# pycell.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/pyo3-0.13.2/src/pycell.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Includes `PyCell` implementation.
    use crate::conversion::{AsPyPointer, FromPyPointer, ToPyObject};
    use crate::exceptions::PyRuntimeError;
    use crate::pyclass::{PyClass, PyClassThreadChecker};
    use crate::pyclass_init::PyClassInitializer;
    use crate::pyclass_slots::{PyClassDict, PyClassWeakRef};
    use crate::type_object::{PyBorrowFlagLayout, PyLayout, PySizedLayout, PyTypeInfo};
    use crate::types::PyAny;
    use crate::{ffi, IntoPy, PyErr, PyNativeType, PyObject, PyResult, Python};
    use std::cell::{Cell, UnsafeCell};
    use std::fmt;
    use std::mem::ManuallyDrop;
    use std::ops::{Deref, DerefMut};

    /// Base layout of PyCell.
    /// This is necessary for sharing BorrowFlag between parents and children.
    #[doc(hidden)]
    #[repr(C)]
    pub struct PyCellBase<T: PyTypeInfo> {
        ob_base: T::Layout,
        borrow_flag: Cell<BorrowFlag>,
    }

    unsafe impl<T> PyLayout<T> for PyCellBase<T>
    where
        T: PyTypeInfo + PyNativeType,
        T::Layout: PySizedLayout<T>,
    {
        const IS_NATIVE_TYPE: bool = true;
    }

    // Thes impls ensures `PyCellBase` can be a base type.
    impl<T> PySizedLayout<T> for PyCellBase<T>
    where
        T: PyTypeInfo + PyNativeType,
        T::Layout: PySizedLayout<T>,
    {
    }

    unsafe impl<T> PyBorrowFlagLayout<T> for PyCellBase<T>
    where
        T: PyTypeInfo + PyNativeType,
        T::Layout: PySizedLayout<T>,
    {
    }

    /// Inner type of `PyCell` without dict slots and reference counter.
    /// This struct has two usages:
    /// 1. As an inner type of `PyRef` and `PyRefMut`.
    /// 2. When `#[pyclass(extends=Base)]` is specified, `PyCellInner<Base>` is used as a base layout.
    #[doc(hidden)]
    #[repr(C)]
    pub struct PyCellInner<T: PyClass> {
        ob_base: T::BaseLayout,
        value: ManuallyDrop<UnsafeCell<T>>,
    }

    impl<T: PyClass> AsPyPointer for PyCellInner<T> {
        fn as_ptr(&self) -> *mut ffi::PyObject {
            (self as *const _) as *mut _
        }
    }

    unsafe impl<T: PyClass> PyLayout<T> for PyCellInner<T> {
        const IS_NATIVE_TYPE: bool = false;
        fn get_super(&mut self) -> Option<&mut T::BaseLayout> {
            Some(&mut self.ob_base)
        }
        fn py_init(&mut self, value: T) {
            self.value = ManuallyDrop::new(UnsafeCell::new(value));
        }
        unsafe fn py_drop(&mut self, py: Python) {
            ManuallyDrop::drop(&mut self.value);
            self.ob_base.py_drop(py);
        }
    }

    // These impls ensures `PyCellInner` can be a base type.
    impl<T: PyClass> PySizedLayout<T> for PyCellInner<T> {}
    unsafe impl<T: PyClass> PyBorrowFlagLayout<T> for PyCellInner<T> {}

    impl<T: PyClass> PyCellInner<T> {
        fn get_ptr(&self) -> *mut T {
            self.value.get()
        }
        fn get_borrow_flag(&self) -> BorrowFlag {
            let base = (&self.ob_base) as *const _ as *const PyCellBase<T::BaseNativeType>;
            unsafe { (*base).borrow_flag.get() }
        }
        fn set_borrow_flag(&self, flag: BorrowFlag) {
            let base = (&self.ob_base) as *const _ as *const PyCellBase<T::BaseNativeType>;
            unsafe { (*base).borrow_flag.set(flag) }
        }
    }

