Python class dynamic lazy properties
I have a Record class that is used to store the results of reading a text file. The file contains a simple database with fields and tags. I would like each record instance to only have properties associated with its database. Basically:
R1 = Record("file1")
R2 = Record("file2")
print(R1.TI) #"Record 1 title"
print(R2.TI) #AttributeError: 'Record' object has no attribute 'TI'
Unfortunately, some of the fields may require a lot of processing to return something useful, and these values may never be needed. So I would like the value to be determined on first call not on object initialization.
Since I only know tags, I only tried:
class tagWrapper(object):
def __init__(self, tag):
self.tag = tag
self.data = None
def __get__(self, instance, owner):
if self.data == None:
try:
#tagToFunc is a dictionary that maps tags to their processing function
self.data = tagToFunc[self.tag](instance._rawDataDict[self.tag])
except KeyError: #I do not know the full list of tags
self.data = instance._rawDataDict[self.tag]
return self.data
class Record(object):
def __init__(self, file):
#Reading file and making _rawDataDict
setattr(self, tag, tagWrapper(tag))
This causes it to R1.TI
create a wrapper object, not the value I want. So I suspect that I am citing something using the get method.
Note. I am trying to make attributes part of individual class instances and not evaluated until needed. I can implement one or the other, but have not been able to figure out how to do it.
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I have a solution that seems to work, albeit rather ugly:
class Record(object):
def __init__(self, file):
self._unComputedTags = set() #needs to be initialized first
#stuff
self._unComputedTags = set(self._fieldDict.keys())
for tag in self._fieldDict:
self.__dict__[tag] = None
def __getattribute__(self, name):
if name == '_unComputedTags':
#This may be unnecessary if I play with things a bit
return object.__getattribute__(self, '_unComputedTags')
if name in self._unComputedTags:
try:
tagVal = tagToFunc[name](self._fieldDict[name])
except KeyError:
tagVal = self._fieldDict[name]
setattr(self, name, tagVal)
self._unComputedTags.remove(name)
return object.__getattribute__(self, name)
I don't like rewriting __getattribute__
, but it works.
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You are implementing the descriptor protocol and the descriptor belongs to the class instead of an instance of the class, so you cannot assign it to an instance attribute.
class Tag(object):
def __init__(self, tag):
self.tag = tag
self.data = None
def __get__(self, instance, owner):
if not instance: # if accessed with the class directly, ie. Record.T1, just return this descriptor
return self
if self.data is None:
print "Reading data"
self.data = range(10)
return self.data
class Record(object):
T1 = Tag('T1')
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