Interactive figures and asynchronous programming#
Matplotlib supports rich interactive figures by embedding figures into a GUI window. The basic interactions of panning and zooming in an Axes to inspect your data is 'baked in' to Matplotlib. This is supported by a full mouse and keyboard event handling system that you can use to build sophisticated interactive graphs.
This guide is meant to be an introduction to the low-level details of how Matplotlib integration with a GUI event loop works. For a more practical introduction to the Matplotlib event API see event handling system, Interactive Tutorial, and Interactive Applications using Matplotlib.
Event loops#
Fundamentally, all user interaction (and networking) is implemented as an infinite loop waiting for events from the user (via the OS) and then doing something about it. For example, a minimal Read Evaluate Print Loop (REPL) is
exec_count = 0
while True:
inp = input(f"[{exec_count}] > ") # Read
ret = eval(inp) # Evaluate
print(ret) # Print
exec_count += 1 # Loop
This is missing many niceties (for example, it exits on the first exception!), but is representative of the event loops that underlie all terminals, GUIs, and servers [1]. In general the Read step is waiting on some sort of I/O -- be it user input or the network -- while the Evaluate and Print are responsible for interpreting the input and then doing something about it.
In practice we interact with a framework that provides a mechanism to
register callbacks to be run in response to specific events rather
than directly implement the I/O loop [2]. For example "when the
user clicks on this button, please run this function" or "when the
user hits the 'z' key, please run this other function". This allows
users to write reactive, event-driven, programs without having to
delve into the nitty-gritty [3] details of I/O. The core event loop
is sometimes referred to as "the main loop" and is typically started,
depending on the library, by methods with names like _exec
,
run
, or start
.
All GUI frameworks (Qt, Wx, Gtk, tk, OSX, or web) have some method of
capturing user interactions and passing them back to the application
(for example Signal
/ Slot
framework in Qt) but the exact
details depend on the toolkit. Matplotlib has a backend for each GUI toolkit we support which uses the
toolkit API to bridge the toolkit UI events into Matplotlib's event
handling system. You can then use
FigureCanvasBase.mpl_connect
to connect your function to
Matplotlib's event handling system. This allows you to directly
interact with your data and write GUI toolkit agnostic user
interfaces.
Command prompt integration#
So far, so good. We have the REPL (like the IPython terminal) that lets us interactively send code to the interpreter and get results back. We also have the GUI toolkit that runs an event loop waiting for user input and lets us register functions to be run when that happens. However, if we want to do both we have a problem: the prompt and the GUI event loop are both infinite loops that each think they are in charge! In order for both the prompt and the GUI windows to be responsive we need a method to allow the loops to 'timeshare' :
let the GUI main loop block the python process when you want interactive windows
let the CLI main loop block the python process and intermittently run the GUI loop
fully embed python in the GUI (but this is basically writing a full application)
Blocking the prompt#
Display all open figures. |
|
Run the GUI event loop for interval seconds. |
|
Start a blocking event loop. |
|
Stop the current blocking event loop. |
The simplest "integration" is to start the GUI event loop in 'blocking' mode and take over the CLI. While the GUI event loop is running you can not enter new commands into the prompt (your terminal may echo the characters typed into the terminal, but they will not be sent to the Python interpreter because it is busy running the GUI event loop), but the figure windows will be responsive. Once the event loop is stopped (leaving any still open figure windows non-responsive) you will be able to use the prompt again. Re-starting the event loop will make any open figure responsive again (and will process any queued up user interaction).
To start the event loop until all open figures are closed, use
pyplot.show
as
pyplot.show(block=True)
To start the event loop for a fixed amount of time (in seconds) use
pyplot.pause
.
If you are not using pyplot
you can start and stop the event loops
via FigureCanvasBase.start_event_loop
and
FigureCanvasBase.stop_event_loop
. However, in most contexts where
you would not be using pyplot
you are embedding Matplotlib in a
large GUI application and the GUI event loop should already be running
for the application.
