pySerial - Is there a way to select on multiple ports at once?

Question

I am developing app that need to communicate with many serial ports. I havnt found a way to do this without using thread per port. Is there a way to do this with single thread?something like select or poll on multiple ports at once? I am using pyserial 2.6

Answer 1

I'm assuming you are using PySerial on a unix like platform...

Since PySerial objects implement fileno() to get the underlying file descriptor, you can pass them straight into select() which will allow you to do deal with multiple PySerial objects at once.

Another alternative would be to open the Serial object in non-blocking (read) mode (by passing timeout=0) and deal with the fact that your reads and writes may return errno.EWOULDBLOCK errors. This is probably the simplest method.

A third alternative would be to use twisted serial ports if you don't mind getting your head round the way twisted does things.

Update

For Windows, pretty much your only alternative other than using threads is to use the in_waiting property. Poll all your serial ports regularly reading in_waiting from them. If there is stuff waiting then you can read that and only that many bytes without blocking.

Unfortunately pyserial doesn't have a "how much free space is there in the output buffer" method which means that when you write to serial ports you are at risk of blocking. If you are implementing a typical serial port protocol the default buffer sizes of a few kilobytes will ensure that this isn't normally a problem.