FAQ
Hello everyone,

I try to set two properties, "value" and "square" in the following code,
and arrange it in such way that setting one property also sets another
one and vice versa. But the code seems to get Python into infinite loop:
import math
class Squared2(object):
def __init__(self, val):
self._internalval=val
self.square=pow(self._internalval,2)

def fgetvalue(self):
return self._internalval

def fsetvalue(self, val):
self._internalval=val
self.square=pow(self._internalval,2)

value = property(fgetvalue, fsetvalue)

def fgetsquare(self):
return self.square
def fsetsquare(self,s):
self.square = s
self.value = math.sqrt(self.square)

square = property(fgetsquare, fsetsquare)

a=Squared2(5)
Traceback (most recent call last):
File "<pyshell#11>", line 1, in <module>
a=Squared2(5)
File "<pyshell#10>", line 5, in __init__
self.square=pow(self._internalval,2)
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare

...

Is there a way to achieve this goal of two mutually setting properties?

## Search Discussions

• at Sep 3, 2008 at 2:31 pm ⇧ On Wed, 03 Sep 2008 15:57:50 +0200, mk wrote:

I try to set two properties, "value" and "square" in the following code,
and arrange it in such way that setting one property also sets another
one and vice versa. But the code seems to get Python into infinite loop:
Is there a way to achieve this goal of two mutually setting properties?
My attempt:
---
import math

class Square(object):
def __init__(self, val):
self._square = pow(val, 2)
self._value = math.sqrt(self.square)

def getsquare(self):
return self._square

def setsquare(self, square):
self._square = square
self._value = math.sqrt(self._square)

square = property(getsquare, setsquare)

def getvalue(self):
return self._value

def setvalue(self, value):
self._value = value
self._square = math.pow(value, 2)

value = property(getvalue, setvalue)

a = Square(5)
print a.square
print a.value
a.value = 10
print a.square
print a.value
a.square = 64
print a.square
print a.value
---

and the result:

\$ python sqval.py
25
5.0
100.0
10
64
8.0
\$

--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/
• at Sep 3, 2008 at 2:34 pm ⇧ On Wed, 3 Sep 2008 14:31:17 +0000 (UTC), Wojtek Walczak wrote:

class Square(object):
def __init__(self, val):
self._square = pow(val, 2)
self._value = math.sqrt(self.square)
^^^^^^^^^^^^^^^^^^^^^^
or just:
self._value = val

:-)

--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/
• at Sep 3, 2008 at 2:42 pm ⇧ mk schrieb:
Hello everyone,

I try to set two properties, "value" and "square" in the following code,
and arrange it in such way that setting one property also sets another
one and vice versa. But the code seems to get Python into infinite loop:
import math
class Squared2(object):
def __init__(self, val):
self._internalval=val
self.square=pow(self._internalval,2)

def fgetvalue(self):
return self._internalval

def fsetvalue(self, val):
self._internalval=val
self.square=pow(self._internalval,2)

value = property(fgetvalue, fsetvalue)

def fgetsquare(self):
return self.square
def fsetsquare(self,s):
self.square = s
self.value = math.sqrt(self.square)

square = property(fgetsquare, fsetsquare)

a=Squared2(5)
Traceback (most recent call last):
File "<pyshell#11>", line 1, in <module>
a=Squared2(5)
File "<pyshell#10>", line 5, in __init__
self.square=pow(self._internalval,2)
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare
self.square = s
File "<pyshell#10>", line 19, in fsetsquare

...

Is there a way to achieve this goal of two mutually setting properties?
Better to make the getter for square return the square of value, and the
setter of square compute the root & set that. Like this:

class Squared2(object):

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

@apply
def squared():
def fset(self, squared):
self.value = math.sqrt(squared)

def fget(self):
return self.value ** 2

return property(**locals())

Diez
• at Sep 3, 2008 at 2:44 pm ⇧ Le Wednesday 03 September 2008 15:57:50 mk, vous avez ?crit?:
I try to set two properties, "value" and "square" in the following code,
and arrange it in such way that setting one property also sets another
one and vice versa. But the code seems to get Python into infinite loop:

?>>> import math
?>>> class Squared2(object):

????????def __init__(self, val):
????????????????self._internalval=val
????????????????self.square=pow(self._internalval,2)
????????????????
????????def fgetvalue(self):
????????????????return self._internalval
????????????????
????????def fsetvalue(self, val):
????????????????self._internalval=val
????????????????self.square=pow(self._internalval,2)
????????????????
????????value = property(fgetvalue, fsetvalue)

