First draft of new kernel registry machinery

IPEP 25

IPython/kernel/kernelspec.py

@@ -0,0 +1,72 b''
+import io
+import json
+import os
+
+pjoin = os.path.join
+
+from IPython.utils.path import get_ipython_dir
+from IPython.utils.py3compat import PY3
+from IPython.utils.traitlets import HasTraits, List, Unicode
+
+USER_KERNEL_DIR = pjoin(get_ipython_dir(), 'kernels')
+
+if os.name == 'nt':
+    programdata = os.environ.get('PROGRAMDATA', None)
+    if programdata:
+        SYSTEM_KERNEL_DIR = pjoin(programdata, 'ipython', 'kernels')
+    else:  # PROGRAMDATA is not defined by default on XP.
+        SYSTEM_KERNEL_DIR = None
+else:
+    SYSTEM_KERNEL_DIR = "/usr/share/ipython/kernels"
+    
+NATIVE_KERNEL_NAME = 'python3' if PY3 else 'python2'
+
+class KernelSpec(HasTraits):
+    argv = List()
+    display_name = Unicode()
+    language = Unicode()
+    codemirror_mode = Unicode()
+    
+    resource_dir = Unicode()
+    
+    def __init__(self, resource_dir, argv, display_name, language,
+                 codemirror_mode=None):
+        super(KernelSpec, self).__init__(resource_dir=resource_dir, argv=argv,
+                display_name=display_name, language=language,
+                codemirror_mode=codemirror_mode)
+        if not self.codemirror_mode:
+            self.codemirror_mode = self.language
+    
+    @classmethod
+    def from_resource_dir(cls, resource_dir):
+        kernel_file = os.path.join(resource_dir, 'kernel.json')
+        with io.open(kernel_file, 'r', encoding='utf-8') as f:
+            kernel_dict = json.load(f)
+        return cls(resource_dir=resource_dir, **kernel_dict)
+
+def _is_kernel_dir(path):
+    return os.path.isdir(path) and os.path.isfile(pjoin(path, 'kernel.json'))
+
+def _list_kernels_in(dir):
+    if dir is None:
+        return {}
+    return {f.lower(): pjoin(dir, f) for f in os.listdir(dir)
+                        if _is_kernel_dir(pjoin(dir, f))}
+
+def list_kernel_specs():
+    """Returns a dict mapping kernels to resource directories."""
+    d = _list_kernels_in(SYSTEM_KERNEL_DIR)
+    d.update(_list_kernels_in(USER_KERNEL_DIR))
+    
+    if NATIVE_KERNEL_NAME not in d:
+        d[NATIVE_KERNEL_NAME] = ''  # TODO: native kernel resource directory
+    return d
+
+def get_kernel_spec(kernel_name):
+    """Returns a KernelSpec instance for the given kernel_name.
+    
+    Raises KeyError if the given kernel name is not found.
+    """
+    d = list_kernel_specs()
+    resource_dir = d[kernel_name.lower()]
+    return KernelSpec.from_resource_dir(resource_dir)
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