1 files changed, 123 insertions, 0 deletions
diff --git a/simplegeneric.py b/simplegeneric.py
new file mode 100644
index 0000000..bd12c14
--- /dev/null
+++ b/simplegeneric.py
@@ -0,0 +1,123 @@
+__all__ = ["generic"]
+try:
+    from types import ClassType, InstanceType
+    classtypes = type, ClassType
+except ImportError:
+    classtypes = type
+    InstanceType = None
+
+def generic(func):
+    """Create a simple generic function"""
+
+    _sentinel = object()
+
+    def _by_class(*args, **kw):
+        cls = args[0].__class__
+        for t in type(cls.__name__, (cls,object), {}).__mro__:
+            f = _gbt(t, _sentinel)
+            if f is not _sentinel:
+                return f(*args, **kw)
+        else:
+            return func(*args, **kw)
+
+    _by_type = {object: func, InstanceType: _by_class}
+    _gbt = _by_type.get
+
+    def when_type(*types):
+        """Decorator to add a method that will be called for the given types"""
+        for t in types:
+            if not isinstance(t, classtypes):
+                raise TypeError(
+                    "%r is not a type or class" % (t,)
+                )
+        def decorate(f):
+            for t in types:
+                if _by_type.setdefault(t,f) is not f:
+                    raise TypeError(
+                        "%r already has method for type %r" % (func, t)
+                    )
+            return f
+        return decorate
+
+    _by_object = {}
+    _gbo = _by_object.get
+
+    def when_object(*obs):
+        """Decorator to add a method to be called for the given object(s)"""
+        def decorate(f):
+            for o in obs:
+                if _by_object.setdefault(id(o), (o,f))[1] is not f:
+                    raise TypeError(
+                        "%r already has method for object %r" % (func, o)
+                    )
+            return f
+        return decorate
+
+
+    def dispatch(*args, **kw):
+        f = _gbo(id(args[0]), _sentinel)
+        if f is _sentinel:
+            for t in type(args[0]).__mro__:
+                f = _gbt(t, _sentinel)
+                if f is not _sentinel:
+                    return f(*args, **kw)
+            else:
+                return func(*args, **kw)
+        else:
+            return f[1](*args, **kw)
+
+    dispatch.__name__       = func.__name__
+    dispatch.__dict__       = func.__dict__.copy()
+    dispatch.__doc__        = func.__doc__
+    dispatch.__module__     = func.__module__
+
+    dispatch.when_type = when_type
+    dispatch.when_object = when_object
+    dispatch.default = func
+    dispatch.has_object = lambda o: id(o) in _by_object
+    dispatch.has_type   = lambda t: t in _by_type
+    return dispatch
+
+
+
+def test_suite():
+    import doctest
+    return doctest.DocFileSuite(
+        'README.txt',
+        optionflags=doctest.ELLIPSIS|doctest.REPORT_ONLY_FIRST_FAILURE,
+    )
+
+if __name__=='__main__':
+    import unittest
+    r = unittest.TextTestRunner()
+    r.run(test_suite())
+
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