Is it possible to set a number to NaN or infinity?

Question

Is it possible to set an element of an array to NaN in Python?

Additionally, is it possible to set a variable to +/- infinity? If so, is there any function to check whether a number is infinity or not?

Lambard

stackoverflow.com/questions/944700 tells you how to check for NaN. For Inf and -Inf you can test with == but that doesn't work for NaN because of the IEEE754 rules for NaN. – Prejudicial

Answer 1

Cast from string using float():

>>> float('NaN')
nan
>>> float('Inf')
inf
>>> -float('Inf')
-inf
>>> float('Inf') == float('Inf')
True
>>> float('Inf') == 1
False

Hsinking

That will teach me not to jump in with a quip before reading the question over a second time!! Sorry! That said, it wouldn't hurt to say so all the same because it's an easy trap to fall into, NaN != NaN – Prejudicial

also note: >>> float('Inf')-float('Inf') ===> nan – Deadlight

note:float('Inf')*0 and float('Inf')/float('Inf') ==> nan. and also float('Inf')*-1 ==> -inf – Inflatable

Answer 2

Yes, you can use numpy for that.

import numpy as np
a = arange(3,dtype=float)

a[0] = np.nan
a[1] = np.inf
a[2] = -np.inf

a # is now [nan,inf,-inf]

np.isnan(a[0]) # True
np.isinf(a[1]) # True
np.isinf(a[2]) # True

Trimming

On python >= 2.6, you can just use math.isnan() and math.isinf() – Holifield

numpy is quite a heavy import if all you want is NaN or inf – Wool

If all you need is NaN or Inf, one could from numpy import nan, inf which has existed since this question was raised. – Tuberous

Answer 3

Is it possible to set a number to NaN or infinity?

Yes, in fact there are several ways. A few work without any imports, while others require import, however for this answer I'll limit the libraries in the overview to standard-library and NumPy (which isn't standard-library but a very common third-party library).

The following table summarizes the ways how one can create a not-a-number or a positive or negative infinity float:

╒══════════╤══════════════╤════════════════════╤════════════════════╕
│   result │ NaN          │ Infinity           │ -Infinity          │
│ module   │              │                    │                    │
╞══════════╪══════════════╪════════════════════╪════════════════════╡
│ built-in │ float("nan") │ float("inf")       │ -float("inf")      │
│          │              │ float("infinity")  │ -float("infinity") │
│          │              │ float("+inf")      │ float("-inf")      │
│          │              │ float("+infinity") │ float("-infinity") │
├──────────┼──────────────┼────────────────────┼────────────────────┤
│ math     │ math.nan     │ math.inf           │ -math.inf          │
├──────────┼──────────────┼────────────────────┼────────────────────┤
│ cmath    │ cmath.nan    │ cmath.inf          │ -cmath.inf         │
├──────────┼──────────────┼────────────────────┼────────────────────┤
│ numpy    │ numpy.nan    │ numpy.PINF         │ numpy.NINF         │
│          │ numpy.NaN    │ numpy.inf          │ -numpy.inf         │
│          │ numpy.NAN    │ numpy.infty        │ -numpy.infty       │
│          │              │ numpy.Inf          │ -numpy.Inf         │
│          │              │ numpy.Infinity     │ -numpy.Infinity    │
╘══════════╧══════════════╧════════════════════╧════════════════════╛

A couple remarks to the table:

• The float constructor is actually case-insensitive, so you can also use float("NaN") or float("InFiNiTy").
• The cmath and numpy constants return plain Python float objects.
• The numpy.NINF is actually the only constant I know of that doesn't require the -.

• It is possible to create complex NaN and Infinity with complex and cmath:

  ╒══════════╤════════════════╤═════════════════╤═════════════════════╤══════════════════════╕
  │   result │ NaN+0j         │ 0+NaNj          │ Inf+0j              │ 0+Infj               │
  │ module   │                │                 │                     │                      │
  ╞══════════╪════════════════╪═════════════════╪═════════════════════╪══════════════════════╡
  │ built-in │ complex("nan") │ complex("nanj") │ complex("inf")      │ complex("infj")      │
  │          │                │                 │ complex("infinity") │ complex("infinityj") │
  ├──────────┼────────────────┼─────────────────┼─────────────────────┼──────────────────────┤
  │ cmath    │ cmath.nan ¹    │ cmath.nanj      │ cmath.inf ¹         │ cmath.infj           │
  ╘══════════╧════════════════╧═════════════════╧═════════════════════╧══════════════════════╛
  

  The options with ¹ return a plain float, not a complex.

is there any function to check whether a number is infinity or not?

