将子流程调用的输出打印到标签tkinter中

Question

I am trying to create a GUI in tkinter. The Gui is basically for Mcp23017 . I am trying to configure the input and output pins so that the user could change them according to their choice.. There is also an option to make the inputs/outputs high or low..

Now I am trying to read a pin using 'i2cget'(using a seperate function).. I need to display the output of this subprocess call into a label on the gui..

This is what my code looks like:

def read_p22():
   output = subprocess.call(['i2cget -y 0x20 0x12 0xFD'],shell=True)
   x=print (output)
   Label(tableframe,textvariable=x).grid(row=2,column=20)
   root.after(5000, read_p22)

When this function excutes(by pressing a button), however it prints a value '1' on the python shell alternatly when I press the button... I dont know how to redirect the output to the label.. Could somebody suggest something?

Update:: On executing the suggested commands:

process = subprocess.Popen(['i2cget -y 0x20 0x12 0xFD'], stdout=PIPE,      stderr=PIPE, shell=True)
output, code = process.communicate()

I printed the 'output' and 'process' and they gave me the follwing respectivley:

b'' <subprocess.Popen object at 0x00000000035CB2B0>

Since nothing is connected to the pin I expect it to return a value of '0'.. I don't know what is the b'' it is giving me ...

Any advice is really appreciated..

Kind Regards, Namita.

Answer 1

What you have is the return code, not the output.

From subprocess.call docs:

Run the command described by args. Wait for command to complete, then return the returncode attribute .

Instead, use subprocess.Popen to open a process, subprocess.PIPE to pipe the output to you, and Popen.communicate get the output:

process = subprocess.Popen(['i2cget -y 0x20 0x12 0xFD'], stdout=PIPE, stderr=PIPE, shell=True)
output, code = process.communicate()
# do stuff with output...

Answer 2

There are multiple things wrong in your code. You should split your problem into tiny tiny subtasks and complete each task independently.

1. Get output from a subprocess as a string variable in Python

subprocess.call() returns an integer (exit status), not string (the output) -- start Python REPL and try to run any command via subprocess and see for yourself: it should help you to understand what is happening in addition to reading the linked documentation.

You could use subprocess.check_output() , to get the output if you don't mind blocking your GUI thread -- nothing will respond until the subprocess returns.

Otherwise (if i2cget does not return in an instant, always), you could do it asynchroniously using threads or .createfilehandler() method .

2. Learn why x = print(output) is useless in Python

It is SyntaxError in Python 2 by default. Otherwise print() function always returns None ie, you could have written x = None (except that output is printed to stdout as a side-effect).

3. Update text in a Label

• create a label only once instead of creating a new label every 5 seconds
• pass StringVar() as a textvariable parameter
• use .set(new_text) to update the text