????????def fgetsquare(self):
????????????????return self.square
????????def fsetsquare(self,s):
????????????????self.square = s
????????????????self.value = math.sqrt(self.square)
????????????????
????????square = property(fgetsquare, fsetsquare)

????????
?>>> a=Squared2(5)

Traceback (most recent call last):
? ?File "<pyshell#11>", line 1, in <module>
? ? ?a=Squared2(5)
? ?File "<pyshell#10>", line 5, in __init__
? ? ?self.square=pow(self._internalval,2)
? ?File "<pyshell#10>", line 19, in fsetsquare
? ? ?self.square = s
? ?File "<pyshell#10>", line 19, in fsetsquare
? ? ?self.square = s
? ?File "<pyshell#10>", line 19, in fsetsquare
? ? ?self.square = s
? ?File "<pyshell#10>", line 19, in fsetsquare
? ? ?self.square = s
? ?File "<pyshell#10>", line 19, in fsetsquare
? ? ?self.square = s
? ?File "<pyshell#10>", line 19, in fsetsquare

...

Is there a way to achieve this goal of two mutually setting properties?
Your square property is not correctly defined, it recurselively call itself,
it should be (I also avoided the extra lookup) :

def fgetsquare(self):
return self._square
def fsetsquare(self,s):
self._square = s
self.value = math.sqrt(s)

then the fsetvalue will be also be called recursively as it use the square
property, you should also write it :

def fsetvalue(self, val):
self._internalval=val
self._square=pow(val,2)

*but*, if you want to add more logic in the setters, you could want to add two
extra methods :

def _setsquare(self, v) :
# some extra logic here
self._square = s

def fsetsquare(self,s):
self._setsquare(s)
self._setvalue = math.sqrt(s)

def _setvalue(self, val):
# some extra logic here
self._internalval=val

def fsetvalue(self, val):
self._setvalue(val)
self._setsquare=pow(val,2)

Note that if one property can really be computed from another, this kind of
thing could be considered as bad design (except if the computation is heavy).

--
_____________

Maric Michaud
• at Sep 3, 2008 at 2:52 pm ⇧ Le Wednesday 03 September 2008 16:44:10 Maric Michaud, vous avez ?crit?:
? ? ? ? ?def _setsquare(self, v) :
? ? ? ? ? ? ? ? ?# some extra logic here
? ? ? ? ? ? ? ? ?self._square = s

? ? ? ? ?def fsetsquare(self,s):
? ? ? ? ? ? ? ? ?self._setsquare(s)
? ? ? ? ? ? ? ? ?self._setvalue = math.sqrt(s)

? ? ? ? ?def _setvalue(self, val):
? ? ? ? ? ? ? ? ?# some extra logic here
? ? ? ? ? ? ? ? ?self._internalval=val

? ? ? ? ?def fsetvalue(self, val):
? ? ? ? ? ? ? ? ?self._setvalue(val)
? ? ? ? ? ? ? ? ?self._setsquare=pow(val,2)
Oh sorry for this last version the setters should be :

def fsetsquare(self,s):
self._setsquare(s)
self._setvalue = math.sqrt(self.square)

def fsetvalue(self, val):
self._setvalue(val)
self._setsquare=pow(self.value, 2)

as we don't know what is done in _setXXX methods.

--
_____________

Maric Michaud
• at Sep 3, 2008 at 3:40 pm ⇧ *but*, if you want to add more logic in the setters, you could want to add two
extra methods :

def _setsquare(self, v) :
# some extra logic here
self._square = s

def fsetsquare(self,s):
self._setsquare(s)
self._setvalue = math.sqrt(s)

def _setvalue(self, val):
# some extra logic here
self._internalval=val

def fsetvalue(self, val):
self._setvalue(val)
self._setsquare=pow(val,2)
Thanks for that, I'll keep that in mind.

Note that if one property can really be computed from another, this kind of
thing could be considered as bad design (except if the computation is heavy).
Hmm, why? Is the line of thinking smth like: because the variables
should be kept to minimum and they should be calculated at the moment
they are needed?
• at Sep 3, 2008 at 4:11 pm ⇧ Le Wednesday 03 September 2008 17:40:43 mk, vous avez ?crit?:
Note that if one property can really be computed from another, this kind
of thing could be considered as bad design (except if the computation is
heavy).
Hmm, why? Is the line of thinking smth like: because the variables
should be kept to minimum and they should be calculated at the moment
they are needed?
Because you have to make extra effort to keep the logical relation between
value and square. self._square is not really needed, and what is not needed
is just extra hassle.

