[Python-bugs-list] [ python-Bugs-721171 ] Tkinter precision loss for doubles

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Bugs item #721171, was opened at 2003-04-14 17:32
Message generated for change (Comment added) made by loewis
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Category: Tkinter
Group: None
Status: Closed
Resolution: Fixed
Priority: 5
Submitted By: Tim Peters (tim_one)
Assigned to: Martin v. Löwis (loewis)
Summary: Tkinter precision loss for doubles

Initial Comment:
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.

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>Comment By: Martin v. Löwis (loewis)
Date: 2003-04-16 21:59

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

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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-04-15 23:04

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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.

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Comment By: Martin v. Löwis (loewis)
Date: 2003-04-15 22:54

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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?

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Comment By: Guido van Rossum (gvanrossum)
Date: 2003-04-14 17:45

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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?

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