IPython/external/qt_for_kernel.py

""" Import Qt in a manner suitable for an IPython kernel.

This is the import used for the `gui=qt` or `matplotlib=qt` initialization.

Import Priority:

if Qt4 has been imported anywhere else:
   use that

if matplotlib has been imported and doesn't support v2 (<= 1.0.1):
    use PyQt4 @v1

Next, ask ETS' QT_API env variable

if QT_API not set:
    ask matplotlib via rcParams['backend.qt4']
    if it said PyQt:
        use PyQt4 @v1
    elif it said PySide:
        use PySide

    else: (matplotlib said nothing)
        # this is the default path - nobody told us anything
        try:
            PyQt @v1
        except:
            fallback on PySide
else:
    use PyQt @v2 or PySide, depending on QT_API
    because ETS doesn't work with PyQt @v1.

"""

import os
import sys

from IPython.utils.version import check_version
from IPython.external.qt_loaders import (load_qt, QT_API_PYSIDE,
                                         QT_API_PYQT, QT_API_PYQT_DEFAULT,
                                         loaded_api)

#Constraints placed on an imported matplotlib
def matplotlib_options(mpl):
    if mpl is None:
        return
    mpqt = mpl.rcParams.get('backend.qt4', None)
    if mpqt is None:
        return None
    if mpqt.lower() == 'pyside':
        return [QT_API_PYSIDE]
    elif mpqt.lower() == 'pyqt4':
        return [QT_API_PYQT_DEFAULT]
    raise ImportError("unhandled value for backend.qt4 from matplotlib: %r" %
                      mpqt)

def get_options():
    """Return a list of acceptable QT APIs, in decreasing order of
    preference
    """
    #already imported Qt somewhere. Use that
    loaded = loaded_api()
    if loaded is not None:
        return [loaded]

    mpl = sys.modules.get('matplotlib', None)

    if mpl is not None and not check_version(mpl.__version__, '1.0.2'):
        #1.0.1 only supports PyQt4 v1
        return [QT_API_PYQT_DEFAULT]

    if os.environ.get('QT_API', None) is None:
        #no ETS variable. Ask mpl, then use either
        return matplotlib_options(mpl) or [QT_API_PYQT_DEFAULT, QT_API_PYSIDE]

    #ETS variable present. Will fallback to external.qt
    return None

api_opts = get_options()
if api_opts is not None:
    QtCore, QtGui, QtSvg, QT_API = load_qt(api_opts)

else: # use ETS variable
    from IPython.external.qt import QtCore, QtGui, QtSvg, QT_API