如何检查numpy数组是否为空？

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

I used the following code, but this fails if the array contains a zero.

if not self.Definition.all():

Is this the solution?

if self.Definition == array( [] ):

You can always take a look at the .size attribute. It is defined as an integer, and is zero (0) when there are no elements in the array:

import numpy as np
a = np.array([])

if a.size == 0:
    # Do something when `a` is empty

http://www.scipy.org/Tentative_NumPy_Tutorial#head-6a1bc005bd80e1b19f812e1e64e0d25d50f99fe2

NumPy’s main object is the homogeneous multidimensional array. In Numpy dimensions are called axes. The number of axes is rank. Numpy’s array class is called ndarray. It is also known by the alias array. The more important attributes of an ndarray object are:

ndarray.ndim
the number of axes (dimensions) of the array. In the Python world, the number of dimensions is referred to as rank.

ndarray.shape
the dimensions of the array. This is a tuple of integers indicating the size of the array in each dimension. For a matrix with n rows and m columns, shape will be (n,m). The length of the shape tuple is therefore the rank, or number of dimensions, ndim.

ndarray.size
the total number of elements of the array. This is equal to the product of the elements of shape.

One caveat, though. Note that np.array(None).size returns 1! This is because a.size is equivalent to np.prod(a.shape), np.array(None).shape is (), and an empty product is 1.

>>> import numpy as np
>>> np.array(None).size
1
>>> np.array(None).shape
()
>>> np.prod(())
1.0

Therefore, I use the following to test if a numpy array has elements:

>>> def elements(array):
    ...     return array.ndim and array.size

>>> elements(np.array(None))
0
>>> elements(np.array([]))
0
>>> elements(np.zeros((2,3,4)))
24

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.