What is this data structure? (b'zzzz', ('192.168.4.16', 43578))

[boylesg]

Obtained via this code

ValuePair = self.Socket.recvfrom(self.nReceiveDataSize)
print(ValuePair)

What is the 'b' about?

'zzzz' is the data I sent from my mobile phone over Wifi access point.

It looks to me like an array of 2 elements where the first element is the string 'zzzz' and the second element is another array.

[jimmo]

It's a bytes string (i.e. a string of bytes). https://docs.python.org/3/library/stdtypes.html#bytes

This is different to a regular string which is a string of characters (which may not actually be bytes, e.g. utf-8 encoded).

You can use ValuePair.decode() to decode bytes into a string.

[jimmo]

Non-blocking sockets raise exceptions if you read and there's no data because you're not supposed to do that.

The idea is that you're supposed to have figured out if there's data e.g. using select() or poll(). This is exactly the same as in C, where you'd be using select/poll/epoll/kqueue.

In Python, the select/poll thing is simplified dramatically using asyncio, however I'm not sure if Micropython's asyncio supports UDP fully yet.

As an exercise, consider how you'd design a version of recvfrom that didn't raise an exception in this situation. Returning None is no better because you still have to handle it.

[boylesg]

Non-blocking sockets raise exceptions if you read and there's no data because you're not supposed to do that.

The idea is that you're supposed to have figured out if there's data e.g. using select() or poll(). This is exactly the same as in C, where you'd be using select/poll/epoll/kqueue.

In Python, the select/poll thing is simplified dramatically using asyncio, however I'm not sure if Micropython's asyncio supports UDP fully yet.

As an exercise, consider how you'd design a version of recvfrom that didn't raise an exception in this situation. Returning None is no better because you still have to handle it.

You HAVE to be able to poll data sources without them necessarily blocking!

Consider an RC control robot thing that does nothing else but sit there as wait for commands to move it is one direction or another - blocking is fine,

But consider a weather station that logs weather data to an SD card but also services HTTP requests to view the data to date.

If the socket blocks until a HTTP request is received then no weather data will ever be logged!!!!!!

[cwalther]

You HAVE to be able to poll data sources without them necessarily blocking!

That is correct, and nothing of what jimmo wrote contradicts it. Read https://docs.micropython.org/en/latest/library/select.html to find out how.

[boylesg]

You HAVE to be able to poll data sources without them necessarily blocking!

That is correct, and nothing of what jimmo wrote contradicts it. Read https://docs.micropython.org/en/latest/library/select.html to find out how.

At least for MCU, programs where debugging tools tend to be rather rudimentary, I am really not a fan of introducing yet more layers of complexity with this 'poll' class.

It starts making things less simple and less convenient to debug.

I really prefer to just have direct control over how long those receive functions block for.

As far as the sort of asynchronous projects I am working on, not returning any data after a poll is not an error condition.

Threads (effectively) might be OK for regular python where you probably have access to proper integrated debuggers in which you can set break points etc.

But I think they should be used vary sparingly (if at all) in MCU projects, where you are stuck with the likes of Thonny.

[cwalther]

For what it’s worth, empirically (on an ESP32) I find that I can use read() instead of recv[from]() to read from a non-blocking UDP socket, and it will return None instead of raising an exception when there is no data available. I don’t see that specified anywhere though (in CPython, makefile(), which gets one the read method, is explicitly specified to be unsupported on non-blocking sockets). Checking the source, it depends on whether the underlying network stack returns EAGAIN/EWOULDBLOCK or some other error.