Python Parallel plotting and and input reading
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)
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
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.
After moving the plotting
while loop in the second edit inside
with context manager and declaring
stop_flag as a global variable inside
fetch_input it worked as expected.
- Use main thread for GUI/graphs
with concurrent.futures.ThreadPoolExecutor() as executor:joins all the threads in the end. So all the code, that should be parallelized must be within context manager or used without context manager.
2nd snippet in question now contains working code.
0 comment threads