    /// `PyCell` is the container type for [`PyClass`](../pyclass/trait.PyClass.html).
    ///
    /// From Python side, `PyCell<T>` is the concrete layout of `T: PyClass` in the Python heap,
    /// which means we can convert `*const PyClass<T>` to `*mut ffi::PyObject`.
    ///
    /// From Rust side, `PyCell<T>` is the mutable container of `T`.
    /// Since `PyCell<T: PyClass>` is always on the Python heap, we don't have the ownership of it.
    /// Thus, to mutate the data behind `&PyCell<T>` safely, we employ the
    /// [Interior Mutability Pattern](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html)
    /// like [std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html).
    ///
    /// `PyCell` implements `Deref<Target = PyAny>`, so you can also call methods from `PyAny`
    /// when you have a `PyCell<T>`.
    ///
    /// # Examples
    ///
    /// In most cases, `PyCell` is hidden behind `#[pymethods]`.
    /// However, you can construct `&PyCell` directly to test your pyclass in Rust code.
    ///
    /// ```
    /// # use pyo3::prelude::*;
    /// #[pyclass]
    /// struct Book {
    ///     #[pyo3(get)]
    ///     name: &'static str,
    ///     author: &'static str,
    /// }
    /// let gil = Python::acquire_gil();
    /// let py = gil.python();
    /// let book = Book {
    ///     name: "The Man in the High Castle",
    ///     author: "Philip Kindred Dick",
    /// };
    /// let book_cell = PyCell::new(py, book).unwrap();
    /// // you can expose PyCell to Python snippets
    /// pyo3::py_run!(py, book_cell, "assert book_cell.name[-6:] == 'Castle'");
    /// ```
    /// You can use `slf: &PyCell<Self>` as an alternative `self` receiver of `#[pymethod]`,
    /// though you rarely need it.
    /// ```
    /// # use pyo3::prelude::*;
    /// use std::collections::HashMap;
    /// #[pyclass]
    /// #[derive(Default)]
    /// struct Counter {
    ///     counter: HashMap<String, usize>
    /// }
    /// #[pymethods]
    /// impl Counter {
    ///     // You can use &mut self here, but now we use &PyCell for demonstration
    ///     fn increment(slf: &PyCell<Self>, name: String) -> PyResult<usize> {
    ///         let mut slf_mut = slf.try_borrow_mut()?;
    ///         // Now a mutable reference exists so we cannot get another one
    ///         assert!(slf.try_borrow().is_err());
    ///         assert!(slf.try_borrow_mut().is_err());
    ///         let counter = slf_mut.counter.entry(name).or_insert(0);
    ///         *counter += 1;
    ///         Ok(*counter)
    ///     }
    /// }
    /// # let gil = Python::acquire_gil();
    /// # let py = gil.python();
    /// # let counter = PyCell::new(py, Counter::default()).unwrap();
    /// # pyo3::py_run!(py, counter, "assert counter.increment('cat') == 1");
    /// ```
    #[repr(C)]
    pub struct PyCell<T: PyClass> {
        inner: PyCellInner<T>,
        thread_checker: T::ThreadChecker,
        // DO NOT CHANGE THE ORDER OF THESE FIELDS WITHOUT CHANGING PyCell::dict_offset()
        // AND PyCell::weakref_offset()
        dict: T::Dict,
        weakref: T::WeakRef,
    }

    impl<T: PyClass> PyCell<T> {
        /// Get the offset of the dictionary from the start of the struct in bytes.
        #[cfg(not(all(Py_LIMITED_API, not(Py_3_9))))]
        pub(crate) fn dict_offset() -> Option<usize> {
            if T::Dict::IS_DUMMY {
                None
            } else {
                Some(
                    std::mem::size_of::<Self>()
                        - std::mem::size_of::<T::Dict>()
                        - std::mem::size_of::<T::WeakRef>(),
                )
            }
        }

