Fix attempted write to attribute of read-only object.

Add new class "cachedproperty" for creating property-like attributes
that cache the return value of a method call.

Also fix few issues in the unit tests to enable them to succeed.

ticket 1959

3 files changed, 38 insertions, 4 deletions

diff --git a/ipalib/dn.py b/ipalib/dn.py
index dc3119d9..6f2f7deb 100644
--- a/ipalib/dn.py
+++ b/ipalib/dn.py
@@ -1092,7 +1092,7 @@ class DN(object):
                 return rdns
         elif isinstance(value, (tuple, list)):
             if len(value) != 2:
-                raise ValueError("tuple or list must be 2-valued, not \"%s\"" % (rdn))
+                raise ValueError("tuple or list must be 2-valued, not \"%s\"" % (value))
             rdn = RDN(value, first_key_match=self.first_key_match)
             return rdn
         else:
diff --git a/ipalib/errors.py b/ipalib/errors.py
index 5b634880..f115f0c4 100644
--- a/ipalib/errors.py
+++ b/ipalib/errors.py
@@ -448,10 +448,10 @@ class RefererError(PublicError):
 
     For example:
 
-    >>> raise RefererError()
+    >>> raise RefererError(referer='referer')
     Traceback (most recent call last):
       ...
-    RefererError: Missing or invalid HTTP Referer
+    RefererError: Missing or invalid HTTP Referer, referer
     """
 
     errno = 911
@@ -1537,7 +1537,7 @@ class DependentEntry(ExecutionError):
     >>> raise DependentEntry(label=u'SELinux User Map', key=u'test', dependent=u'test1')
     Traceback (most recent call last):
       ...
-    DependentEntry: Not registered yet
+    DependentEntry: test cannot be deleted because SELinux User Map test1 requires it
 
     """
 
diff --git a/ipalib/util.py b/ipalib/util.py
index ffa27594..d575329e 100644
--- a/ipalib/util.py
+++ b/ipalib/util.py
@@ -27,6 +27,7 @@ import time
 import socket
 import re
 from types import NoneType
+from weakref import WeakKeyDictionary
 
 from ipalib import errors
 from ipalib.text import _
@@ -272,3 +273,36 @@ def validate_hostname(hostname):
     if not all(regex_name.match(part) for part in hostname.split(".")):
         raise ValueError(_('hostname parts may only include letters, numbers, and - ' \
                            '(which is not allowed as the last character)'))
+
+class cachedproperty(object):
+    """
+    A property-like attribute that caches the return value of a method call.
+
+    When the attribute is first read, the method is called and its return
+    value is saved and returned. On subsequent reads, the saved value is
+    returned.
+
+    Typical usage:
+    class C(object):
+        @cachedproperty
+        def attr(self):
+            return 'value'
+    """
+    __slots__ = ('getter', 'store')
+
+    def __init__(self, getter):
+        self.getter = getter
+        self.store = WeakKeyDictionary()
+
+    def __get__(self, obj, cls):
+        if obj is None:
+            return None
+        if obj not in self.store:
+            self.store[obj] = self.getter(obj)
+        return self.store[obj]
+
+    def __set__(self, obj, value):
+        raise AttributeError("can't set attribute")
+
+    def __delete__(self, obj):
+        raise AttributeError("can't delete attribute")