Tkinter precision loss for doubles #38303

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1. tim-one commented on Apr 14, 2003

   

   tim-one

   on Apr 14, 2003

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   DoubleVar.set() (I think) goes thru PyObject_Str(),
   causing precision loss for doubles:

   >>> import Tkinter
   >>> import math
   >>> x = Tkinter.DoubleVar(Tkinter.Tk())
   >>> x.set(math.pi)
   >>> x.get()
   3.1415926535900001   # not good
   >>> math.pi
   3.1415926535897931   # original precision
   >>> eval(str(math.pi))
   3.1415926535900001   # reproduces Tk result
   >>>

   Reported on c.l.py. Guido says you may know how to
   create a "native" Tcl/Tk double in this case instead.
2. tim-one

   closed this as completedon Apr 14, 2003
3. tim-one

   assigned

   loewis

   on Apr 14, 2003
4. tim-one

   added

   topic-tkinter

   on Apr 14, 2003
5. tim-one

   closed this as completedon Apr 14, 2003
6. tim-one

   assigned

   loewis

   on Apr 14, 2003
7. tim-one

   added

   topic-tkinter

   on Apr 14, 2003
8. gvanrossum commented on Apr 14, 2003

   

   gvanrossum

   on Apr 14, 2003

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   If a proper fix using Tcl float/double objects doesn't work
   for some reason, maybe the attached fix (_tkinter.fix) would
   work?
9. loewis commented on Apr 15, 2003

   

   loewismannequin

   on Apr 15, 2003

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   I have now fixed this for _tkinter.c 1.154, by passing all
   .set arguments with the same object type to Tcl, and
   retrieving all Tcl variables with their native type.

   This causes slight semantic changes (like the one being
   reported as a bug here); most notably:

   >>> y=Tkinter.StringVar(Tkinter.Tk())
   >>> y.set(3.14)
   >>> y.get()
   3.1400000000000001

   So a StringVar may not return a string anymore if .set did
   not enter a string. Does that need to get fixed?
10. gvanrossum commented on Apr 15, 2003

    

    gvanrossum

    on Apr 15, 2003

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    Thanks!

    Re StringVar: I think StringVar should coerce the result to
    str(), just like IntVar coerces to int(), etc. Maybe
    Variable should grow get() method that does no coercion, and
    it should no longer be marked as an internal class.

    BTW there are a few places in Tkinter docstrings that
    promise 0 or 1 where a bool is now returned.
11. loewis commented on Apr 16, 2003

    

    loewismannequin

    on Apr 16, 2003

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    In Tkinter.py 1.172, StringVars now always returns strings
    (potentially Unicode); doc strings have been changed to
    mention bools.
12. ezio-melotti

    transferred this issue fromon Apr 9, 2022