        /// Get the offset of the weakref list from the start of the struct in bytes.
        #[cfg(not(all(Py_LIMITED_API, not(Py_3_9))))]
        pub(crate) fn weakref_offset() -> Option<usize> {
            if T::WeakRef::IS_DUMMY {
                None
            } else {
                Some(std::mem::size_of::<Self>() - std::mem::size_of::<T::WeakRef>())
            }
        }
    }

    unsafe impl<T: PyClass> PyNativeType for PyCell<T> {}

    impl<T: PyClass> PyCell<T> {
        /// Make a new `PyCell` on the Python heap and return the reference to it.
        ///
        /// In cases where the value in the cell does not need to be accessed immediately after
        /// creation, consider [`Py::new`](../instance/struct.Py.html#method.new) as a more efficient
        /// alternative.
        pub fn new(py: Python, value: impl Into<PyClassInitializer<T>>) -> PyResult<&Self>
        where
            T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
        {
            unsafe {
                let initializer = value.into();
                let self_ = initializer.create_cell(py)?;
                FromPyPointer::from_owned_ptr_or_err(py, self_ as _)
            }
        }

        /// Immutably borrows the value `T`. This borrow lasts untill the returned `PyRef` exists.
        ///
        /// # Panics
        ///
        /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
        /// [`try_borrow`](#method.try_borrow).
        pub fn borrow(&self) -> PyRef<'_, T> {
            self.try_borrow().expect("Already mutably borrowed")
        }

        /// Mutably borrows the value `T`. This borrow lasts untill the returned `PyRefMut` exists.
        ///
        /// # 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(&self) -> PyRefMut<'_, T> {
            self.try_borrow_mut().expect("Already borrowed")
        }

        /// Immutably borrows the value `T`, returning an error if the value is currently
        /// mutably borrowed. This borrow lasts untill the returned `PyRef` exists.
        ///
        /// This is the non-panicking variant of [`borrow`](#method.borrow).
        ///
        /// # Examples
        ///
        /// ```
        /// # use pyo3::prelude::*;
        /// #[pyclass]
        /// struct Class {}
        /// let gil = Python::acquire_gil();
        /// let py = gil.python();
        /// let c = PyCell::new(py, Class {}).unwrap();
        /// {
        ///     let m = c.borrow_mut();
        ///     assert!(c.try_borrow().is_err());
        /// }
        ///
        /// {
        ///     let m = c.borrow();
        ///     assert!(c.try_borrow().is_ok());
        /// }
        /// ```
        pub fn try_borrow(&self) -> Result<PyRef<'_, T>, PyBorrowError> {
            self.thread_checker.ensure();
            let flag = self.inner.get_borrow_flag();
            if flag == BorrowFlag::HAS_MUTABLE_BORROW {
                Err(PyBorrowError { _private: () })
            } else {
                self.inner.set_borrow_flag(flag.increment());
                Ok(PyRef { inner: &self.inner })
            }
        }

        /// Mutably borrows the value `T`, returning an error if the value is currently borrowed.
        /// This borrow lasts untill the returned `PyRefMut` exists.
        ///
        /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
        ///
        /// # Examples
        ///
        /// ```
        /// # use pyo3::prelude::*;
        /// #[pyclass]
        /// struct Class {}
        /// let gil = Python::acquire_gil();
        /// let py = gil.python();
        /// let c = PyCell::new(py, Class {}).unwrap();
        /// {
        ///     let m = c.borrow();
        ///     assert!(c.try_borrow_mut().is_err());
        /// }
        ///
        /// assert!(c.try_borrow_mut().is_ok());
        /// ```
        pub fn try_borrow_mut(&self) -> Result<PyRefMut<'_, T>, PyBorrowMutError> {
            self.thread_checker.ensure();
            if self.inner.get_borrow_flag() != BorrowFlag::UNUSED {
                Err(PyBorrowMutError { _private: () })
            } else {
                self.inner.set_borrow_flag(BorrowFlag::HAS_MUTABLE_BORROW);
                Ok(PyRefMut { inner: &self.inner })
            }
        }

