.show() and/or.exec() from Pyqt5 not working in Spyder - python-3.x

Question

I have Python 3.6.3 and Spyder 3.2.4, both installed along with Pyqt5 yesterday on a computer with a new installation of Windows 10. I am having a problem running some previously written Pyqt5 code in Spyder. The below code shows a minimum code snippet that reproduces the problem.
The Spyder IPython console hangs indefinitely on running the code, and never opens the window. Ctrl-C does not stop execution, so I have to kill Spyder. If I try to run it line by line, I observe that "w.show()" executes but does nothing, while "app.exec()" is the line that hangs. In contrast, if I instead run it from the command line console, "w.show()" makes a nonfunctional window appear, while "app.exec()" makes the window functional. All of the non-pyqt code I have run in Spyder executes just fine.
I'd prefer to develop code in Spyder because running code from the command line often takes 30-60s to start (presumably from the imports.)
I could run the code in Spyder on my old (now broken) system, so clearly it is a problem with my installation or computer. But I am at a loss as to what. I have already tried disabling my virus software, updating all of the relevant packages with pip, resetting Spyder from the Anaconda prompt, and restarting the computer. Any other ideas?
import sys
from PyQt5.QtWidgets import QApplication, QWidget
if not QApplication.instance():
app = QApplication(sys.argv)
else:
app = QApplication.instance()
print("app started")
w = QWidget()
w.resize(250, 250)
w.move(200, 200)
w.setWindowTitle('Test window')
print('app setting')
w.show()
print("shown")
app.exec_()
#sys.exit(app.exec_())
print("exit")

Answer 1

When running this in python it needs to be modified as below. I've modified your script a little to be able to let it run for you some of the main PyQt conventions. See also my inline comments to make sense of what you're trying to accomplish with your print lines. Enjoy!
# -*- coding: utf-8 -*-
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QApplication, QWidget
class MyMainUI(QtWidgets.QMainWindow):
def __init__(self, parent=None):
print ' Start Main'
super(MyMainUI, self).__init__(parent)
self.setWindowTitle('Test window')
self.resize(250, 250)
self.move(200, 200)
MySpidercodeHook()
def MySpiderCodeHook(self):
...link/run code...
if __name__ == '__main__':
app = QApplication(sys.argv) # no QApplication.instance() > returns -1 or error.
# Your setting up something that cannot yet be peeked
# into at this stage.
print("app started")
w = MyMainUI() # address your main GUI application
# not solely a widgetname..that is confusing...
x = 200
y = 200
# w.move(x, y) # normally this setting is done within the mainUI.
# See also PyQT conventions.
print 'app setting: moving window to position : %s' % ([x, y])
w.show()
print("shown")
app.exec_()
#sys.exit(app.exec_()) # if active then print "exit" is not shown..
# now is while using app.exec_()
#sys.exit(-1) # will returns you '-1' value as handle. Can be any integer.
print("exit")
sys.stdout.flush() # line used to print "printing text" when you're in an IDE
# like Komodo EDIT.