How to get NaN when I divide by zero

Question

When I do floating point division in Python, if I divide by zero, I get an exception:

>>> 1.0/0.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: float division

I'd really like to get NaN or Inf instead (because the NaN or Inf will propagate through the rest of my calculation correctly and not kill my program).

How can I do this?

@beerbajay: `0.0 / 0.0` —> `nan`, `1.0 / 0.0` —> `inf`, `-1.0 / 0.0` —> `-inf`. — Sven Marnach, Apr 04 '12 at 13:14
Actually very interesting would be to change the Python behavior so it really works for `1.0 / 0` without explicitly retyping everything or putting try-except everywhere. — Fenikso, Apr 04 '12 at 13:30

score 80 · Accepted Answer · answered Apr 04 '12 at 13:09

80

The easiest way to get this behaviour is to use numpy.float64 instead of Python default float type:

>>> import numpy
>>> numpy.float64(1.0) / 0.0
inf

Of course this requires NumPy. You can use numpy.seterr() to fine-tune the error handling.

answered Apr 04 '12 at 13:09

Sven Marnach

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This worked great. You can even pass `numpy.float64` values to SWIG-wrapped C libraries without any problems. – Ken Bloom Apr 04 '12 at 20:23
20

What's the point of even having `float('inf')` and `float('nan')` built in to Python if you have to use a 3rd-party library to get the expected behaviour? i.e., it seems like typing `float('inf')` explicitly is the only time I can actually count on seeing an 'inf' result returned by Python. – Brandin Jan 18 '16 at 14:18
The first time you run this, it will throw an error. – a06e Nov 24 '17 at 13:40
3

@becko: It prints a warning, which is quite different from throwing an error. You can control warnings using the built-in `warnings` module. – Sven Marnach Nov 24 '17 at 17:58
@Brandin E.g. `1e200 * 1e200` results in `inf`. But I agree, I would also like to either have IEEE 754 behavior all-round or no `inf` and `nan` at all. – Feuermurmel Apr 26 '20 at 09:47

score 28 · Answer 2 · edited Oct 19 '16 at 09:52

28

Method 1:

try:
    value = a/b
except ZeroDivisionError:
    value = float('Inf')

Method 2:

if b != 0:
    value = a / b
else:
    value = float('Inf')

But be aware that the value could as well be -Inf, so you should make a more distinctive test. Nevertheless, this above should give you the idea how to do it.

edited Oct 19 '16 at 09:52

Augustin

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answered Apr 04 '12 at 13:12

glglgl

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Oh! Somehow I thought this was a new question. Anyway, it's there as of version 3.5 – Neil G Feb 17 '16 at 07:23
@glglgl I've checked this and I get `inf`, `-inf` and ` ` anyone know why that might be the ZeroDivisionError is always raised? – ctrl-alt-delete Feb 25 '16 at 10:56
1

Note: 0/0 == NaN; the result of a/b is +Inf or -Inf if a != 0 and b == 0 – damphat Oct 17 '17 at 13:56

score 10 · Answer 3 · answered Apr 04 '12 at 13:13

You could try using the 'decimal' module:

>>> from decimal import *
>>> setcontext(ExtendedContext)
>>> inf = Decimal(1) / Decimal(0)
>>> print(inf)
Infinity
>>> neginf = Decimal(-1) / Decimal(0)
>>> print(neginf)
-Infinity
>>> print(neginf + inf)
NaN
>>> print(neginf * inf)
-Infinity
>>> print(dig / 0)
Infinity

score -6 · Answer 4 · edited Apr 04 '12 at 13:21

-6

If i understand your problem properly then this should be the solution:

try:
   1.0/0.0
except:    
   return 'inf'

you can modified it according to various python exception handling method available

edited Apr 04 '12 at 13:21

Shawn Chin

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answered Apr 04 '12 at 13:19

Bhagirath Bhardwaj

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Better `float('inf')` instead of `'inf'` - you'll get a float then, not a string... – glglgl Apr 05 '12 at 10:59
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Better `except ZeroDivisionError`, `except` alone also catches `KeyboardInterrupt` etc. – Marian Nov 07 '12 at 13:34

score -6 · Answer 5 · edited Jul 15 '15 at 12:13

-6

I used a wrapper function in a python program of mine for a simple division that was returning ZeroDivisionError when the sensors I was using weren't plugged in. It simply returns 0 (zero), which in real-world terms is what I wanted. Probably gets messy with more variables, however...

def calculation(a, b):
    if a == 0:
        return 0
    elif b == 0:
        return 0
    else:
        return a/b

edited Jul 15 '15 at 12:13

Glorfindel

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answered Jul 15 '15 at 12:03

Colin MacRae

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Sorry, I forgot to revise this for what was being asked, but all you have to do is change the zero's to "NaN" I believe. – Colin MacRae Jul 15 '15 at 15:17
This is not a general solution. E.g. I would not expect a / 0.0 to give 0, nor would I expect it to give NaN. – Brandin Jan 18 '16 at 14:55

How to get NaN when I divide by zero

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