Modifying the value of a float-like object

David Smith dns4 at
Tue Apr 14 17:31:54 CEST 2009

Eric.Le.Bigot at wrote:
> It looks like what is needed here are a kind of "mutable float".  Is
> there a simple way of creating such a type?  I don't mind changing the
> value through x.value = 1.23 instead of x = 1.23... :)
> On Apr 14, 3:03 pm, Eric.Le.Bi... at wrote:
>> Hello,
>> Is there a way to easily build an object that behaves exactly like a
>> float, but whose value can be changed?  The goal is to maintain a list
>> [x, y,…] of these float-like objects, and to modify their value on the
>> fly (with something like x.value = 3.14) so that any expression like "x
>> +y" uses the new value.
>> I thought of two solutions, both of which I can't make to work:
>> 1) Use a class that inherits from float.  This takes care of the
>> "behave like float" part.  But is it possible to change the value of
>> the float associated with an instance?  That is, is it possible to
>> do:  "x = MyFloat(1.23); x.change_value(3.14)" so that x's float value
>> becomes 3.14?
>> 2) The other possibility I thought of was: use a class that defines a
>> 'value' member (x.value).  This takes care of the "value can be
>> changed" part.  But is it possible/easy to make it fully behave like a
>> float (including when passed to functions like math.sin)?
>> Alternatively, I'd be happy with a way of handling numerical
>> uncertainties in Python calculations (such as in "calculate the value
>> and uncertainty of a*sin(b) knowing that a=3.0 +/- 0.1 and b=1.00 +/-
>> 0.01").
>> Any idea would be much appreciated!

I think you'll have to describe your use case a little better.  I don't
see why you'd need a mutable float.  As long as the reference x is
visible to the other parts of your code, when that code uses x, it'll
always get the right instance of a float object.


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