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

Javier nospam at
Fri Aug 19 23:06:11 EDT 2011

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... 

lzlu123 <lzlu123 at> 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?

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