PySide Qt script doesn't launch from Spyder but works from shell

问题

I have a weird bug in my project that uses PySide for its Qt GUI, and in response I'm trying to test with simpler code that sets up the environment.

Here is the code I am testing with: https://stackoverflow.com/a/6906552/130164

When I launch that from my shell (python test.py), it works perfectly. However, when I run that script in Spyder, I get the following error:

Traceback (most recent call last):
  File "/home/test/Desktop/test/test.py", line 31, in <module>
    app = QtGui.QApplication(sys.argv)
RuntimeError: A QApplication instance already exists.

If it helps, I also get the following warning:

/usr/lib/pymodules/python2.6/matplotlib/__init__.py:835: UserWarning:  This call to matplotlib.use() has no effect
because the the backend has already been chosen;
matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

Why does that code work when launched from my shell but not from Spyder?

Update: Mata answered that the problem happens because Spyder uses Qt, which makes sense. For now, I've set up execution in Spyder using the "Execute in an external system terminal" option, which doesn't cause errors but doesn't allow debugging, either. Does Spyder have any built-in workarounds to this?

回答1:

As Spyder also is a Qt application, it starts it's own QApplication. In the same process only one QApplication can exist, that's why you get the first error.

Sypder also uses matplotlib, and probably, therfore it already will have imported some of the mentioned modules, so that's why you get the second error.

So when usin it like that, you can't create your own QApplication or call matplotlib.use(). Or maybe it will work if you wrap these calls in try/except.

回答2:

I have the same problem, and somewhere on stackoverflow was a solution.

Instead of

qApp = QtGui.QApplication(sys.argv)

Use

qApp = QtGui.QApplication.instance()
if qApp is None:
        qApp = QtGui.QApplication(sys.argv)

回答3:

It won't work in Spyder if you try to launch the application into an interactive console because that console is specially configured to import several scientific libraries, automatically show() matplotlib figures, and a few other details. Type scientific at the Spyder console prompt for more details. The result is effectively that a Qt application event loop is already running there.

To get your application to run inside of Spyder:

Make sure Spyder is configured to open external consoles with PySide and not PyQt. This can be set from Tools>Preferences>Console>External modules>Qt-Python bindings library selection.
With your script active in the editor, hit F6 to open the Run Settings dialog. Select the "Execute in a new dedicated Python interpreter" radio button instead of executing in the current interactive interpreter. Click OK. Now run the script by hitting F5. Debug the script by hitting Ctrl+F5.

来源：https://stackoverflow.com/questions/10709262/pyside-qt-script-doesnt-launch-from-spyder-but-works-from-shell

标签

python

pyqt

pyside

spyder