[python-dtcwt] 390/497: add switchable backend support

[Ghislain Vaillant]

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ghisvail-guest pushed a commit to branch debian/sid
in repository python-dtcwt.

commit f3278a494f4ffbc8fe2683528fc5c799d463bb9a
Author: Rich Wareham <rjw57 at cam.ac.uk>
Date:   Mon Feb 10 16:16:32 2014 +0000

    add switchable backend support
---
 dtcwt/__init__.py | 124 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 123 insertions(+), 1 deletion(-)

diff --git a/dtcwt/__init__.py b/dtcwt/__init__.py
index dff5ee4..97c70f4 100644
--- a/dtcwt/__init__.py
+++ b/dtcwt/__init__.py
@@ -1,8 +1,130 @@
-from .numpy import Transform1d, Transform2d, Transform3d, Pyramid
+import os
+import sys
+
+import dtcwt.numpy
+import dtcwt.opencl
 
 __all__ = [
     'Transform1d',
     'Transform2d',
     'Transform3d',
     'Pyramid',
+
+    'backend_name',
+    'push_backend',
+    'pop_backend',
+    'preserve_backend_stack',
 ]
+
+# An array of dictionaries. Each dictionary stores the top-level module
+# variables for that backend.
+_BACKEND_STACK = []
+
+# Possible backends keyed by name
+_AVAILABLE_BACKENDS = {
+    'numpy': {
+        'Transform1d': dtcwt.numpy.Transform1d,
+        'Transform2d': dtcwt.numpy.Transform2d,
+        'Transform3d': dtcwt.numpy.Transform3d,
+        'Pyramid': dtcwt.numpy.Pyramid,
+    },
+    'opencl': {
+        'Transform1d': dtcwt.numpy.Transform1d,
+        'Transform2d': dtcwt.opencl.Transform2d,
+        'Transform3d': dtcwt.numpy.Transform3d,
+        'Pyramid': dtcwt.opencl.Pyramid,
+    },
+}
+
+def _update_from_current_backend():
+    # Update values from backend
+    for k,v in _BACKEND_STACK[-1][1].items():
+        setattr(dtcwt, k, v)
+    dtcwt.backend_name = _BACKEND_STACK[-1][0]
+
+class _BackendGuard(object):
+    def __init__(self, stack):
+        self._stack = list(stack)
+
+    def __enter__(self):
+        return _BACKEND_STACK
+
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        dtcwt._BACKEND_STACK = self._stack
+        # only re-raise if it's *not* the exception that was
+        # passed to throw(), because __exit__() must not raise
+        # an exception unless __exit__() itself failed.  But
+        # throw() has to raise the exception to signal
+        # propagation, so this fixes the impedance mismatch 
+        # between the throw() protocol and the __exit__()
+        # protocol.
+        #
+        if sys.exc_info()[1] is not exc_value:
+            raise
+
+def preserve_backend_stack():
+    """Return a generator object which can be used to preserve the backend
+    stack even when an exception has been raise. For example:
+
+    .. code-block:: python
+
+        # current backend is NumPy
+        assert dtcwt.backend_name == 'numpy'
+
+        with dtcwt.preserve_backend_stack():
+            dtcwt.push_backend('opencl')
+            # ... things which may raise an exception
+
+        # current backend is NumPy even if an exception was thrown
+        assert dtcwt.backend_name == 'numpy'
+
+    """
+    return _BackendGuard(_BACKEND_STACK)
+
+def push_backend(name):
+    """Switch backend implementation to *name*. Push the previous backend onto
+    the backend stack. The previous backend may be restored via
+    :py:func:`dtcwt.pop_backend`.
+
+    :param name: string identifying which backend to switch to
+    :raises ValueError: if *name* does not correspond to a known backend
+
+    *name* may take one of the following values:
+
+    * ``numpy``: the default NumPy backend. See :py:mod:`dtcwt.numpy`.
+    * ``opencl``: a backend which uses OpenCL where available. See
+      :py:mod:`dtcwt.opencl`.
+
+    """
+    try:
+        _BACKEND_STACK.append((name, _AVAILABLE_BACKENDS[name]))
+    except KeyError:
+        raise ValueError('No such backend: {0}'.format(name))
+    _update_from_current_backend()
+
+def pop_backend():
+    """Restore the backend after a call to :py:func:`push_backend`. Calls to
+    :py:func:`pop_backend` and :py:func:`pop_backend` may be neted. This
+    function will undo the most recent call to :py:func:`push_backend`.
+
+    :raise IndexError: if one attempts to pop more backends than one has pushed.
+
+    """
+    # It is an error to pop off the default backend
+    if len(_BACKEND_STACK) <= 1:
+        raise IndexError('Cannot pop default backend')
+
+    _BACKEND_STACK.pop()
+    _update_from_current_backend()
+
+backend_name = None
+"""A string providing a short human-readable name for the DTCWT backend
+currently being used. This corresponds to the *name* parameter passed to
+:py:func:`dtcwt.push_backend`. The *default* backend is ``numpy`` but can be
+overridden by setting the DTCWT_BACKEND environment variable to a valid backend
+name.
+"""
+
+# Default to the numpy backend unless DTCWT_BACKEND environment variable is
+# set.
+push_backend(os.getenv('DTCWT_BACKEND', 'numpy'))

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