Doesn't it clear that your code is more hard to maintain than the
alternative :

class Squared(object):

def __init__(self, val):
self._val=val

def fgetvalue(self):
return self._val
def fsetvalue(self, val):
self._val=val
value = property(fgetvalue, fsetvalue)

def fgetsquare(self):
return self.value ** 2
def fsetsquare(self,s):
self.value = math.sqrt(s)
square = property(fgetsquare, fsetsquare)

--
_____________

Maric Michaud
• at Sep 3, 2008 at 5:38 pm ⇧ Maric Michaud a ?crit :
Le Wednesday 03 September 2008 17:40:43 mk, vous avez ?crit :
Note that if one property can really be computed from another, this kind
of thing could be considered as bad design (except if the computation is
heavy).
Hmm, why? Is the line of thinking smth like: because the variables
should be kept to minimum and they should be calculated at the moment
they are needed?
Because you have to make extra effort to keep the logical relation between
value and square. self._square is not really needed, and what is not needed
is just extra hassle.

Doesn't it clear that your code is more hard to maintain than the
alternative :

class Squared(object):

def __init__(self, val):
self._val=val

def fgetvalue(self):
return self._val
def fsetvalue(self, val):
self._val=val
value = property(fgetvalue, fsetvalue)

def fgetsquare(self):
return self.value ** 2
def fsetsquare(self,s):
self.value = math.sqrt(s)
square = property(fgetsquare, fsetsquare)
FWIW, if there's no computation on getting or setting value, you can
make it a plain attribute.

But while it's quite clear that in this example use case it would be
better to have only one property (weither square or value, depending on
which is the most often use), things are not always that simple in real
world code, and - as you mentionned - there may be times where you have
interdependant properties and really want to avoid recomputing the same
thing over and over. Now there's no one size fits all solution here -
it's an optimization problem, and as such depends on real use cases.
• at Sep 3, 2008 at 11:20 pm ⇧ Le Wednesday 03 September 2008 19:38:13 Bruno Desthuilliers, vous avez ?crit?:
FWIW, if there's no computation on getting or setting value, you can
make it a plain attribute.
Yes, this is the problem with these tiny examples, we don't know at which
point we must simplify them...
But while it's quite clear that in this example use case it would be
better ?to have only one property (weither square or value, depending on
which is the most often use), things are not always that simple in real
world code, and - as you mentionned - there may be times where you have
interdependant properties and really want to avoid recomputing the same
thing over and over. Now there's no one size fits all solution here -
it's an optimization problem, and as such depends on real use cases.
Yes, my advice was exactly in that sense, it's an optimization problem,
duplicating such a data is a caching mechanism, and should be done knowingly,
in acceptance of all the complication it comes with.

--
_____________

Maric Michaud
• at Sep 3, 2008 at 2:50 pm ⇧ mk a ?crit :
Hello everyone,

I try to set two properties, "value" and "square" in the following code,
and arrange it in such way that setting one property also sets another
one and vice versa. But the code seems to get Python into infinite loop:
import math
class Squared2(object):
def __init__(self, val):
self._internalval=val
self.square=pow(self._internalval,2)
the 'internal' prefix is already implied by the '_'. And your __init__
code is a useless duplication of fsetvalue, so just use the property and
get rid of copy-pasted code.

def fgetvalue(self):
return self._internalval
the '_' prefix already means 'internal'. The convention here would be to
name the attribute '_value' (to make clear it's the implementation
support for the 'value' property). Also, your property getter and setter
should also be marked as implementation using the '_' prefix - they are
implementation detail, not part of your class API.
def fsetvalue(self, val):
self._internalval=val
self.square=pow(self._internalval,2)

value = property(fgetvalue, fsetvalue)

def fgetsquare(self):
return self.square
def fsetsquare(self,s):
self.square = s
Hem... Notice something here ?
self.value = math.sqrt(self.square)

square = property(fgetsquare, fsetsquare)
Your fsetsquare implementation is broken - it calls itself recursively.
You have to use different names for the property and the 'implementation
attribute' for the property. But even if you fix this, you'll have
another infinite recursion between the two setters.

The simplest solution : don't call one property from the other, do
direct attribute access within the setters:

import math

class Squared2(object):
def __init__(self, value):
self.value=value

def _fgetvalue(self):
return self._value
def _fsetvalue(self, value):
self._value=value
self._square=pow(value,2)
value = property(_fgetvalue, _fsetvalue)

def _fgetsquare(self):
return self._square
def _fsetsquare(self,square):
self._square = square
self._value = math.sqrt(square)
square = property(_fgetsquare, _fsetsquare)