"""
NamedValueAccess provides functions, a mix-in class and a wrapper class
all for accessing Python objects by named attributes. You can use which
ever of the three approaches best suites your needs and style.
NOTES
If Python provided a root class 'Object' in the same tradition as other
OOP languages such as Smalltalk, Objective-C and Java, then we could
dispense with the global functions and simply stick with the mix-in.
TO DO
* The mix-in's valueForKey() could be out of slight alignment with the
function, since they have different implementations. However, the test
cases pass for both right now.
* Should the valueForKey() function provide for caching of bindings in
the same manner than the mix-in does?
If not, should the mix-in allow an option to *not* cache bindings?
* hasValueForKey() function? (We already have a method in the mix-in)
* valuesForNames() in the mix-in:
* Change parameter 'keys' to 'names'
* Use NoDefault instead of None in the parameters
* Revisit doc string and test cases
* Docs: More improvs to doc strings.
* Testing: increase coverage
* Rename? class NamedValueAccess+ible:
* Benchmarking: Set this up in a new file:
Testing/BenchNamedValueAccess.py
so we can experment with caching vs. not and other techniques.
PAST DESIGN DECISIONS
* Only if a name binds to a method is it invoked. Another approach is
to invoke any value that is __call__able, but that is unPythonic: If
obj.foo is a class or a function then obj.foo gives that class or
function, not the result of invoking it. Method is the only
convenience we provide, because that's one of the major points of
providing this.
CREDIT
Chuck Esterbrook <echuck@mindspring.com>
Tavis Rudd <tavis@calrudd.com>
"""
import types
import string, sys
from time import time
from MiscUtils import NoDefault
technique = 1
class NamedValueAccessError(LookupError): pass
class ValueForKeyError(NamedValueAccessError): pass
class NamedValueAccess:
"""
This class is intended to be ancestor class such that you can say:
from NamedValueAccess import *
age = someObj.valueForName("age")
name = someObj.valueForName("info.fields.name")
This can be useful in setups where you wish to textually refer to the objects
in a program, such as an HTML template processed in the context of an
object-oriented framework.
Keys can be matched to either methods or ivars and with or without underscores.
valueForName() can also traverse bona fide dictionaries (DictType).
You can safely import * from this module. Only the NamedValueAccess class is exported
(other than typical things like string and sys).
There is no __init__() method and never will be.
You can run the test suite by running this module as a program.
You'll see the terms 'key' and 'name' in the class and its documentation. A 'key'
is a single identifier such as 'foo'. A name could be key, or a qualified key,
such as 'foo.bar.boo'. Names are generally more convenient and powerful, while
key-oriented methods are more efficient and provide the atomic functionality that
name-oriented methods are built upon. From a usage point of view, you normally
just use the 'name' methods and forget about the 'key'.
@@ 2000-05-21 ce: This class causes problems when used in WebKit for logging.
Perhaps circular references?
Involving self?
Having to do with methods bound to their objects?
@@ 2000-03-03 ce: document ivars
@@ 2000-04-24 ce: Some classes like UserDict need to use getitem()
instead of getattr() and don't need to deal with _bindingForGetKey().
@@ 2000-05-31 ce: Rename this class to NamedValues, NamedValueAccess, ValuesByName
@@ This class probably needs to be in MiscUtils, as it's being used in that way
while MiddleKit was intended for "enterprise/business objects".
"""
def hasValueForKey(self, key):
""" Returns true if the key is available, although that does not
guarantee that there will not be errors caused by retrieving the key. """
return self._bindingForGetKey(key)!=None
def valueForKey(self, key, default=NoDefault):
""" Suppose key is 'foo'. This method returns the value with the following precedence:
1. Methods before non-methods
2. Public attributes before private attributes
More specifically, this method then returns one of the following:
* self.foo()
* self._foo()
* self.foo
* self._foo
...or default, if it was specified,
otherwise invokes and returns result of valueForUnknownKey().
Note that valueForUnknownKey(), normally returns an exception.
See valueForName() which is a more advanced version of this method that allows
multiple, qualified keys.
"""
binding = self._bindingForGetKey(key)
if not binding:
if default is NoDefault:
return self.valueForUnknownKey(key, default)
else:
return default
if type(binding) is types.MethodType:
if technique:
result = binding(self)
else:
result = binding()
return result
else:
return getattr(self, binding)
def hasValueForName(self, keysString):
try:
value = self.valueForName(keysString)
except NamedValueAccessError:
return 0
return 1
def valueForName(self, keysString, default=None):
""" Returns the value for the given keysString. This is the more advanced version of
valueForKey(), which can only handle single names. This method can handle
'foo', 'foo1.foo2', 'a.b.c.d', etc. It will traverse dictionaries if needed. """
keys = string.split(keysString, '.')
return self.valueForKeySequence(keys, default)
def valueForKeySequence(self, listOfKeys, default=None):
return _valueForKeySequence(self, listOfKeys, default)
def valuesForNames(self, keys, default=None, defaults=None, forgive=0, includeNames=0):
""" Returns a list of values that match the given keys, each of which is passed
through valueForName() and so could be of the form 'a.b.c'.
keys is a sequence. default is any kind of object. defaults is a sequence.
forgive and includeNames is a flag.