        /// Immutably borrows the value `T`, returning an error if the value is
        /// currently mutably borrowed.
        ///
        /// # Safety
        ///
        /// This method is unsafe because it does not return a `PyRef`,
        /// thus leaving the borrow flag untouched. Mutably borrowing the `PyCell`
        /// while the reference returned by this method is alive is undefined behaviour.
        ///
        /// # Examples
        ///
        /// ```
        /// # use pyo3::prelude::*;
        /// #[pyclass]
        /// struct Class {}
        /// let gil = Python::acquire_gil();
        /// let py = gil.python();
        /// let c = PyCell::new(py, Class {}).unwrap();
        ///
        /// {
        ///     let m = c.borrow_mut();
        ///     assert!(unsafe { c.try_borrow_unguarded() }.is_err());
        /// }
        ///
        /// {
        ///     let m = c.borrow();
        ///     assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
        /// }
        /// ```
        pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, PyBorrowError> {
            self.thread_checker.ensure();
            if self.inner.get_borrow_flag() == BorrowFlag::HAS_MUTABLE_BORROW {
                Err(PyBorrowError { _private: () })
            } else {
                Ok(&*self.inner.value.get())
            }
        }

        /// Replaces the wrapped value with a new one, returning the old value,
        ///
        /// # Panics
        ///
        /// Panics if the value is currently borrowed.
        #[inline]
        pub fn replace(&self, t: T) -> T {
            std::mem::replace(&mut *self.borrow_mut(), t)
        }

        /// Replaces the wrapped value with a new one computed from `f`, returning the old value.
        ///
        /// # Panics
        ///
        /// Panics if the value is currently borrowed.
        pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
            let mut_borrow = &mut *self.borrow_mut();
            let replacement = f(mut_borrow);
            std::mem::replace(mut_borrow, replacement)
        }

        /// Swaps the wrapped value of `self` with the wrapped value of `other`.
        ///
        /// # Panics
        ///
        /// Panics if the value in either `PyCell` is currently borrowed.
        #[inline]
        pub fn swap(&self, other: &Self) {
            std::mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
        }

        /// Allocates a new PyCell given a type object `subtype`. Used by our `tp_new` implementation.
        /// Requires `T::BaseLayout: PyBorrowFlagLayout<T::BaseType>` to ensure `self` has a borrow flag.
        pub(crate) unsafe fn internal_new(
            py: Python,
            subtype: *mut ffi::PyTypeObject,
        ) -> PyResult<*mut Self>
        where
            T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
        {
            let base = T::new(py, subtype);
            if base.is_null() {
                return Err(PyErr::fetch(py));
            }
            let base = base as *mut PyCellBase<T::BaseNativeType>;
            (*base).borrow_flag = Cell::new(BorrowFlag::UNUSED);
            let self_ = base as *mut Self;
            (*self_).dict = T::Dict::new();
            (*self_).weakref = T::WeakRef::new();
            (*self_).thread_checker = T::ThreadChecker::new();
            Ok(self_)
        }
    }

    unsafe impl<T: PyClass> PyLayout<T> for PyCell<T> {
        const IS_NATIVE_TYPE: bool = false;
        fn get_super(&mut self) -> Option<&mut T::BaseLayout> {
            Some(&mut self.inner.ob_base)
        }
        fn py_init(&mut self, value: T) {
            self.inner.value = ManuallyDrop::new(UnsafeCell::new(value));
        }
        unsafe fn py_drop(&mut self, py: Python) {
            ManuallyDrop::drop(&mut self.inner.value);
            self.dict.clear_dict(py);
            self.weakref.clear_weakrefs(self.as_ptr(), py);
            self.inner.ob_base.py_drop(py);
        }
    }

    impl<T: PyClass> AsPyPointer for PyCell<T> {
        fn as_ptr(&self) -> *mut ffi::PyObject {
            self.inner.as_ptr()
        }
    }

