Python Parallel plotting and and input reading

Hello everyone,

I am seeking help with understanding why multithreading does not work correctly in this example:

import time
import matplotlib.pyplot as plt
import concurrent.futures

voltage = list()
stop_flag = False
fig, ax = plt.subplots()
fig.show()

def plotter(ax, arr):
    while True:
        if stop_flag:
            print("stream interupted")
            return
        arr.append(1)
        ax.clear()
        ax.plot(arr)
        ax.get_figure().canvas.draw()
        ax.get_figure().canvas.flush_events()
        time.sleep(1)

with concurrent.futures.ThreadPoolExecutor() as executor:
    plot_thread = executor.submit(plotter, ax, voltage)

for _ in range(15):
    res = input("Type: ")
    if res == "q":
        stop_flag = True
        break
    print(res)

Expected behaviour: Graph is being spawned by matplotlib and the line at y=1 is continuously growing in the x direction. In parallel terminal waits for user input. If input is q, the function plotter exits and everything finishes.

Actual behaviour: Graph is appearing, but not updating at all, while user input is being processed and reprinted to console. It seems that one point does exist on the graph. When pressing q or reaching 15 iterations program terminates.

I do not understand why parallel thread does not run and updates the graph independently from for loop.

Edit: Changing plotting to the main program and running input loop in another thread also does not work:

import time
import matplotlib.pyplot as plt
import concurrent.futures
import matplotlib
matplotlib.use("tkAgg")  # lock tk backend

voltage = list()
stop_flag = False


fig, ax = plt.subplots()
fig.show()
def fetch_input():
    global stop_flag
    for _ in range(15):
        res = input("Type: ")
        if res == "q":
            stop_flag = True
            break
        print(res)

with concurrent.futures.ThreadPoolExecutor() as executor:
    new_thread = executor.submit(fetch_input)

    while True:
        if stop_flag:
            print("main loop interupted")
            break
        voltage.append(1)
        ax.clear()
        ax.plot(voltage)
        ax.get_figure().canvas.draw()
        ax.get_figure().canvas.flush_events()
        plt.pause(1)  # run eventloop for 1 second
        # time.sleep(1)

First the function fetching the input starts and continues until it finished and only then the figure appears. It is even weirder now, since fig.show() happens in the beginning before the thread is even started. So I would expect to see at least empty figure together with input query in the terminal.

posted almost 2 years ago

CC BY-SA 4.0

2y ago

kek‭

36 reputation 1 1 5 3