Background:
I'm familiar with C's select() function. I've been using this function for many purposes. Most of them, if not all, for reading and writing to pipes, files, etc. I must say that I've never used the error list, but this is not involved in the key question.
Question:
Does Python's select() behave as follows?
It turns out to me that select() on Python behaves a different way despite the straightforward interface to C select(). It seems that select() returns the very first time a file is ready for reading. If you read the file while leaving some bytes to be read, calling select() will block. But, if you call select() again after a previous call to select() was returned without any read call between these two calls, select() will return as expected. For example:
import select
# Open the file (yes, playing around with joysticks)
file = open('/dev/input/js0', 'r')
# Hold on the select() function waiting
select.select([file], [], [])
# Say 16 bytes are sent to the file, select() will return.
([<open file '/dev/input/js0', mode 'r' at 0x7ff2949c96f0>], [], [])
# Call select() again, and select() will indeed return.
select.select([file], [], [])
([<open file '/dev/input/js0', mode 'r' at 0x7ff2949c96f0>], [], [])
# read 8 bytes. There are 8 bytes left for sure. Calling again file.read(8) will empty the queue and would be pointless for this example
file.read(8)
'<\t\x06\x01\x00\x00\x81\x01'
# call select() again, and select() will block
select.select([file], [], [])
# Should it block? there are 8 bytes on the file to be read.
If this is the behaviour of select() in Python, I'm okay with that, I could handle it. Not what I expected, though, but it's fine. I know what I can do with it.
But if this is not the behaviour of select() I would appreciate someone to tell me what I'm doing wrong. What I read about select() is what the Python docs say: "select() returns if any file in the read|write|error list is ready for read|write|error.". That's OK no lies there. Maybe the questions should be:
- When a file is considered to be ready for reading in python?
- Does it means a file that has never been read?
- Does it means a file with bytes to be read?