Trac #25546: Install Jupyter kernel in the correct prefix

The Sage Jupyter kernel is unconditionally installed in `sys.prefix`.
However, this should instead be the `install_data` directory. This
defaults to the installation prefix, which in turn defaults to
`sys.prefix`. So the default won't change, but it allows customization.

The reason for choosing `install_data` is to be compatible with other
Jupyter packages, which use the `data_files` option to `setup()` for
installing Jupyter files (see for example https://github.com/jupyter-
widgets/ipywidgets/blob/master/widgetsnbextension/setup.py#L203).
Those files are installed in the `install_data` directory.

URL: https://trac.sagemath.org/25546
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Vincent Klein

src/sage/repl/ipython_kernel/install.py

@@ -13,26 +13,34 @@
     SAGE_DOC, SAGE_LOCAL, SAGE_EXTCODE,
     SAGE_VERSION
 )
-from jupyter_core.paths import ENV_JUPYTER_PATH
-JUPYTER_PATH = ENV_JUPYTER_PATH[0]
 
 
 class SageKernelSpec(object):
 
-    def __init__(self):
+    def __init__(self, prefix=None):
         """
         Utility to manage SageMath kernels and extensions
 
+        INPUT:
+
+        - ``prefix`` -- (optional, default: ``sys.prefix``)
+          directory for the installation prefix
+
         EXAMPLES::
 
             sage: from sage.repl.ipython_kernel.install import SageKernelSpec
             sage: spec = SageKernelSpec()
             sage: spec._display_name    # random output
             'SageMath 6.9'
+            sage: spec.kernel_dir == SageKernelSpec(sys.prefix).kernel_dir
+            True
         """
         self._display_name = 'SageMath {0}'.format(SAGE_VERSION)
-        self.nbextensions_dir = os.path.join(JUPYTER_PATH, "nbextensions")
-        self.kernel_dir = os.path.join(JUPYTER_PATH, "kernels", self.identifier())
+        if prefix is None:
+            from sys import prefix
+        jupyter_dir = os.path.join(prefix, "share", "jupyter")
+        self.nbextensions_dir = os.path.join(jupyter_dir, "nbextensions")
+        self.kernel_dir = os.path.join(jupyter_dir, "kernels", self.identifier())
         self._mkdirs()
 
     def _mkdirs(self):
@@ -236,7 +244,7 @@ def _symlink_resources(self):
         )
 
     @classmethod
-    def update(cls):
+    def update(cls, *args, **kwds):
         """
         Configure the Jupyter notebook for the SageMath kernel
 
@@ -250,7 +258,7 @@ def update(cls):
             sage: spec = SageKernelSpec()
             sage: spec.update()  # not tested
         """
-        instance = cls()
+        instance = cls(*args, **kwds)
         instance.use_local_mathjax()
         instance.use_local_jsmol()
         instance.use_local_threejs()

src/setup.py

@@ -895,7 +895,10 @@ def install_kernel_spec(self):
             use ``data_files`` for this.
         """
         from sage.repl.ipython_kernel.install import SageKernelSpec
-        SageKernelSpec.update()
+        # Jupyter packages typically use the data_files option to
+        # setup() to install kernels and nbextensions. So we should use
+        # the install_data directory for installing our Jupyter files.
+        SageKernelSpec.update(prefix=self.install_data)
 
     def clean_stale_files(self):
         """