If default is not None, then it is substituted when a key is not found.
Otherwise, if defaults is not None, then it's corresponding/parallel value
for the current key is substituted when a key is not found.
Otherwise, if forgive=1, then unknown keys simply don't produce any values.
Otherwise, if default and defaults are None, and forgive=0, then the unknown
keys will probably raise an exception through self.valueForUnknownKey() although
that method can always return a final, default value.
if keys is None, then None is returned. If keys is an empty list, then None
is returned.
Often these last four arguments are specified by key.
Examples:
names = ['origin.x', 'origin.y', 'size.width', 'size.height']
obj.valuesForNames(names)
obj.valuesForNames(names, default=0.0)
obj.valuesForNames(names, defaults=[0.0, 0.0, 100.0, 100.0])
obj.valuesForNames(names, forgive=0)
@@ 2000-03-04 ce: includeNames is only supported when forgive=1.
It should be supported for the other cases.
It should be documented.
It should be included in the test cases.
"""
if keys is None:
return None
if len(keys) is 0:
return []
results = []
if default is not None:
results = map(lambda key, myself=self, mydefault=default: myself.valueForName(key, mydefault), keys)
elif defaults is not None:
if len(keys) is not len(defaults):
raise NamedValueAccessError, 'Keys and defaults have mismatching lengths (%d and %d).' % (len(keys), len(defaults))
results = map(lambda key, default, myself=self: myself.valueForName(key, default), keys, defaults)
elif forgive:
results = []
uniqueObject = 'uni' + 'que'
for key in keys:
value = self.valueForName(key, uniqueObject)
if value is not uniqueObject:
if includeNames:
results.append((key, value))
else:
results.append(value)
else:
results = map(lambda key, myself=self: myself.valueForName(key), keys)
return results
def setValueForKey(self, key, value):
""" Suppose key is 'foo'. This method sets the value with the following precedence:
1. Public attributes before private attributes
2. Methods before non-methods
More specifically, this method then uses one of the following:
@@ 2000-03-04 ce: fill in
...or invokes handleUnknownSetKey().
"""
raise NotImplementedError
def resetKeyBindings(self):
if hasattr(self, '_kvGetBindings'):
self._kvGetBindings = {}
def valueForUnknownKey(self, key, default):
raise NamedValueAccessError, key
handleUnknownSetKey(self, key):
def _bindingForGetKey(self, key):
""" Bindings are cached.
Bindings are methods or strings.
"""
if not hasattr(self, '_kvGetBindings'):
self._kvGetBindings = {}
if self._kvGetBindings.has_key(key):
return self._kvGetBindings[key]
found = None
if hasattr(self, key):
found = getattr(self, key)
'0: found = ', found, type(found)
if type(found) is not types.MethodType:
found = key
elif technique:
found = getattr(self.__class__, key)
self._kvGetBindings[key] = found
'1: found = ', found, type(found)
if type(found) is not types.MethodType:
underKey = '_' + key
if hasattr(self, underKey):
underAttr = getattr(self, underKey)
if found==None:
if type(underAttr) is types.MethodType:
if technique:
value = getattr(self.__class__, underKey)
else:
value = underAttr
else:
value = underKey
found = self._kvGetBindings[key] = value
else:
if type(underAttr) is types.MethodType:
if technique:
underAttr = getattr(self.__class__, underKey)
found = self._kvGetBindings[key] = underAttr
'2: found = ', found, type(found)
return found
class NamedValueAccessWrapper(NamedValueAccess):
"""
This provides a wrapper around an existing object which will respond
to the methods of NamedValueAccess. By using the wrapper, you can
stick with objects and methods such as obj.valueForName('x.y') (as
opposed to functions like valueForName()) and refrain from modifying
the existing class hierarchy with NamedValueAccess.
Example:
wrapper = NamedValueAccessWrapper(obj)
print wrapper.valueForName('manager.name')
"""
def __init__(self, object):
self._object = object
def hasValueForKey(self, key):
try:
value = self.valueForKey(ley)
except NamedValueAccessError:
return 0
else:
return 1
def valueForKey(self, key, default=NoDefault):
return valueForKey(self._object)
def valueForName(self, key, default=NoDefault):
return valueForName(self._object)
def _valueForKeySequence(obj, listOfKeys, default=None):
""" This is a recursive function used to implement NamedValueAccess.valueForKeySequence.
