reraise ImportError when importing kwant.continuum

Previously we used the ExtensionUnavailable proxy that would
raise an exception when accessed. Now because kwant.continuum
is loaded lazily we can just raise an exception. We choose to
re-raise ImportError with a better error message.

kwant/_common.py

@@ -41,30 +41,6 @@ class UserCodeError(Exception):
     pass
 
 
-class ExtensionUnavailable:
-    """Class that replaces unavailable extension modules in the Kwant namespace.
-
-    Some extensions for Kwant (e.g. 'kwant.continuum') require additional
-    dependencies that are not required for core functionality. When the
-    additional dependencies are not installed an instance of this class will
-    be inserted into Kwant's root namespace to simulate the presence of the
-    extension and inform users that they need to install additional
-    dependencies.
-
-    See https://mail.python.org/pipermail/python-ideas/2012-May/014969.html
-    for more details.
-    """
-
-    def __init__(self, name, dependencies):
-        self.name = name
-        self.dependencies = ', '.join(dependencies)
-
-    def __getattr__(self, _):
-        msg = ("'{}' is not available because one or more of the following "
-               "dependencies are not installed: {}")
-        raise RuntimeError(msg.format(self.name, self.dependencies))
-
-
 def ensure_isinstance(obj, typ, msg=None):
     if isinstance(obj, typ):
         return

kwant/continuum/__init__.py

@@ -6,18 +6,15 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-import sys
-
-from .._common import ExtensionUnavailable
-
 try:
     from .discretizer import discretize, discretize_symbolic, build_discretized
     from ._common import sympify, lambdify
     from ._common import momentum_operators, position_operators
-except ImportError:
-    sys.modules[__name__] = ExtensionUnavailable(__name__, ('sympy',))
+except ImportError as error:
+    msg = ("'kwant.continuum' is not available because one or more of its "
+           "dependencies is not installed.")
+    raise ImportError(msg) from error
 
-del sys, ExtensionUnavailable
 
 __all__ = ['discretize', 'discretize_symbolic', 'build_discretized',
            'sympify', 'lambdify']