[Idle-dev] Bug Report on float number calculation in IDLE

[Terry Reedy]

On 12/19/2015 5:20 AM, Grey wrote:
 > I have found a strange issue on float number calculation in IDEL
 > of Python 2.7X。

IDLE does not run you code. As far as running code goes, IDLE is an 
input-output front-end run by Python. It displays what Python outputs.
If you had run the same code in Python directly, you would have seen the 
same result.

 >>> num = .1 + .1 + .1 + .1 +.1 +.1 + .1 + .1 + .1
 >>> num
0.8999999999999999
 >>> print(num)
0.9

Interactive echo (>>> num) prints repr(num), which is as accurate as 
possible.  Print prints str(num), which in 2.7 is rounded to a 'nice' 
form.  Because this was confusing, it was changed in 3.0.

 >>> repr(num)
'0.8999999999999999'
 >>> str(num)
'0.9'

 > 1，When I tried to calculate the sum of a bunch of float numbers
 > in IDLE of Python 2.7.6 I found that the result was not
 > accurate(well, I have knew that it is up to the storage format of
 > data by searching the information on Internet).

Issues of round-off error with floats are well documented.  I believe 
there is an entry in the Python FAQ for this.

Summary: no bug, and nothing to do with IDLE

-- 
Terry Jan Reedy