Making the SpiDev file descriptor accessible.

[sbelbin]

We have automated script that executes various test cases and much of it is implemented in Python script which uses aSpiDev object to perform IOCTL with Raspberry Pi devices.

The automated script needs to run at 1024 Hz (eventually to 2048 Hz) and thus the critical portions are implemented in C code and invoked via Cython as a bridging interface (adapter) between Python script and C code.

Missing is capability of the SpiDev object to provide its underlying file descriptor (fd) that is missing.

The C code won't compete with Python script for the file descriptor such as both using it at the same, nor does the C code close the the file descriptor, but leaves that to Python script via SpiDev.close or upon destruction of the SpiDev object.

Regards,
Steven

[Gadgetoid]

Is this because you want to take the fd from an SpiDev instance and pass it into your own C code?

Seems like a reasonable and low-impact feature- though it’s been a long time since I touched the code 😬

[sbelbin]

That is correct.

I've got an idea what needs to be changed.

But, my policy is that it's best to ask. There might be reasons as not to provide the file descriptor property or even better someone has tried in the past then removed it.

[fschrempf]

I guess if someone would like to implement this, it could be done. But on the other hand I'm wondering if it is really a good idea as there is no way to ensure that there is no interference. This would create an additional use-case that needs to be supported and maintained.

My suggestion would be that you use C code for the SpiDev handling (instead of py-spdev) altogether and only keep your test scripting as a Python wrapper.

[yochem]

Isn't this what SpiDev_fileno does?

py-spidev/spidev_module.c

Lines 913 to 923 in 60375bd

	PyDoc_STRVAR(SpiDev_fileno_doc,
	"fileno() -> integer \"file descriptor\"\n\n"
	"This is needed for lower-level file interfaces, such as os.read().\n");
	static PyObject *
	SpiDev_fileno(SpiDevObject *self)
	{
	PyObject *result = Py_BuildValue("i", self->fd);
	Py_INCREF(result);
	return result;
	}

[sbelbin]

@yochem, the code is almost there just missing the a getter of that fd' or filenoas a property via the Python class. Case in point, there is one to get/set themode` and other properties.

https://github.com/doceme/py-spidev/blob/60375bd992a6dae570593051bf802d4f8bb0264c/spidev_module.c#L1313C1-L1335C3

I think that it's a question of added one more entry to expose it as a property.

---

@fschrempf, I understand that points that you've made. If py-spidev were an application, then I'd probably agree to avoid exposing that property.

However, I view pyi-spidev as a tool to enable as a low-level tool for test engineers & developers to write script so that can rapidly iterate on the implementation low-level suite of tests or utilities that developers use internally.

From that perspective Python is a key enabler for those test engineers and developers whom are more learnt & comfortable using Python than using C.

Our case exposing the file descriptor is beneficial so that the majority of the testing infrastructure is codified in Python and in our case uses Cython as an interop shim to invoke some functions in C library that is also used in a higher level application code. Hence, we are validating this C library.

[yochem]

Do I understand it correctly that you want it as a property instead of a method? Because it's an available method:

py-spidev/spidev_module.c

Lines 1469 to 1470 in 60375bd

	{"fileno", (PyCFunction)SpiDev_fileno, METH_NOARGS,
	SpiDev_fileno_doc},

[sbelbin]

Exposing as a property would be more intuitive for more developers, since I was looking for a property such as fd or something similar to that.

I didn't consider that getting a file descriptor as a method via fileno until you shed light on the matter in your previous post.

Therefore, as a property such as fd is more convenient & intuitive.

[yochem]

It's the standard way to return the underlying file descriptor in Python: https://docs.python.org/3/library/io.html#io.IOBase.fileno