lazy dictionary class

koji/util.py

@@ -113,6 +113,73 @@ def dslice(dict, keys, strict=True):
     return ret
 
 
+class LazyValue(object):
+    """Used to represent a value that is generated by a function call at access time
+    """
+
+    def __init__(self, func, args, kwargs=None, cache=False):
+        if kwargs is None:
+            kwargs = {}
+        self.func = func
+        self.args = args
+        self.kwargs = kwargs
+        self.cache = cache
+
+    def get(self):
+        if hasattr(self, '_value'):
+            return self._value
+        value = self.func(*self.args, **self.kwargs)
+        if self.cache:
+            self._value = value
+        return value
+
+
+def lazy_eval(value):
+    if isinstance(value, LazyValue):
+        return value.get()
+    return value
+
+
+class LazyDict(dict):
+    """A container for lazy data
+
+    fields can refer to function calls, which can optionally be cached
+    """
+
+    def __getitem__(self, key):
+        return lazy_eval(super(LazyDict, self).__getitem__(key))
+
+    def lazyset(self, key, func, args, kwargs=None, cache=False):
+        self[key] = LazyValue(func, args, kwargs=kwargs, cache=cache)
+
+    def get(self, *args, **kwargs):
+        return lazy_eval(super(LazyDict, self).get(*args, **kwargs))
+
+    def copy(self):
+        return LazyDict(self)
+
+    def values(self):
+        return [lazy_eval(val) for val in super(LazyDict, self).values()]
+
+    def items(self):
+        return [(key, lazy_eval(val)) for key, val in super(LazyDict, self).items()]
+
+    def itervalues(self):
+        for val in super(LazyDict, self).itervalues():
+            yield lazy_eval(val)
+
+    def iteritems(self):
+        for key, val in super(LazyDict, self).iteritems():
+            yield key, lazy_eval(val)
+
+    def pop(self, key, *args, **kwargs):
+        return lazy_eval(super(LazyDict, self).pop(key, *args, **kwargs))
+
+    def popitem(self):
+        key, val = super(LazyDict, self).popitem()
+        return key, lazy_eval(val)
+
+
 def rmtree(path):
     """Delete a directory tree without crossing fs boundaries"""
     st = os.lstat(path)