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?

Luís de Sousa · Accepted Answer · 2024-04-08 13:28:08Z

67

You simply have to explicitly declare the data type:

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

edited Apr 8, 2024 at 13:28

Luís de Sousa

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answered Oct 18, 2013 at 18:10

Jaime

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8 Comments

Akavall Over a year ago

I had a feeling that I was missing something simple...Thank You

Andrzej Pronobis Over a year ago

The problem with this approach is that it only works for float for which nan is defined and not for dtype int.

Babak Over a year ago

Justy a heads-up that np.float is deprecated. use dtype=float instead

matanox Over a year ago

Interestingly this turns them to plain python float nan, not np.nan. I wonder why as it may seem a little irregular, even though both types of nan qualify the same under np.isnan() . This is so even when using dtype=np.float64 which is explicitly a numpy data type. This conversion is not even explicitly documented for np.array()

Gideon Kogan Over a year ago

DeprecationWarning: np.float is a deprecated alias for the builtin float

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Udo Klein · Accepted Answer · 2013-10-18 18:07:55Z

7

What about

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

answered Oct 18, 2013 at 18:07

Udo Klein

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2 Comments

alexey Over a year ago

I'd say x is None as == might be more quirky

Esteban Over a year ago

What worked for me was my_array = np.array(list(map(lambda x: np.nan if x is None else x, my_list)))

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