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Q&A 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...

1 answer · posted 2y ago by kek‭ · edited 2y ago by kek‭

Question python multithreading concurrency matplotlib

#4: Nominated for promotion by

Alexei‭ · 2023-02-04T06:36:01Z (almost 2 years ago)

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#3: Post edited by

kek‭ · 2023-02-03T21:17:29Z (almost 2 years ago)
fixed the code in the 2nd snippet to working condition

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Hello everyone,
I am seeking help with understanding why multithreading does not work correctly in this example:
```python
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:
```python
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():
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.

Hello everyone,
I am seeking help with understanding why multithreading does not work correctly in this example:
```python
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:
```python
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.

#2: Post edited by

kek‭ · 2023-02-03T17:44:26Z (almost 2 years ago)

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Hello everyone,
I am seeking help with understanding why multithreading does not work correctly in this example:
```python
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.~~

Hello everyone,
I am seeking help with understanding why multithreading does not work correctly in this example:
```python
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:
```python
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():
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.

#1: Initial revision by

kek‭ · 2023-02-03T12:03:45Z (almost 2 years ago)

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Python Parallel plotting and and input reading

Hello everyone,

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

```python
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.

python multithreading concurrency matplotlib

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