Away from the prompt, this technique can be very useful if you want to write a script that pauses for user interaction, or displays a figure between polling for additional data. See Scripts and functions for more details.
Input hook integration#
While running the GUI event loop in a blocking mode or explicitly handling UI events is useful, we can do better! We really want to be able to have a usable prompt and interactive figure windows.
We can do this using the 'input hook' feature of the interactive prompt. This hook is called by the prompt as it waits for the user to type (even for a fast typist the prompt is mostly waiting for the human to think and move their fingers). Although the details vary between prompts the logic is roughly
start to wait for keyboard input
start the GUI event loop
as soon as the user hits a key, exit the GUI event loop and handle the key
repeat
This gives us the illusion of simultaneously having interactive GUI windows and an interactive prompt. Most of the time the GUI event loop is running, but as soon as the user starts typing the prompt takes over again.
This time-share technique only allows the event loop to run while python is otherwise idle and waiting for user input. If you want the GUI to be responsive during long running code it is necessary to periodically flush the GUI event queue as described in Explicitly spinning the event Loop. In this case it is your code, not the REPL, which is blocking the process so you need to handle the "time-share" manually. Conversely, a very slow figure draw will block the prompt until it finishes drawing.
Full embedding#
It is also possible to go the other direction and fully embed figures (and a Python interpreter) in a rich native application. Matplotlib provides classes for each toolkit which can be directly embedded in GUI applications (this is how the built-in windows are implemented!). See Embedding Matplotlib in graphical user interfaces for more details.
Scripts and functions#
Flush the GUI events for the figure. |
|
Request a widget redraw once control returns to the GUI event loop. |
|
Blocking call to interact with a figure. |
|
Blocking call to interact with a figure. |
|
Display all open figures. |
|
Run the GUI event loop for interval seconds. |
There are several use-cases for using interactive figures in scripts:
capture user input to steer the script
progress updates as a long running script progresses
streaming updates from a data source
Blocking functions#
If you only need to collect points in an Axes you can use
Figure.ginput
. However if you have written some custom event
handling or are using widgets
you will need to manually run the GUI
event loop using the methods described above.
You can also use the methods described in Blocking the prompt
to suspend run the GUI event loop. Once the loop exits your code will
resume. In general, any place you would use time.sleep
you can use
pyplot.pause
instead with the added benefit of interactive figures.
For example, if you want to poll for data you could use something like
fig, ax = plt.subplots()
ln, = ax.plot([], [])
while True:
x, y = get_new_data()
ln.set_data(x, y)
plt.pause(1)
which would poll for new data and update the figure at 1Hz.
Explicitly spinning the event Loop#
Flush the GUI events for the figure. |
|
Request a widget redraw once control returns to the GUI event loop. |
If you have open windows that have pending UI
events (mouse clicks, button presses, or draws) you can explicitly
process those events by calling FigureCanvasBase.flush_events
.
This will run the GUI event loop until all UI events currently waiting
have been processed. The exact behavior is backend-dependent but
typically events on all figure are processed and only events waiting
to be processed (not those added during processing) will be handled.
For example
import time
import matplotlib.pyplot as plt
import numpy as np
plt.ion()
fig, ax = plt.subplots()
th = np.linspace(0, 2*np.pi, 512)
ax.set_ylim(-1.5, 1.5)
ln, = ax.plot(th, np.sin(th))
def slow_loop(N, ln):
for j in range(N):
time.sleep(.1) # to simulate some work
ln.figure.canvas.flush_events()
slow_loop(100, ln)
While this will feel a bit laggy (as we are only processing user input every 100ms whereas 20-30ms is what feels "responsive") it will respond.
If you make changes to the plot and want it re-rendered you will need
to call draw_idle
to request that the canvas be
re-drawn. This method can be thought of draw_soon in analogy to
asyncio.loop.call_soon
.