Besides supporting inheritors of NamedValueAccess, this function also supports
dictionaries, which is why it's not found in the class.
"""
if type(obj) is types.DictType:
try:
value = obj[listOfKeys[0]]
except:
if default is None:
raise NamedValueAccessError, 'Unknown key (%s) in dictionary.' % listOfKeys[0]
else:
return default
else:
value = obj.valueForKey(listOfKeys[0], default)
if len(listOfKeys)>1:
return _valueForKeySequence(value, listOfKeys[1:], default)
else:
return value
def _dict_valueForKey(obj, key, default=NoDefault):
"""
Returns the value for a given key of the dictionary-like object.
This is a private, custom function built in support of valueForKey().
"""
try:
value = obj[key]
except AttributeError, e:
substring = "instance has no attribute '__getitem__'"
if e.args[0][-len(substring):]==substring:
if default is NoDefault:
return None
else:
return
else:
raise
except KeyError, e:
if e.args[0]==key:
if default is NoDefault:
raise ValueForKeyError, key
else:
return default
else:
raise
else:
return value
def valueForKey(obj, key, default=NoDefault):
"""
Returns the value of the object named by the given key.
Suppose key is 'foo'. This method returns the value with the
following precedence:
1. Methods before non-methods
2. Attributes before keys (__getitem__)
3. Public things before private things
(private being denoted by a preceding underscore)
More specifically, this method returns one of the following:
* obj.valueForKey(key) # only if the method exists
* obj.foo()
* obj._foo()
* obj.foo
* obj._foo
* obj['foo']
* obj.valueForUnknownKey(key)
* default # only if specified
If all of these fail, a ValueForKeyError is raised.
NOTES
* If the object provides a valueForKey() method, that method will be
invoked to do the work.
* valueForKey() works on dictionaries and dictionary-like objects.
* valueForUnknownKey() provides a hook by which objects can
delegate or chain their keyed value access to other objects.
The key and default arguments are passed to it and it should
generally respect the typical treatment of the the default
argument as found throughout Webware and described in the Style
Guidelines.
* See valueForName() which is a more advanced version of this
function that allows multiple, qualified keys.
"""
assert obj is not None
assert type(key) is types.StringType
valueForKeyMeth = getattr(obj, 'valueForKey', None)
if valueForKeyMeth:
return valueForKeyMeth(key, default)
attr = None
method = None
value = None
unknown = 0
if type(obj) is types.DictType:
if default is NoDefault:
try:
return obj[key]
except KeyError:
raise ValueForKeyError, key
else:
return obj.get(key, default)
else:
try:
klass = obj.__class__
except AttributeError:
klass = None
method = None
else:
method = getattr(klass, key, None)
if not method:
underKey = '_' + key
method = klass and getattr(klass, underKey, None) or None
if not method:
attr = getattr(obj, key, NoDefault)
if attr is NoDefault:
attr = getattr(obj, underKey, NoDefault)
if attr is NoDefault:
if klass is not None:
getitem = getattr(klass, '__getitem__', None)
if getitem:
try:
value = getitem(obj, key)
except KeyError:
unknown = 1
if not unknown:
if method:
return method(obj)
if attr is not NoDefault:
return attr
valueForUnknownKey = getattr(obj, 'valueForUnknownKey', None)
if valueForUnknownKey:
return valueForUnknownKey(key, default)
if default!=NoDefault:
return default
else:
raise ValueForKeyError, key
def valueForName(obj, name, default=NoDefault):
"""
Returns the value of the object that is named. The name can use
dotted notation to traverse through a network/graph of objects.
Since this function relies on valueForKey() for each individual
component of the name, you should be familiar with the semantics
of that notation.
Example: valueForName(obj, 'department.manager.salary')
"""
names = string.split(name, '.')
for name in names:
obj = valueForKey(obj, name, default)
if obj is default:
return obj
return obj
def _enhanceUserDict():
from UserDict import UserDict
if not NamedValueAccess in UserDict.__bases__:
UserDict.__bases__ = UserDict.__bases__ + (NamedValueAccess,)
def _UserDict_hasValueForKey(self, key):
return self.has_key(key)
def _UserDict_valueForKey(self, key, default=NoDefault):
if default is NoDefault:
if self.has_key(key):
return self[key]
else:
raise ValueForKeyError, key
else:
return self.get(key, default)
setattr(UserDict, 'hasValueForKey', _UserDict_hasValueForKey)
setattr(UserDict, 'valueForKey', _UserDict_valueForKey)
_enhanceUserDict()