    impl<T: PyClass> ToPyObject for &PyCell<T> {
        fn to_object(&self, py: Python<'_>) -> PyObject {
            unsafe { PyObject::from_borrowed_ptr(py, self.as_ptr()) }
        }
    }

    impl<T: PyClass> AsRef<PyAny> for PyCell<T> {
        fn as_ref(&self) -> &PyAny {
            unsafe { self.py().from_borrowed_ptr(self.as_ptr()) }
        }
    }

    impl<T: PyClass> Deref for PyCell<T> {
        type Target = PyAny;

        fn deref(&self) -> &PyAny {
            unsafe { self.py().from_borrowed_ptr(self.as_ptr()) }
        }
    }

    impl<T: PyClass + fmt::Debug> fmt::Debug for PyCell<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self.try_borrow() {
                Ok(borrow) => f.debug_struct("RefCell").field("value", &borrow).finish(),
                Err(_) => {
                    struct BorrowedPlaceholder;
                    impl fmt::Debug for BorrowedPlaceholder {
                        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                            f.write_str("<borrowed>")
                        }
                    }
                    f.debug_struct("RefCell")
                        .field("value", &BorrowedPlaceholder)
                        .finish()
                }
            }
        }
    }

    /// Wraps a borrowed reference to a value in a `PyCell<T>`.
    ///
    /// See the [`PyCell`](struct.PyCell.html) documentation for more.
    /// # Example
    /// You can use `PyRef` as an alternative of `&self` receiver when
    /// - You need to access the pointer of `PyCell`.
    /// - You want to get super class.
    /// ```
    /// # use pyo3::prelude::*;
    /// #[pyclass(subclass)]
    /// struct Parent {
    ///     basename: &'static str,
    /// }
    /// #[pyclass(extends=Parent)]
    /// struct Child {
    ///     name: &'static str,
    ///  }
    /// #[pymethods]
    /// impl Child {
    ///     #[new]
    ///     fn new() -> (Self, Parent) {
    ///         (Child { name: "Caterpillar" }, Parent { basename: "Butterfly" })
    ///     }
    ///     fn format(slf: PyRef<Self>) -> String {
    ///         // We can get *mut ffi::PyObject from PyRef
    ///         use pyo3::AsPyPointer;
    ///         let refcnt = unsafe { pyo3::ffi::Py_REFCNT(slf.as_ptr()) };
    ///         // We can get &Self::BaseType by as_ref
    ///         let basename = slf.as_ref().basename;
    ///         format!("{}(base: {}, cnt: {})", slf.name, basename, refcnt)
    ///     }
    /// }
    /// # let gil = Python::acquire_gil();
    /// # let py = gil.python();
    /// # let sub = PyCell::new(py, Child::new()).unwrap();
    /// # pyo3::py_run!(py, sub, "assert sub.format() == 'Caterpillar(base: Butterfly, cnt: 3)'");
    /// ```
    pub struct PyRef<'p, T: PyClass> {
        inner: &'p PyCellInner<T>,
    }

    impl<'p, T: PyClass> PyRef<'p, T> {
        /// Returns `Python` token.
        /// This function is safe since PyRef has the same lifetime as a `GILGuard`.
        pub fn py(&self) -> Python {
            unsafe { Python::assume_gil_acquired() }
        }
    }

    impl<'p, T, U> AsRef<U> for PyRef<'p, T>
    where
        T: PyClass + PyTypeInfo<BaseType = U, BaseLayout = PyCellInner<U>>,
        U: PyClass,
    {
        fn as_ref(&self) -> &T::BaseType {
            unsafe { &*self.inner.ob_base.get_ptr() }
        }
    }

