Read file with timeout in Python

Question

Within Linux, there is a file, /sys/kernel/debug/tracing/trace_pipe, which as the name says, is a pipe. So, let\'s say I want to read the first 50 bytes from it

Answer 1

Just adding this as note, for better formatting:

@Alfe's answer in my case:

$ sudo python -c 'import os, select; 
f=open("/sys/kernel/debug/tracing/trace_pipe","r"); print f; 
rrdy, wrdy, xrdy = select.select([f], [], [], 1); print rrdy, wrdy, xrdy ; 
timeout= "timed out" if (rrdy==[]) else "" ; 
print timeout; 
print os.read(f.fileno(), 50) if timeout=="" else ""; 
f.close() '

If there is something in the file, I get response like:

[] [] []

            Xorg-1033  [001] 12570.075859: 

If there is nothing in the file, I get:

[] [] []
timed out

Note that the select documentation isn't explicit that the timeout parameter is in seconds - but that floating point values (e.g. 0.5) also work.

@GabiMe's answer:

$ sudo python -c 'import os; 
filno = os.open("/sys/kernel/debug/tracing/trace_pipe", os.O_RDONLY|os.O_NONBLOCK); 
f=os.fdopen(filno, "r"); print f; 
print "A", f.read(50); 
print "B", os.read(f.fileno(), 50); 
f.close() '

If there is something in the file, I get response like:

', mode 'r' at 0xb77b6e90>
A             bash-13777 [000] 13694.404519: sys_exi
B            Timer-31065 [001] 13694.404830: sys_exi

If there is nothing in the file, I get:

', mode 'r' at 0xb77c1e90>
A
Traceback (most recent call last):
  File "", line 1, in 
IOError: [Errno 11] Resource temporarily unavailable

... so one must run this in a try block, to catch the IOError, if there is nothing in the file... (both os.read and f.read will raise this exception)