Casting PyObjectRef to PyString












1














I have a function that accepts a Python list. I expect the list to be made of strings. How do I extract them?.



Arch Linux, Python 3.7, nightly rust 1.33.0, pyo3 0.5.2.



fn f(list: &PyList) -> PyResult<SomeClass> {

    for obj in list.iter() {

        let rust_string = PyString::from_object(obj, "ASCII", "strict")?.to_string()?;

        // fails with `TypeError: decoding str is not supported`



        if PyString::is_exact_instance(obj) {

            let py_str: PyString = unsafe {std::mem::transmute(str)};

            let rust_str = py_str.to_string()?;

            // panics with failed assertion in PyString.as_bytes()

        }

    }

    ...

}



// Python call site

f(["string1", "string2"])





rust pyo3







share|improve this question




















  • 1




    From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
    – Sven Marnach
    Dec 27 '18 at 21:39










  • On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
    – Sven Marnach
    Dec 27 '18 at 21:43










  • Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
    – user7810882
    Dec 27 '18 at 21:49
















1














I have a function that accepts a Python list. I expect the list to be made of strings. How do I extract them?.



Arch Linux, Python 3.7, nightly rust 1.33.0, pyo3 0.5.2.



fn f(list: &PyList) -> PyResult<SomeClass> {

    for obj in list.iter() {

        let rust_string = PyString::from_object(obj, "ASCII", "strict")?.to_string()?;

        // fails with `TypeError: decoding str is not supported`



        if PyString::is_exact_instance(obj) {

            let py_str: PyString = unsafe {std::mem::transmute(str)};

            let rust_str = py_str.to_string()?;

            // panics with failed assertion in PyString.as_bytes()

        }

    }

    ...

}



// Python call site

f(["string1", "string2"])





rust pyo3







share|improve this question




















  • 1




    From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
    – Sven Marnach
    Dec 27 '18 at 21:39










  • On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
    – Sven Marnach
    Dec 27 '18 at 21:43










  • Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
    – user7810882
    Dec 27 '18 at 21:49














1












1








1







I have a function that accepts a Python list. I expect the list to be made of strings. How do I extract them?.



Arch Linux, Python 3.7, nightly rust 1.33.0, pyo3 0.5.2.



fn f(list: &PyList) -> PyResult<SomeClass> {

    for obj in list.iter() {

        let rust_string = PyString::from_object(obj, "ASCII", "strict")?.to_string()?;

        // fails with `TypeError: decoding str is not supported`



        if PyString::is_exact_instance(obj) {

            let py_str: PyString = unsafe {std::mem::transmute(str)};

            let rust_str = py_str.to_string()?;

            // panics with failed assertion in PyString.as_bytes()

        }

    }

    ...

}



// Python call site

f(["string1", "string2"])





rust pyo3







share|improve this question















I have a function that accepts a Python list. I expect the list to be made of strings. How do I extract them?.



Arch Linux, Python 3.7, nightly rust 1.33.0, pyo3 0.5.2.



fn f(list: &PyList) -> PyResult<SomeClass> {

    for obj in list.iter() {

        let rust_string = PyString::from_object(obj, "ASCII", "strict")?.to_string()?;

        // fails with `TypeError: decoding str is not supported`



        if PyString::is_exact_instance(obj) {

            let py_str: PyString = unsafe {std::mem::transmute(str)};

            let rust_str = py_str.to_string()?;

            // panics with failed assertion in PyString.as_bytes()

        }

    }

    ...

}



// Python call site

f(["string1", "string2"])





rust pyo3




rust pyo3






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Dec 27 '18 at 21:07

























asked Dec 27 '18 at 20:15









user7810882

5629




5629








  • 1




    From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
    – Sven Marnach
    Dec 27 '18 at 21:39










  • On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
    – Sven Marnach
    Dec 27 '18 at 21:43










  • Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
    – user7810882
    Dec 27 '18 at 21:49














  • 1




    From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
    – Sven Marnach
    Dec 27 '18 at 21:39










  • On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
    – Sven Marnach
    Dec 27 '18 at 21:43










  • Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
    – user7810882
    Dec 27 '18 at 21:49








1




1




From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
– Sven Marnach
Dec 27 '18 at 21:39




From a quick look at the documentation, it looks like downcasting is done using the PyObject::extract() method, something like let rust_str: &str = obj.extract(py)?, where py is the GIL wrapper. I haven't tried it, though.
– Sven Marnach
Dec 27 '18 at 21:39












On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
– Sven Marnach
Dec 27 '18 at 21:43




On second look, the example on the start page of the documentation looks even more promising – you can use the pyfn attribute to have the conversion from Python to Rust types handled for you implicitly.
– Sven Marnach
Dec 27 '18 at 21:43












Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
– user7810882
Dec 27 '18 at 21:49




Thanks. The first one works. let py_str: &PyString = obj.extract()?;. let str = py_str.to_string()?;
– user7810882
Dec 27 '18 at 21:49












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