ValueError when checking if variable is None or numpy array

Question

I d like to check if variable is None or numpy array  I ve implemented check a function to do this   def check a a       if not a          print  please initialize a   a   None check a a  a   np array  1 2   check a a    But  this code raises ValueError  What is the straight forward way   ValueError                                Traceback  most recent call last   lt ipython-input-41-0201c81c185e gt  in  lt module gt          6 check a a        7 a   np array  1 2   ---- gt  8 check a a    lt ipython-input-41-0201c81c185e gt  in check a a        1 def check a a   ---- gt  2     if not a        3         print  please initialize a        4        5 a   None  ValueError  The truth value of an array with more than one element is ambiguous  Use a any   or a all

User · Answer

To stick to    without consideration of the other type  the following is also possible  type a     type None

User · Answer

You can see if object has shape or not  def check array x       try          x shape         return True     except          return False

User · Answer

Using not a to test whether a is None assumes that the other possible values of a have a truth value of True  However  most NumPy arrays don t have a truth value at all  and not cannot be applied to them   If you want to test whether an object is None  the most general  reliable way is to literally use an is check against None   if a is None          else            This doesn t depend on objects having a truth value  so it works with NumPy arrays   Note that the test has to be is  not     is is an object identity test     is whatever the arguments say it is  and NumPy arrays say it s a broadcasted elementwise equality comparison  producing a boolean array    gt  gt  gt  a   numpy arange 5   gt  gt  gt  a    None array  False  False  False  False  False    gt  gt  gt  if a    None          pass     Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ValueError  The truth value of an array with more than one element is ambiguous   Use a any   or a all       On the other side of things  if you want to test whether an object is a NumPy array  you can test its type     Careful - the type is np ndarray  not np array  np array is a factory function  if type a  is np ndarray          else            You can also use isinstance  which will also return True for subclasses of that type  if that is what you want   Considering how terrible and incompatible np matrix is  you may not actually want this     Again  ndarray  not array  because array is a factory function  if isinstance a  np ndarray           else

User · Answer

If you are trying to do something very similar  a is not None  the same issue comes up  That is  Numpy complains that one must use a any or a all    A workaround is to do   if not  a is None       pass   Not too pretty  but it does the job

[python] ValueError when checking if variable is None or numpy.array

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