Convert python list with None values to numpy array with nan values

Question

I am trying to convert a list that contains numeric values and None values to numpy.array, such that None is replaces with numpy.nan.

For example:

my_list = [3,5,6,None,6,None]

# My desired result: 
my_array = numpy.array([3,5,6,np.nan,6,np.nan])

Naive approach fails:

>>> my_list
[3, 5, 6, None, 6, None]
>>> np.array(my_list)
array([3, 5, 6, None, 6, None], dtype=object) # very limited 
>>> _ * 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

>>> my_array # normal array can handle these operations
array([  3.,   5.,   6.,  nan,   6.,  nan])
>>> my_array * 2
array([  6.,  10.,  12.,  nan,  12.,  nan])

What is the best way to solve this problem?

score 57 · Accepted Answer · answered Oct 18 '13 at 18:10

57

You simply have to explicitly declare the data type:

>>> my_list = [3, 5, 6, None, 6, None]
>>> np.array(my_list, dtype=np.float)
array([  3.,   5.,   6.,  nan,   6.,  nan])

answered Oct 18 '13 at 18:10

Jaime

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I had a feeling that I was missing something simple...Thank You – Akavall Oct 18 '13 at 18:12
4

The problem with this approach is that it only works for float for which nan is defined and not for dtype int. – Andrzej Pronobis May 20 '15 at 02:14
Please see my question http://stackoverflow.com/questions/30339214/initializing-numpy-masked-array-from-python-int-list-with-none-values on that problem exactly. – Andrzej Pronobis May 20 '15 at 02:35

score 7 · Answer 2 · answered Oct 18 '13 at 18:07

7

What about

my_array = np.array(map(lambda x: numpy.nan if x==None else x, my_list))

answered Oct 18 '13 at 18:07

Udo Klein

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I'd say `x is None` as == might be more [quirky](https://stackoverflow.com/questions/14247373/python-none-comparison-should-i-use-is-or) – alexey Oct 19 '17 at 04:40
What worked for me was `my_array = np.array(list(map(lambda x: np.nan if x is None else x, my_list)))` – Esteban Jan 18 '23 at 18:46

Convert python list with None values to numpy array with nan values

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