UserWarning: Starting a Matplotlib GUI outside of the main thread

UserWarning: Starting a Matplotlib GUI outside of the main thread

$ python script.py
UserWarning: Starting a Matplotlib GUI outside of the main thread will likely fail.

Why this happens

Matplotlib’s GUI backends (like TkAgg, Qt5Agg) are generally not thread-safe and must be run from the main thread of the application. Running plotting commands in a background thread can cause crashes or this warning.

Fix

Perform all plotting and GUI operations in the main thread. If you need to plot from a thread, use a non-interactive backend or queue data to the main thread for plotting.

Wrong code

import threading
import matplotlib.pyplot as plt

def plot():
    plt.plot([1, 2])
    plt.show()

t = threading.Thread(target=plot)
t.start()

Fixed code

# Run plotting in main thread
import matplotlib.pyplot as plt

def prepare_data():
    return [1, 2]

data = prepare_data()
plt.plot(data)
plt.show()