    impl<'p, T, U> PyRef<'p, T>
    where
        T: PyClass + PyTypeInfo<BaseType = U, BaseLayout = PyCellInner<U>>,
        U: PyClass,
    {
        /// Get `PyRef<T::BaseType>`.
        /// You can use this method to get super class of super class.
        ///
        /// # Examples
        /// ```
        /// # use pyo3::prelude::*;
        /// #[pyclass(subclass)]
        /// struct Base1 {
        ///     name1: &'static str,
        /// }
        /// #[pyclass(extends=Base1, subclass)]
        /// struct Base2 {
        ///     name2: &'static str,
        ///  }
        /// #[pyclass(extends=Base2)]
        /// struct Sub {
        ///     name3: &'static str,
        ///  }
        /// #[pymethods]
        /// impl Sub {
        ///     #[new]
        ///     fn new() -> PyClassInitializer<Self> {
        ///         PyClassInitializer::from(Base1{ name1: "base1" })
        ///             .add_subclass(Base2 { name2: "base2" })
        ///             .add_subclass(Self { name3: "sub" })
        ///     }
        ///     fn name(slf: PyRef<Self>) -> String {
        ///         let subname = slf.name3;
        ///         let super_ = slf.into_super();
        ///         format!("{} {} {}", super_.as_ref().name1, super_.name2, subname)
        ///     }
        /// }
        /// # let gil = Python::acquire_gil();
        /// # let py = gil.python();
        /// # let sub = PyCell::new(py, Sub::new()).unwrap();
        /// # pyo3::py_run!(py, sub, "assert sub.name() == 'base1 base2 sub'")
        /// ```
        pub fn into_super(self) -> PyRef<'p, U> {
            let PyRef { inner } = self;
            std::mem::forget(self);
            PyRef {
                inner: &inner.ob_base,
            }
        }
    }

    impl<'p, T: PyClass> Deref for PyRef<'p, T> {
        type Target = T;

        #[inline]
        fn deref(&self) -> &T {
            unsafe { &*self.inner.get_ptr() }
        }
    }

    impl<'p, T: PyClass> Drop for PyRef<'p, T> {
        fn drop(&mut self) {
            let flag = self.inner.get_borrow_flag();
            self.inner.set_borrow_flag(flag.decrement())
        }
    }

    impl<T: PyClass> IntoPy<PyObject> for PyRef<'_, T> {
        fn into_py(self, py: Python<'_>) -> PyObject {
            unsafe { PyObject::from_borrowed_ptr(py, self.inner.as_ptr()) }
        }
    }

    impl<'a, T: PyClass> std::convert::TryFrom<&'a PyCell<T>> for crate::PyRef<'a, T> {
        type Error = PyBorrowError;
        fn try_from(cell: &'a crate::PyCell<T>) -> Result<Self, Self::Error> {
            cell.try_borrow()
        }
    }

    impl<'a, T: PyClass> AsPyPointer for PyRef<'a, T> {
        fn as_ptr(&self) -> *mut ffi::PyObject {
            self.inner.as_ptr()
        }
    }

    impl<T: PyClass + fmt::Debug> fmt::Debug for PyRef<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    /// Wraps a mutable borrowed reference to a value in a `PyCell<T>`.
    ///
    /// See the [`PyCell`](struct.PyCell.html) and [`PyRef`](struct.PyRef.html) documentations for more.
    pub struct PyRefMut<'p, T: PyClass> {
        inner: &'p PyCellInner<T>,
    }

    impl<'p, T: PyClass> PyRefMut<'p, T> {
        /// Returns `Python` token.
        /// This function is safe since PyRefMut has the same lifetime as a `GILGuard`.
        pub fn py(&self) -> Python {
            unsafe { Python::assume_gil_acquired() }
        }
    }

    impl<'p, T, U> AsRef<U> for PyRefMut<'p, T>
    where
        T: PyClass + PyTypeInfo<BaseType = U, BaseLayout = PyCellInner<U>>,
        U: PyClass,
    {
        fn as_ref(&self) -> &T::BaseType {
            unsafe { &*self.inner.ob_base.get_ptr() }
        }
    }

    impl<'p, T, U> AsMut<U> for PyRefMut<'p, T>
    where
        T: PyClass + PyTypeInfo<BaseType = U, BaseLayout = PyCellInner<U>>,
        U: PyClass,
    {
        fn as_mut(&mut self) -> &mut T::BaseType {
            unsafe { &mut *self.inner.ob_base.get_ptr() }
        }
    }

