How to make statements running in strictly sequential fashion like in an interactive shell

lzlu123 lzlu123 at gmail.com
Mon Aug 22 16:33:26 CEST 2011


On Aug 19, 11:06 pm, Javier <nos... at nospam.com> wrote:
> Never used it, but I think you can try this:
>
> Pexpect - a Pure Python Expect-like module
> Pexpect is a pure Python Expect-like module. Pexpect makes Python...www.noah.org/python/pexpect/
>
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> lzlu123 <lzlu... at gmail.com> wrote:
> > I have an instrument that has a RS232 type serial comm port and I need
> > to connect to and control. I use Python 3.2 in Windows XP, plus
> > pySerial module. I have a problem when I execute a script consisting
> > of statements that open the comm port, configure it, write strings to
> > and receive strings from it. Thoese strings aer either commands
> > pertinent to the instrument (control) or responses from the instrument
> > (response).
>
> > When those statements are executed in a python interpreter
> > interactively (at >>>), I get what I expect and the results are good
> > and correct. However, when I execute the script, either being invoked
> > within the interpreter or run file, I don???t get what I want. The
> > statements in the script is the same as what I use in the interactive
> > interpreter.
>
> > Why do I get the strange behavior and how can I change the script to
> > make it to behave like in interactive interpreter?
>
> > ----------------------script-----------------------
> > def read(comport):
>
> >    wrt_str=b'movt 3000'+b'\r\n'
> >    ret_str=comport.write(wrt_str)
>
> >    wrt_str=b'scan'+b'\r\n'
> >    ret_str=comport.write(wrt_str)
>
> >    rsp_str=comport.readlines() #########1
>
> >    wrt_str=b'hllo'+b'\r\n'
> >    ret_str=comport.write(wrt_str)
>
> >    rsp_str=comport.readlines()#########2
> > ----------------------------------------------------------
>
> > The problem is with the lines above with #######. In interactive mode,
> > there is about 1 second delay at #1, and 9 seonds delay at #2. I get
> > correct responses there. However, if I execute the script above, there
> > is no delay at all and I get incorrect results (garbage). I set the
> > read timeout to 0 in comm port set up, as
>
> > comport.timeout=0
> > So the comport should be in blocking mode if it waits for the end of
> > line or end of file.
>
> > I tried many things, like exec (execfile in 2.7), but at no avail.
>
> > I have an update to the original post I made a few days ago. I think I
> > know what the problem is and want to know if anyone has a solution:
>
> > After putting "print" and "time.sleep(delay)" after every statement, I
> > found when the script is running, it appears going around the pyserial
> > statement, such as "comport.write(..)" or "comport.readlines(...)"
> > while the pyserial command is executing (appearing as waiting and
> > busying doing some thing, you know serial port is slow). So for
> > example, when I exec all statements in a python interactive shell, I
> > am not able to type and run a new statement if the previous one is not
> > returned. Let's ay, if comport.readlines() is not returning, I can't
> > type and run the next comport.write(...) statemtn. However, in a
> > script that is running, if the comport.readlines() is busy reading,
> > the next statement is running, if the next statement happens to be a
> > comport.write() which will abort the reading.
>
> > Is there any way to force the python script to behave like running
> > exactly sequentially?

I am using Python 3.2 in Windows in which Pexpect appeared
problematic.



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