Discrepancy with division calculation in Python class

Question

I have a Python 3 class method for rescaling values that looks like this:

class A(object): """docstring for A""" def __init__(self): super(A, self).__init__() def rescale(self, old_min, old_max, new_min, new_max, value): """rescales a value given a current old_min and old_max to the desired new_min and new_max""" scale = (old_max - old_min) / (new_max - new_min) rescaled_value = new_min + ((value - old_min) / (scale)) return rescaled_value

Using Python 3, this method works like this:

>>> import A >>> x = A() >>> x.rescale(1,10,1,100,5) 45.0

In Python 2.7, this code does not work:

>>> from __future__ import division >>> x = A() >>> x.rescale(1,10,1,100,5) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "bunsen.py", line 35, in rescale rescaled_value = new_min + ((value - old_min) / (scale)) ZeroDivisionError: integer division or modulo by zero

If I manually do this math in Python 2.7 I get the correct answer:

>>> from __future__ import division >>> scale = (10 - 1) / (100 - 1) >>> rescaled_value = 1 + ((5 - 1) / (scale)) >>> rescaled_value 45.0

Can anyone point out why this method does not work in Python 2.7?

I doubt you executed the last code manually in 2.7. Integer division in 2.7 will yield an integer, not float, and 9/99 will result in 0. Have a look at python.org/dev/peps/pep-0238 — Felix Kling
– Felix Kling, Commented Feb 3, 2012 at 12:41
@FelixKling See my edit. I originally forgot from __future__ import division — drbunsen
– drbunsen, Commented Feb 3, 2012 at 12:45
@dr.bunsen: Please note that, yes, you will get the error with the code you show. But the answers point out that you need to have the from __future__ import division in the file that defines A.rescale(). I do not believe you have that. — Lennart Regebro
– Lennart Regebro, Commented Feb 3, 2012 at 17:43

wRAR · Accepted Answer · 2012-02-03 12:45:14Z

8

You need to set from __future__ import division in the file that contains the divisions, i.e. in the file with A.

answered Feb 3, 2012 at 12:45

wRAR

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5 Comments

drbunsen Over a year ago

I have from __future__ import division in the file and I do not believe that this is the problem.

wRAR Over a year ago

Well, I've said exactly that.

jsbueno Over a year ago

@wRAR - this is most certainly the correct answer. Failing to have his import in the file containing the class defintion for A would yield the problem you are reporting

wRAR Over a year ago

For some reason the OP deleted his comment stating that my suggestion fiexed his problem.

Lennart Regebro Over a year ago

I've tested the code as given, and it works under 2.7 as well as Python 3. Removing the __future__ makes it fail as described in the question. I believe you are correct. This is the problem.

BenMorel · Accepted Answer · 2014-06-17 20:15:03Z

For python 2.7 you can either:

 x.rescale(1.0, 10.0, 1.0, 100.0, 5.0)

Or you could explicitly cast to float in division inside method.

 scale = float((old_max - old_min)) / (new_max - new_min)

Or another way is to import from __future__ import division.

This is because in python 2.x integer divided by integer will result in an integer, in your case 0.

EDIT after your comment:

Make sure you do the

 from __future__ import division

IN module A, as there is where the computations are done not like you did there.

Ethan Furman · Accepted Answer · 2012-02-03 14:25:38Z

Using this code in 2.7:

from __future__ import division class A(object): """docstring for A""" def __init__(self): super(A, self).__init__() def rescale(self, old_min, old_max, new_min, new_max, value): """rescales a value given a current old_min and old_max to the desired new_min and new_max""" scale = (old_max - old_min) / (new_max - new_min) rescaled_value = new_min + ((value - old_min) / (scale)) return rescaled_value x = A() print x.rescale(1,10,1,100,5)

gives me:

45.0

If I remove the __future__ import I get this:

Traceback (most recent call last): File "test.py", line 15, in <module> print x.rescale(1,10,1,100,5) File "test.py", line 11, in rescale rescaled_value = new_min + ((value - old_min) / (scale)) ZeroDivisionError: integer division or modulo by zero

You must have the from __future__ import division line in the module that defines A. Imports only affect the module they are contained in.

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