Python while loop not updating

Question

I'm very new to Python.

I'm trying to display the current temperature from a sensor, which I get from a bash script. I use sed to replace the string in the python script (temp.py), which prints that string to an LCD Display.

Unfortunately, when I run temp.py, it only shows the temperature it has at the moment I run it, but it will not update, even though the lcd.py script is constantly changing (it only shows one temperature).

while True:
    lcd_string("Temp: 25.123 *C",LCD_LINE_1,1)
    time.sleep(1)

25.123 is changing constantly in temp.py, but not the different temperatures are not shown in the LCD

Is there any way to get it to update?

@Ramirous, I really suggest you another aproach.. It wont work because unless you adit the script the last one was executed with a fixed text end is still running eith the .pyc bytecode — Hamlett
– Hamlett, Commented Jul 12, 2016 at 2:29
It is indented, sorry I didn't show it as is. I have a temp. sensor, I can see the temperature changing (cold water to hot water), and I see 25.123 changing constantly in temp.py due to the instruction given by sed from another script — Ramirous
– Ramirous, Commented Jul 12, 2016 at 2:29

Ned Batchelder · Accepted Answer · 2016-07-12 02:28:56Z

5

Changing the .py file on disk won't change the running program. The .py file is only read once, when the program starts. After that, the .py file isn't used again until the program is run again.

answered Jul 12, 2016 at 2:28

Ned Batchelder

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Comments

Joran Beasley · Accepted Answer · 2016-07-12 02:29:15Z

2

have your sed script constantly update a file called "label.txt" (you probably dont even need sed ... as an aside, why arent you just interfacing the sensor from python?)

then in your temp.py script

while True:
    lcd_string(open("label.txt").read(),LCD_LINE_1,1)
    time.sleep(1)

that is probably the easiest way to just make it work ... (See ned's answer as to why your original implementation did not work)

answered Jul 12, 2016 at 2:29

Joran Beasley

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Hamlett · Accepted Answer · 2016-07-12 13:49:25Z

1

The .py file generates a bytecode to run. See the .pyc file beside your temp.py. So, once the script is running it won't load your change (the one made with sed command) in temp.py until you run it again.

I really suggest you another approach where you don't use an infinite loop but a single script that takes an argument every time it is executed and send the value to _lcd_string_ method.

Something like:

import sys

temp = sys.argv[1]
lcd_string("Temp: %s *C" % temp,LCD_LINE_1,1)

and instead of sed you can run:

python temp.py <your temperature>

edited Jul 12, 2016 at 13:49

answered Jul 12, 2016 at 2:33

Hamlett

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Comments

Jacky Wang · Accepted Answer · 2016-07-12 02:36:58Z

0

A bad idea is to reload lcd.py every iteration, the newly value updated by sed will works after saving(the lcd.py file). The code as follows,

while True:
    reload(lcd)  # <-------- 
    lcd_string("Temp: 25.123 *C",LCD_LINE_1,1)
    time.sleep(1)

Note it is not recommended in practice.

answered Jul 12, 2016 at 2:36

Jacky Wang

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1 Comment

Joran Beasley Over a year ago

im pretty sure this is running in lcd.py ... can you reload the script that you are in like this? Ive never seen this done... (I know you can reload imported modules like this)

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