Re #5939. Add a mechanism to avoid blocking MantidPlot while running.

We replace the IPython method that actually executes the entered code
with one that then calls the original one on a separate thread so that
we can continue to process GUI events while the command runs.

Code/Mantid/MantidPlot/ipython_widget/mantid_ipython_widget.py

@@ -1,7 +1,40 @@
+import threading
+import types
+
+from PyQt4 import QtGui
+
 from IPython.qt.console.rich_ipython_widget import RichIPythonWidget
 from IPython.qt.inprocess import QtInProcessKernelManager
 
+
+def our_run_code(self, code_obj):
+    """ Method which we replace the run_code method of IPython's InteractiveShell class.
+        It call's the original method (renamed to ipython_run_code) on a separate thread
+        so that we can avoid locking up the whole of MantidPlot while a command runs
+
+        Parameters
+        ----------
+        code_obj : code object
+          A compiled code object, to be executed
+
+        Returns
+        -------
+        False : Always, as it doesn't seem to matter.
+    """
+    t = threading.Thread(target=self.ipython_run_code, args=[code_obj])
+    t.start()
+    while t.is_alive():
+        QtGui.QApplication.processEvents()
+    # We don't capture the return value of the ipython_run_code method but as far as I can tell
+    #   it doesn't make any difference what's returned
+    return 0
+
+
 class MantidIPythonWidget(RichIPythonWidget):
+    """ Extends IPython's qt widget to include setting up and in-process kernel as well as the
+        Mantid environment, plus our trick to avoid blocking the event loop while processing commands.
+        This widget is set in the QDockWidget that houses the script interpreter within ApplicationWindow.
+    """
 
     def __init__(self, *args, **kw):
         super(MantidIPythonWidget, self).__init__(*args, **kw)
@@ -16,7 +49,15 @@ def __init__(self, *args, **kw):
         from os import path
         mantidplotpath = path.split(path.dirname(__file__))[0] # It's the directory above this one
         mantidplotrc = path.join(mantidplotpath, 'mantidplotrc.py')
-        kernel.shell.run_line_magic('run',mantidplotrc)
+        shell = kernel.shell
+        shell.run_line_magic('run',mantidplotrc)
+
+        # These 3 lines replace the run_code method of IPython's InteractiveShell class (of which the
+        # shell variable is a derived instance) with our method defined above. The original method
+        # is renamed so that we can call it from within the our_run_code method.
+        f = shell.run_code
+        shell.run_code = types.MethodType(our_run_code, shell)
+        shell.ipython_run_code = f
 
         kernel_client = kernel_manager.client()
         kernel_client.start_channels()