    impl<'p, T, U> PyRefMut<'p, T>
    where
        T: PyClass + PyTypeInfo<BaseType = U, BaseLayout = PyCellInner<U>>,
        U: PyClass,
    {
        /// Get `PyRef<T::BaseType>`.
        /// See  [`PyRef::into_super`](struct.PyRef.html#method.into_super) for more.
        pub fn into_super(self) -> PyRefMut<'p, U> {
            let PyRefMut { inner } = self;
            std::mem::forget(self);
            PyRefMut {
                inner: &inner.ob_base,
            }
        }
    }

    impl<'p, T: PyClass> Deref for PyRefMut<'p, T> {
        type Target = T;

        #[inline]
        fn deref(&self) -> &T {
            unsafe { &*self.inner.get_ptr() }
        }
    }

    impl<'p, T: PyClass> DerefMut for PyRefMut<'p, T> {
        #[inline]
        fn deref_mut(&mut self) -> &mut T {
            unsafe { &mut *self.inner.get_ptr() }
        }
    }

    impl<'p, T: PyClass> Drop for PyRefMut<'p, T> {
        fn drop(&mut self) {
            self.inner.set_borrow_flag(BorrowFlag::UNUSED)
        }
    }

    impl<T: PyClass> IntoPy<PyObject> for PyRefMut<'_, T> {
        fn into_py(self, py: Python) -> PyObject {
            unsafe { PyObject::from_borrowed_ptr(py, self.inner.as_ptr()) }
        }
    }

    impl<'a, T: PyClass> AsPyPointer for PyRefMut<'a, T> {
        fn as_ptr(&self) -> *mut ffi::PyObject {
            self.inner.as_ptr()
        }
    }

    impl<'a, T: PyClass> std::convert::TryFrom<&'a PyCell<T>> for crate::PyRefMut<'a, T> {
        type Error = PyBorrowMutError;
        fn try_from(cell: &'a crate::PyCell<T>) -> Result<Self, Self::Error> {
            cell.try_borrow_mut()
        }
    }

    impl<T: PyClass + fmt::Debug> fmt::Debug for PyRefMut<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&*(self.deref()), f)
        }
    }

    #[derive(Copy, Clone, Eq, PartialEq)]
    struct BorrowFlag(usize);

    impl BorrowFlag {
        const UNUSED: BorrowFlag = BorrowFlag(0);
        const HAS_MUTABLE_BORROW: BorrowFlag = BorrowFlag(usize::max_value());
        const fn increment(self) -> Self {
            Self(self.0 + 1)
        }
        const fn decrement(self) -> Self {
            Self(self.0 - 1)
        }
    }

    /// An error returned by [`PyCell::try_borrow`](struct.PyCell.html#method.try_borrow).
    ///
    /// In Python, you can catch this error by `except RuntimeError`.
    pub struct PyBorrowError {
        _private: (),
    }

    impl fmt::Debug for PyBorrowError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("PyBorrowError").finish()
        }
    }

    impl fmt::Display for PyBorrowError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt("Already mutably borrowed", f)
        }
    }

    impl From<PyBorrowError> for PyErr {
        fn from(other: PyBorrowError) -> Self {
            PyRuntimeError::new_err(other.to_string())
        }
    }

    /// An error returned by [`PyCell::try_borrow_mut`](struct.PyCell.html#method.try_borrow_mut).
    ///
    /// In Python, you can catch this error by `except RuntimeError`.
    pub struct PyBorrowMutError {
        _private: (),
    }

    impl fmt::Debug for PyBorrowMutError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("PyBorrowMutError").finish()
        }
    }

    impl fmt::Display for PyBorrowMutError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt("Already borrowed", f)
        }
    }

    impl From<PyBorrowMutError> for PyErr {
        fn from(other: PyBorrowMutError) -> Self {
            PyRuntimeError::new_err(other.to_string())
        }
    }