[python] How can I check whether a numpy array is empty or not?

Why would we want to check if an array is empty? Arrays don't grow or shrink in the same that lists do. Starting with a 'empty' array, and growing with np.append is a frequent novice error.

Using a list in if alist: hinges on its boolean value:

In [102]: bool([])                                                                       
Out[102]: False
In [103]: bool([1])                                                                      
Out[103]: True

But trying to do the same with an array produces (in version 1.18):

In [104]: bool(np.array([]))                                                             
/usr/local/bin/ipython3:1: DeprecationWarning: The truth value 
   of an empty array is ambiguous. Returning False, but in 
   future this will result in an error. Use `array.size > 0` to 
   check that an array is not empty.
  #!/usr/bin/python3
Out[104]: False

In [105]: bool(np.array([1]))                                                            
Out[105]: True

and bool(np.array([1,2]) produces the infamous ambiguity error.

edit

The accepted answer suggests size:

In [11]: x = np.array([])
In [12]: x.size
Out[12]: 0

But I (and most others) check the shape more than the size:

In [13]: x.shape
Out[13]: (0,)

Another thing in its favor is that it 'maps' on to an empty list:

In [14]: x.tolist()
Out[14]: []

But there are other other arrays with 0 size, that aren't 'empty' in that last sense:

In [15]: x = np.array([[]])
In [16]: x.size
Out[16]: 0
In [17]: x.shape
Out[17]: (1, 0)
In [18]: x.tolist()
Out[18]: [[]]
In [19]: bool(x.tolist())
Out[19]: True

np.array([[],[]]) is also size 0, but shape (2,0) and len 2.

While the concept of an empty list is well defined, an empty array is not well defined. One empty list is equal to another. The same can't be said for a size 0 array.

The answer really depends on

• what do you mean by 'empty'?
• what are you really test for?