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

lzlu123 lzlu123 at
Fri Aug 19 11:00:21 EDT 2011

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

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

Why do I get the strange behavior and how can I change the script to
make it to behave like in interactive interpreter?

def read(comport):

    wrt_str=b'movt 3000'+b'\r\n'


    rsp_str=comport.readlines() #########1



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

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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