add check_version utility

using LooseVersion and explicitly catching TypeError, rather than the NumericalVersion subclass.

IPython/external/qt.py

@@ -7,7 +7,7 b' Do not use this if you need PyQt with the old QString/QVariant API.'
 """
 
 import os
-from IPython.utils.version import NumericalVersion as V
+from IPython.utils.version import check_version
 # Available APIs.
 QT_API_PYQT = 'pyqt'
 QT_API_PYSIDE = 'pyside'
@@ -26,7 +26,7 b' if QT_API is None:'
     pyside_found = False
     try:
         import PySide
-        if V(PySide.__version__) < V('1.0.3'):
+        if not check_version(PySide.__version__, '1.0.3'):
             # old PySide, fallback on PyQt
             raise ImportError
         # we can't import an incomplete pyside and pyqt4
@@ -49,7 +49,7 b' if QT_API is None:'
                       "present.\nThis will likely crash, please install " \
                       "PySide completely, remove PySide or PyQt4 or set " \
                       "the QT_API environment variable to pyqt or pyside"
-            if V(QtCore.PYQT_VERSION_STR) < V('4.7'):
+            if not check_version(QtCore.PYQT_VERSION_STR, '4.7'):
                 # PyQt 4.6 has issues with null strings returning as None
                 raise ImportError
             QT_API = QT_API_PYQT
@@ -63,7 +63,7 b' elif QT_API == QT_API_PYQT:'
 # Now peform the imports.
 if QT_API == QT_API_PYQT:
     from PyQt4 import QtCore, QtGui, QtSvg
-    if V(QtCore.PYQT_VERSION_STR) < V('4.7'):
+    if not check_version(QtCore.PYQT_VERSION_STR, '4.7'):
         raise ImportError("IPython requires PyQt4 >= 4.7, found %s"%QtCore.PYQT_VERSION_STR)
 
     # Alias PyQt-specific functions for PySide compatibility.
@@ -72,7 +72,7 b' if QT_API == QT_API_PYQT:'
 
 elif QT_API == QT_API_PYSIDE:
     import PySide
-    if V(PySide.__version__) < V('1.0.3'):
+    if not check_version(PySide.__version__, '1.0.3'):
         raise ImportError("IPython requires PySide >= 1.0.3, found %s"%PySide.__version__)
     from PySide import QtCore, QtGui, QtSvg
 

IPython/external/qt_for_kernel.py

@@ -32,10 +32,10 b' import os'
 import sys
 
 from IPython.utils.warn import warn
-from IPython.utils.version import NumericalVersion as V
+from IPython.utils.version import check_version
 
 matplotlib = sys.modules.get('matplotlib')
-if matplotlib and V(matplotlib.__version__) <= V('1.0.1'):
+if matplotlib and not check_version(matplotlib.__version__, '1.0.2'):
     # 1.0.1 doesn't support pyside or v2, so stick with PyQt @v1,
     # and ignore everything else
     from PyQt4 import QtCore, QtGui

IPython/utils/version.py

@@ -53,4 +53,17 b' def version_tuple(vs):'
     regular integer element.
     """
     return tuple(NumericalVersion(vs).version)
+
+#
+def check_version(v, check):
+    """check version string v >= check
+
+    If dev/prerelease tags result in TypeError for string-number comparison,
+    it is assumed that the dependency is satisfied.
+    Users on dev branches are responsible for keeping their own packages up to date.
+    """
+    try:
+        return LooseVersion(v) >= LooseVersion(check)
+    except TypeError:
+        return True
     
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