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Recipe 577248 revision 2 

#!/usr/bin/env python
# -*- coding: utf-8 -*-

NOVALUE = object()

class Maybe(object):
    _has_value = False
    _value = None
    def __init__(self, value):
        if value is not NOVALUE:
            self._has_value = True
            self._value = value

    def __nonzero__(self):
        return self.has_value

    @property
    def has_value(self):
        return self._has_value

    @property
    def value(self):
        return self._value

# optional sugar factories
def Some(value):
    return Maybe(value)

def NoValue():
    return Maybe(NOVALUE)

if __name__ == "__main__":
    class MaybeSupportingRepository(object):
        def __init__(self, *args):
            self._d = dict(args)

        def get(self, key):
            return Maybe(self._d.get(key, NOVALUE))
 
    repo = MaybeSupportingRepository( ("a", 1), ("b", 2), ("c", 3) )

    key = "x"
    maybe_v = repo.get(key)
    if maybe_v:
        print "There's a value for %s: %s" % (key, maybe_v.value)
    else:
        print "There's no value for %s" % key

    key = "a"
    maybe_v = repo.get(key)
    if maybe_v.has_value:
        print "There's a value for %s: %s" % (key, maybe_v.value)
    else:
        print "There's no value for %s" % key

Diff to Previous Revision

--- revision 1 2010-05-29 12:16:25
+++ revision 2 2010-07-21 11:08:11
@@ -1,11 +1,13 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
+
+NOVALUE = object()
 
 class Maybe(object):
     _has_value = False
     _value = None
     def __init__(self, value):
-        if value is not None:
+        if value is not NOVALUE:
             self._has_value = True
             self._value = value
 
@@ -25,7 +27,7 @@
     return Maybe(value)
 
 def NoValue():
-    return Maybe(None)
+    return Maybe(NOVALUE)
 
 if __name__ == "__main__":
     class MaybeSupportingRepository(object):
@@ -33,7 +35,7 @@
             self._d = dict(args)
 
         def get(self, key):
-            return Maybe(self._d.get(key))
+            return Maybe(self._d.get(key, NOVALUE))
  
     repo = MaybeSupportingRepository( ("a", 1), ("b", 2), ("c", 3) )

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