We can add this to our example above as
def slow_loop(N, ln):
for j in range(N):
time.sleep(.1) # to simulate some work
if j % 10:
ln.set_ydata(np.sin(((j // 10) % 5 * th)))
ln.figure.canvas.draw_idle()
ln.figure.canvas.flush_events()
slow_loop(100, ln)
The more frequently you call FigureCanvasBase.flush_events
the more
responsive your figure will feel but at the cost of spending more
resources on the visualization and less on your computation.
Stale artists#
Artists (as of Matplotlib 1.5) have a stale attribute which is
True
if the internal state of the artist has changed since the last
time it was rendered. By default the stale state is propagated up to
the Artists parents in the draw tree, e.g., if the color of a Line2D
instance is changed, the Axes
and Figure
that
contain it will also be marked as "stale". Thus, fig.stale
will
report if any artist in the figure has been modified and is out of sync
with what is displayed on the screen. This is intended to be used to
determine if draw_idle
should be called to schedule a re-rendering
of the figure.
Each artist has a Artist.stale_callback
attribute which holds a callback
with the signature
def callback(self: Artist, val: bool) -> None:
...
which by default is set to a function that forwards the stale state to the artist's parent. If you wish to suppress a given artist from propagating set this attribute to None.
Figure
instances do not have a containing artist and their
default callback is None
. If you call pyplot.ion
and are not in
IPython
we will install a callback to invoke
draw_idle
whenever the
Figure
becomes stale. In IPython
we use the
'post_execute'
hook to invoke
draw_idle
on any stale figures
after having executed the user's input, but before returning the prompt
to the user. If you are not using pyplot
you can use the callback
Figure.stale_callback
attribute to be notified when a figure has
become stale.
Idle draw#
Render the |
|
Request a widget redraw once control returns to the GUI event loop. |
|
Flush the GUI events for the figure. |
In almost all cases, we recommend using
backend_bases.FigureCanvasBase.draw_idle
over
backend_bases.FigureCanvasBase.draw
. draw
forces a rendering of
the figure whereas draw_idle
schedules a rendering the next time
the GUI window is going to re-paint the screen. This improves
performance by only rendering pixels that will be shown on the screen. If
you want to be sure that the screen is updated as soon as possible do
fig.canvas.draw_idle()
fig.canvas.flush_events()
Threading#
Most GUI frameworks require that all updates to the screen, and hence their main event loop, run on the main thread. This makes pushing periodic updates of a plot to a background thread impossible. Although it seems backwards, it is typically easier to push your computations to a background thread and periodically update the figure on the main thread.
In general Matplotlib is not thread safe. If you are going to update
Artist
objects in one thread and draw from another you should make
sure that you are locking in the critical sections.
Eventloop integration mechanism#
CPython / readline#
The Python C API provides a hook, PyOS_InputHook
, to register a
function to be run ("The function will be called when Python's
interpreter prompt is about to become idle and wait for user input
from the terminal."). This hook can be used to integrate a second
event loop (the GUI event loop) with the python input prompt loop.
The hook functions typically exhaust all pending events on the GUI
event queue, run the main loop for a short fixed amount of time, or
run the event loop until a key is pressed on stdin.
Matplotlib does not currently do any management of PyOS_InputHook
due
to the wide range of ways that Matplotlib is used. This management is left to
downstream libraries -- either user code or the shell. Interactive figures,
even with Matplotlib in 'interactive mode', may not work in the vanilla python
repl if an appropriate PyOS_InputHook
is not registered.
Input hooks, and helpers to install them, are usually included with
the python bindings for GUI toolkits and may be registered on import.
IPython also ships input hook functions for all of the GUI frameworks
Matplotlib supports which can be installed via %matplotlib
. This
is the recommended method of integrating Matplotlib and a prompt.
IPython / prompt_toolkit#
With IPython >= 5.0 IPython has changed from using CPython's readline
based prompt to a prompt_toolkit
based prompt. prompt_toolkit
has the same conceptual input hook, which is fed into prompt_toolkit
via the
IPython.terminal.interactiveshell.TerminalInteractiveShell.inputhook()
method. The source for the prompt_toolkit
input hooks lives at
IPython.terminal.pt_inputhooks
.
Footnotes