &Py<T>) -> bool {
        self.0 == o.0
    }
}

/// If the GIL is held this increments `self`'s reference count.
/// Otherwise this registers the [`Py`]`<T>` instance to have its reference count
/// incremented the next time PyO3 acquires the GIL.
impl<T> Clone for Py<T> {
    fn clone(&self) -> Self {
        unsafe {
            gil::register_incref(self.0);
        }
        Self(self.0, PhantomData)
    }
}

/// Dropping a `Py` instance decrements the reference count on the object by 1.
impl<T> Drop for Py<T> {
    fn drop(&mut self) {
        unsafe {
            gil::register_decref(self.0);
        }
    }
}

impl<'a, T> FromPyObject<'a> for Py<T>
where
    T: PyTypeInfo,
    &'a T::AsRefTarget: FromPyObject<'a>,
    T::AsRefTarget: 'a + AsPyPointer,
{
    /// Extracts `Self` from the source `PyObject`.
    fn extract(ob: &'a PyAny) -> PyResult<Self> {
        unsafe {
            ob.extract::<&T::AsRefTarget>()
                .map(|val| Py::from_borrowed_ptr(ob.py(), val.as_ptr()))
        }
    }
}

/// Py<T> can be used as an error when T is an Error.
///
/// However for GIL lifetime reasons, cause() cannot be implemented for Py<T>.
/// Use .as_ref() to get the GIL-scoped error if you need to inspect the cause.
impl<T> std::error::Error for Py<T>
where
    T: std::error::Error + PyTypeInfo,
    T::AsRefTarget: std::fmt::Display,
{
}

impl<T> std::fmt::Display for Py<T>
where
    T: PyTypeInfo,
    T::AsRefTarget: std::fmt::Display,
{
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        Python::with_gil(|py| std::fmt::Display::fmt(self.as_ref(py), f))
    }
}

impl<T> std::fmt::Debug for Py<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_tuple("Py").field(&self.0.as_ptr()).finish()
    }
}

/// A commonly-used alias for `Py<PyAny>`.
///
/// This is an owned reference a Python object without any type information. This value can also be
/// safely sent between threads.
///
/// See the documentation for [`Py`](struct.Py.html).
pub type PyObject = Py<PyAny>;

impl PyObject {
    /// Casts the PyObject to a concrete Python object type.
    ///
    /// This can cast only to native Python types, not types implemented in Rust. For a more
    /// flexible alternative, see [`Py::extract`](struct.Py.html#method.extract).
    pub fn cast_as<'p, D>(&'p self, py: Python<'p>) -> Result<&'p D, PyDowncastError>
    where
        D: PyTryFrom<'p>,
    {
        D::try_from(unsafe { py.from_borrowed_ptr::<PyAny>(self.as_ptr()) })
    }
}

#[cfg(test)]
mod tests {
    use super::{Py, PyObject};
    use crate::types::PyDict;
    use crate::Python;

    #[test]
    fn test_call_for_non_existing_method() {
        Python::with_gil(|py| {
            let obj: PyObject = PyDict::new(py).into();
            assert!(obj.call_method0(py, "asdf").is_err());
            assert!(obj
                .call_method(py, "nonexistent_method", (1,), None)
                .is_err());
            assert!(obj.call_method0(py, "nonexistent_method").is_err());
            assert!(obj.call_method1(py, "nonexistent_method", (1,)).is_err());
        });
    }

    #[test]
    fn py_from_dict() {
        let dict: Py<PyDict> = Python::with_gil(|py| {
            let native = PyDict::new(py);
            Py::from(native)
        });

        assert_eq!(Python::with_gil(|py| dict.get_refcnt(py)), 1);
    }

    #[test]
    fn pyobject_from_py() {
        Python::with_gil(|py| {
            let dict: Py<PyDict> = PyDict::new(py).into();
            let cnt = dict.get_refcnt(py);
            let p: PyObject = dict.into();
            assert_eq!(p.get_refcnt(py), cnt);
        });
    }
}