[issue27697] Obscure bug in the int() function

[Berker Peksag]

Changes by Berker Peksag :


--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue27697] Obscure bug in the int() function

[Ammar Askar]

Ammar Askar added the comment:

You've ran into a classic floating point number limitation. 
Please read the following doc page: 
https://docs.python.org/2/tutorial/floatingpoint.html

The problem comes when you multiply the number by 100, which is what causes the 
precision loss and drops it by a penny.

>>> 1108431.38 * 100
110843137.
>>> int(110843137.)
110843137

If this is for currency, I would suggest you use the decimal module 
https://docs.python.org/2/library/decimal.html

--
nosy: +ammar2

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[issue27697] Obscure bug in the int() function

[nathan snobelen]

New submission from nathan snobelen:

Hi, I've noticed a bug in int() when converting a specific range of numbers it 
will incorrectly round the last digit down.

We have some payment code which formats numbers for processing in our system 
and we noticed that the payment of 1108431.38 was dropped by a penny to 
1108431.37.  I looked into it and found that it is dropped when it is 
multiplied by 100 (to remove the decimal) and then converted back to an int.

Note, this bug only applies to the following range of numbers: 1108431.38 - 
1108431.41.  Any other number I tried was unaffected. 

The following code will replicate the bug:

import math

amount1 = 110843138.0
amount2 = 1108431.38 * 100

print "Orig amount1 " + str(amount1)
print "Orig amount2 " + str(amount2)

print "Converted amount1 " + str(int(amount1))
print "Converted amount2 " + str(int(amount2))

Try it, and you will see that "amount1" remains correct, but "amount2" is 
affected.  Multiplying by 100 seems to trigger it... however note that even 
after it has been multiplied by 100 it is still correct... it's only when you 
then apply the int() function that the penny drops.

So it would appear that something is wrong in the int() function.

Cheers,
Nathan

--
components: Library (Lib)
messages: 272062
nosy: nathan snobelen
priority: normal
severity: normal
status: open
title: Obscure bug in the int() function
type: behavior
versions: Python 2.7

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