BUG: fix IPython's %pylab mode detection

There is no pyplot module in the system registry, as the module is
imported as matplotlib.pyplot.

This code tries to detect when matplotlib.pyplot.show() is invoked in
a script run via the %run IPython's magic while in interactive mode as
enabled by the %pylab or %matplotlib IPyhton's magic commands.  In these
modes, matplotlib.pyplot.show() should not block.

Failing do detect these modes, a simple test.py script as this

  import matplotlib.pyplot as plt
  plt.figure()
  plt.show()

run in IPython as follows

  %matplotlib
  %run test.py

is not expected to block on matplotlib.pyplot.show().  Without this
patch it does.  Additionally, the redraw implemented by IPython at the
end of %run caused an empty figure to be displayed when the script
terminates.

Fixing the module lookup in sys.modules fixes the issue.

Fixes: 86f26a0.
Fixes: matplotlib#25485.

Author: dnicolodi

lib/matplotlib/backend_bases.py

@@ -2930,8 +2930,8 @@ def pyplot_show(cls, *, block=None):
             # Hack: Are we in IPython's %pylab mode?  In pylab mode, IPython
             # (>= 0.10) tacks a _needmain attribute onto pyplot.show (always
             # set to False).
-            ipython_pylab = hasattr(
-                getattr(sys.modules.get("pyplot"), "show", None), "_needmain")
+            pyplot_show = getattr(sys.modules.get("matplotlib.pyplot"), "show", None)
+            ipython_pylab = hasattr(pyplot_show, "_needmain")
             block = not ipython_pylab and not is_interactive()
         if block:
             cls.start_main_loop()
@@ -3645,8 +3645,8 @@ def show(cls, *, block=None):
             # Hack: Are we in IPython's %pylab mode?  In pylab mode, IPython
             # (>= 0.10) tacks a _needmain attribute onto pyplot.show (always
             # set to False).
-            ipython_pylab = hasattr(
-                getattr(sys.modules.get("pyplot"), "show", None), "_needmain")
+            pyplot_show = getattr(sys.modules.get("matplotlib.pyplot"), "show", None)
+            ipython_pylab = hasattr(pyplot_show, "_needmain")
             block = not ipython_pylab and not is_interactive()
         if block:
             cls.mainloop()