Summary

TimedAnimation resets the event timer at the end of its _step() method:

As far as I can tell, this is by design. The original commit included this comment:

To me, this was suprising and it took me a while to get to the bottom of it. From the documentation I took that the event loop calls the animation target at exactly the interval specified. However, the current implementation means that it actually calls it at interval + time_in_callback. In case the callback does some heavy lifting, like processing data or communicating with an instrument, this can add significant overhead to the real interval between frames.

import matplotlib.pyplot as plt
import numpy as np
from matplotlib import animation

def update(frame):
    if len(frame) < 2:
        return []
    diff = np.diff(frame)
    avg.set_text(avg_txt.format(diff.mean()*1e3))
    cur.set_text(cur_txt.format(diff[-1]*1e3))
    return [avg, cur]


def frame():
    times = deque(maxlen=100)
    while True:
        time.sleep(0.1)
        times.append(time.perf_counter())
        yield times
        
fig, ax = plt.subplots(figsize=(1,1))
ax.axis('off')
avg_txt = 'avg: {:.3g} ms'
cur_txt = 'cur: {:.3g} ms'
avg = ax.text(0.5, 0.75, avg_txt.format(0), va='center', ha='center')
cur = ax.text(0.5, 0.25, cur_txt.format(0), va='center', ha='center')

anim = animation.FuncAnimation(fig, update, frame, repeat=False,
                               cache_frame_data=False, interval=200)

With the current behavior, this results in an effective interval of 300 ms. Commenting out the line resetting the interval results in the effective interval I would expect from interval=200, 200 ms.

Proposed fix

There might be reasonable usecases for resetting the interval, but I cannot see any right now. Hence, I would propose to leave the timer running uninterrupted, i.e., dropping