matplotlib globals cause problems with `pyplot()` if multiple sessions

I’m not actually sure if this is a documentation suggestion or bug report or what, but it seems worth noting:

I run this with streamlit (0.52.2):

import time
import streamlit as st
import matplotlib.pyplot as plt

while True:
    plt.scatter([1, 2, 3], [1, 2, 3])
    st.pyplot()
    time.sleep(.2)

I open one browser window, and everything is as expected (I see a long series of plots). If I view the app in another browser tab, then one of the tabs gets this traceback:

IndexError: list index out of range
Traceback:
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/ScriptRunner.py", line 324, in _run_script
    exec(code, module.__dict__)
  File "/Users/davidchudzicki/streamlit_end_code_block/b.py", line 8, in <module>
    st.pyplot()
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/__init__.py", line 152, in wrapped_method
    return method.__get__(dg)(*args, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/DeltaGenerator.py", line 135, in wrapped_method
    return dg._enqueue_new_element_delta(marshall_element, delta_type, last_index)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/DeltaGenerator.py", line 392, in _enqueue_new_element_delta
    rv = marshall_element(msg.delta.new_element)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/DeltaGenerator.py", line 133, in marshall_element
    return method(dg, element, *args, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/DeltaGenerator.py", line 1305, in pyplot
    pyplot.marshall(element, fig, clear_figure, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/streamlit/elements/pyplot.py", line 57, in marshall
    fig.savefig(image, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/pyplot.py", line 723, in savefig
    fig.canvas.draw_idle()   # need this if 'transparent=True' to reset colors
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/backend_bases.py", line 1914, in draw_idle
    self.draw(*args, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/backends/backend_agg.py", line 388, in draw
    self.figure.draw(self.renderer)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/figure.py", line 1709, in draw
    renderer, self, artists, self.suppressComposite)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/image.py", line 135, in _draw_list_compositing_images
    a.draw(renderer)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/axes/_base.py", line 2607, in draw
    self._update_title_position(renderer)
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/axes/_base.py", line 2548, in _update_title_position
    ax.xaxis.get_ticks_position() in choices):
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/axis.py", line 2146, in get_ticks_position
    self._get_ticks_position()]
  File "/Users/davidchudzicki/.virtualenvs/streamlit/lib/python3.6/site-packages/matplotlib/axis.py", line 1833, in _get_ticks_position
    minor = self.minorTicks[0]

If I do this instead, there are no problems:

import time
import streamlit as st
import matplotlib.pyplot as plt

while True:
    fig, ax = plt.subplots()
    ax.scatter([1, 2, 3], [1, 2, 3])
    st.pyplot(fig)
    time.sleep(.2)

Not too surprising. (Seems hard to correctly support the matplotlib global objects…) But maybe it’s worth adding warning? (Or even eventually deprecating the no-argument pyplot.)

[dchudz]

Yeah. For what it’s worth, your Option 1 seems right to me.

I was initially a little put off from Streamlit when I saw this error in an app someone tweeted. It took me some time to realize that the problem was limited to matplotlib global state, and not a deeper issue with Streamlit itself.

If you stop supporting the global-state API you’ll avoid that kind of perception.

[jrhone]

Sorry, should have mentioned it’s a private link.

We’re leaning towards allowing the functionality and displaying a warning.