import sys
import traceback

class PropertyLoadDemmandNew(object):

    def __init__(self,attr,haveprop=False):
        self.attr = attr
        self._haveprop = haveprop

    @property
    def prop(self):
        try:
            return self._prop
        except AttributeError:
            requestloadondemmand = AttributeError("'{}' object has no attribute 'prop'".format(self.__class__.__name__))
            if self._haveprop:
                setattr(requestloadondemmand,'load_for',self)
            raise requestloadondemmand

    def __getattr__(self,name):
        if name == "prop":
            _,request,_ = sys.exc_info()
            if getattr(request,'load_for',None) == self:
                self._prop = "prop data loaded"
                return self._prop
        # in addition this could be reduced to raise keyword only
        raise AttributeError("'{}' object has no attribute '{}'".format(self.__class__.__name__,name))

class PropertyLoadDemmandCurrent(object):

    def __init__(self,attr):
        self.attr = attr

    @property
    def prop(self):
        try:
            return self._prop
        except AttributeError:
            requestloadondemmand = AttributeError("'{}' object has no attribute 'prop'".format(self.__class__.__name__))
            setattr(requestloadondemmand,'load_for',self)
            raise requestloadondemmand

    def __getattr__(self,name):
        if name == "prop":
           self._prop = "prop data loaded"
           return self._prop
        raise AttributeError("'{}' object has no attribute '{}'".format(self.__class__.__name__,name))


class PropertyLoadDemmandUnintended(object):

    def __init__(self,attr):
        self.attr = attr

    @property
    def prop(self):
        try:
            return self._prop
        except AttributeError:
            requestloadondemmand = AttributeError("'{}' object has no attribute 'prop'".format(self.__class__.__name__))
            setattr(requestloadondemmand,'load_for',self)
            raise requestloadondemmand

    @property
    def prop2(self):
        return self.attr + self._compute_some_value()

    def __getattr__(self,name):
        if name == "prop":
           self._prop = "prop data loaded"
           return self._prop
        raise AttributeError("'{}' object has no attribute '{}'".format(self.__class__.__name__,name))

    def _compute_some_value(self):
        return "computed complex value" + self.attr2
        


print("version 1 as working now")
print("========================")
test = PropertyLoadDemmandCurrent("hello")
try:
    print(test.attr,test.prop)
except:
    traceback.print_exc()

# this would not report prop missing if prop.getter could use AttributeError exception to convey that it
# wants prop to be loaded
print("\nversion 2 AttributeError raised by prop.getter not cleared before __getattr__ call (haveprop=True)")
print("==================================================================================================")
test = PropertyLoadDemmandNew("hello",haveprop=True)
try:
    print(test.attr,test.prop)
except:
    traceback.print_exc()

# this would insist that prop is not available as selected by haveprop parameter
print("\nversion 2 AttributeError raised by prop.getter not cleared before __getattr__ call (haveprop=False)")
print("===================================================================================================")
test = PropertyLoadDemmandNew("hello",haveprop = False)
try:
    print(test.attr,test.prop)
except:
    traceback.print_exc()

# the follwoint would properly report missing attr2 instead of prop2
print("\nexample of code called by getter triggering unintended attribute error (simplified)")
print("===================================================================================")
test = PropertyLoadDemmandUnintended("hello")
try:
    print(test.attr,test.prop,test.prop2)
except:
    traceback.print_exc()