Yes there is - in fact there are several functions for NaN, Infinity, and neither Nan nor Inf. However these predefined functions are not built-in, they always require an import:

╒══════════╤═════════════╤════════════════╤════════════════════╕
│      for │ NaN         │ Infinity or    │ not NaN and        │
│          │             │ -Infinity      │ not Infinity and   │
│ module   │             │                │ not -Infinity      │
╞══════════╪═════════════╪════════════════╪════════════════════╡
│ math     │ math.isnan  │ math.isinf     │ math.isfinite      │
├──────────┼─────────────┼────────────────┼────────────────────┤
│ cmath    │ cmath.isnan │ cmath.isinf    │ cmath.isfinite     │
├──────────┼─────────────┼────────────────┼────────────────────┤
│ numpy    │ numpy.isnan │ numpy.isinf    │ numpy.isfinite     │
╘══════════╧═════════════╧════════════════╧════════════════════╛

Again a couple of remarks:

• The cmath and numpy functions also work for complex objects, they will check if either real or imaginary part is NaN or Infinity.
• The numpy functions also work for numpy arrays and everything that can be converted to one (like lists, tuple, etc.)
• There are also functions that explicitly check for positive and negative infinity in NumPy: numpy.isposinf and numpy.isneginf.
• Pandas offers two additional functions to check for NaN: pandas.isna and pandas.isnull (but not only NaN, it matches also None and NaT)

• Even though there are no built-in functions, it would be easy to create them yourself (I neglected type checking and documentation here):

  def isnan(value):
      return value != value  # NaN is not equal to anything, not even itself
  
  infinity = float("infinity")
  
  def isinf(value):
      return abs(value) == infinity 
  
  def isfinite(value):
      return not (isnan(value) or isinf(value))
  

To summarize the expected results for these functions (assuming the input is a float):

╒════════════════╤═══════╤════════════╤═════════════╤══════════════════╕
│          input │ NaN   │ Infinity   │ -Infinity   │ something else   │
│ function       │       │            │             │                  │
╞════════════════╪═══════╪════════════╪═════════════╪══════════════════╡
│ isnan          │ True  │ False      │ False       │ False            │
├────────────────┼───────┼────────────┼─────────────┼──────────────────┤
│ isinf          │ False │ True       │ True        │ False            │
├────────────────┼───────┼────────────┼─────────────┼──────────────────┤
│ isfinite       │ False │ False      │ False       │ True             │
╘════════════════╧═══════╧════════════╧═════════════╧══════════════════╛

Is it possible to set an element of an array to NaN in Python?

In a list it's no problem, you can always include NaN (or Infinity) there:

>>> [math.nan, math.inf, -math.inf, 1]  # python list
[nan, inf, -inf, 1]

However if you want to include it in an array (for example array.array or numpy.array) then the type of the array must be float or complex because otherwise it will try to downcast it to the arrays type!

>>> import numpy as np
>>> float_numpy_array = np.array([0., 0., 0.], dtype=float)
>>> float_numpy_array[0] = float("nan")
>>> float_numpy_array
array([nan,  0.,  0.])

>>> import array
>>> float_array = array.array('d', [0, 0, 0])
>>> float_array[0] = float("nan")
>>> float_array
array('d', [nan, 0.0, 0.0])

>>> integer_numpy_array = np.array([0, 0, 0], dtype=int)
>>> integer_numpy_array[0] = float("nan")
ValueError: cannot convert float NaN to integer

Chorizo

Note: math.isnan does not work with complex numbers. Use math.isnan(x.real) or math.isnan(x.imag) instead. – Hohenstaufen

Answer 4

When using Python 2.4, try

inf = float("9e999")
nan = inf - inf

I am facing the issue when I was porting the simplejson to an embedded device which running the Python 2.4, float("9e999") fixed it. Don't use inf = 9e999, you need convert it from string. -inf gives the -Infinity.

Mining

Answer 5

Or you can calculate them

Python 3.9 on Windows 10
>>> import sys
>>> Inf = sys.float_info.max * 10
>>> Inf
inf
>>> NaN = Inf - Inf
>>> NaN
nan

Mechanician