python how to identify if a variable is an array or a scalar

Question

I have a function that takes the argument NBins  I want to make a call to this function with a scalar 50 or an array  0  10  20  30   How can I identify within the function  what the length of NBins is  or said differently  if it is a scalar or a vector    I tried this    gt  gt  gt  N  2 3 5   gt  gt  gt  P   5  gt  gt  gt  len N  3  gt  gt  gt  len P  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  object of type  int  has no len    gt  gt  gt     As you see  I can t apply len to P  since it s not an array     Is there something like isarray or isscalar in python   thanks

User · Answer

While   jamylak s approach is the better one  here is an alternative approach   gt  gt  gt  N  2 3 5   gt  gt  gt  P   5  gt  gt  gt  type P  in  tuple  list  False  gt  gt  gt  type N  in  tuple  list  True

User · Answer

Combining  jamylak and  jpaddison3 s answers together  if you need to be robust against numpy arrays as the input and handle them in the same way as lists  you should use  import numpy as np isinstance P   list  tuple  np ndarray     This is robust against subclasses of list  tuple and numpy arrays   And if you want to be robust against all other subclasses of sequence as well  not just list and tuple   use  import collections import numpy as np isinstance P   collections Sequence  np ndarray     Why should you do things this way with isinstance and not compare type P  with a target value  Here is an example  where we make and study the behaviour of NewList  a trivial subclass of list    gt  gt  gt  class NewList list           isThisAList               gt  gt  gt  x   NewList  0 1    gt  gt  gt  y   list  0 1    gt  gt  gt  print x  0  1   gt  gt  gt  print y  0  1   gt  gt  gt  x  y True  gt  gt  gt  type x   lt class    main   NewList  gt   gt  gt  gt  type x  is list False  gt  gt  gt  type y  is list True  gt  gt  gt  type x    name    NewList   gt  gt  gt  isinstance x  list  True   Despite x and y comparing as equal  handling them by type would result in different behaviour  However  since x is an instance of a subclass of list  using isinstance x list  gives the desired behaviour and treats x and y in the same manner

User · Answer

Simply use size instead of len    gt  gt  gt  from numpy import size  gt  gt  gt  N    2  3  5   gt  gt  gt  size N  3  gt  gt  gt  N   array  2  3  5    gt  gt  gt  size N  3  gt  gt  gt  P   5  gt  gt  gt  size P  1

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Is there an equivalent to isscalar   in numpy  Yes   gt  gt  gt  np isscalar 3 1  True  gt  gt  gt  np isscalar  3 1   False  gt  gt  gt  np isscalar False  True  gt  gt  gt  np isscalar  abcd   True

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gt  gt  gt  N  2 3 5   gt  gt  gt  P   5  gt  gt  gt  type P   type 0  True  gt  gt  gt  type  1 2    type N  True  gt  gt  gt  type P   type  1 2   False

User · Answer

Since the general guideline in Python is to ask for forgiveness rather than permission  I think the most pythonic way to detect a string scalar from a sequence is to check if it contains an integer  try      1 in a     print     is a sequence  format a   except TypeError      print     is a scalar or string  format a

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Another alternative approach  use of class name property    N    2 3 5  P   5  type N    name       list  True  type P    name       int  True  type N    name   in   list    tuple   True   No need to import anything

User · Answer

Previous answers assume that the array is a python standard list   As someone who uses numpy often  I d recommend a very pythonic test of   if hasattr N     len

User · Answer

I am surprised that such a basic question doesn t seem to have an immediate answer in python  It seems to me that nearly all proposed answers use some kind of type checking  that is usually not advised in python and they seem restricted to a specific case  they fail with different numerical types or generic iteratable objects that are not tuples or lists     For me  what works better is importing numpy and using array size  for example      gt  gt  gt  a 1  gt  gt  gt  np array a  Out 1   array 1    gt  gt  gt  np array a  size Out 2   1   gt  gt  gt  np array  1 2   size Out 3   2   gt  gt  gt  np array  125   Out 4   1   Note also    gt  gt  gt  len np array  1 2     Out 5   2   but    gt  gt  gt  len np array a   --------------------------------------------------------------------------- TypeError                                 Traceback  most recent call last   lt ipython-input-40-f5055b93f729 gt  in  lt module gt    ---- gt  1 len np array a    TypeError  len   of unsized object

User · Answer

To answer the question in the title  a direct way to tell if a variable is a scalar is to try to convert it to a float  If you get TypeError  it s not   N    1  2  3  try      float N  except TypeError      print  it is not a scalar   else      print  it is a scalar

User · Answer

You can check data type of variable   N    2 3 5  P   5 type P    It will give you out put as data type of P    lt type  int  gt    So that you can differentiate that it is an integer or an array

User · Answer

preds test 0  is of shape  128 128 1  Lets check its data type using isinstance   function isinstance takes 2 arguments  1st argument is data 2nd argument is data type isinstance preds test 0   np ndarray  gives Output as True  It means preds test 0  is an array

User · Answer

Here is the best approach I have found  Check existence of   len   and   getitem      You may ask why  The reasons includes    The popular method isinstance obj  abc Sequence  fails on some objects including PyTorch s Tensor because they do not implement   contains    Unfortunately  there is nothing in Python s collections abc that checks for only   len   and   getitem   which I feel are minimal methods for array-like objects  It works on list  tuple  ndarray  Tensor etc     So without further ado   def is array like obj  string is array False  tuple is array True       result   hasattr obj     len     and hasattr obj     getitem          if result and not string is array and isinstance obj   str  abc ByteString            result   False     if result and not tuple is array and isinstance obj  tuple           result   False     return result   Note that I ve added default parameters because most of the time you might want to consider strings as values  not arrays  Similarly for tuples

User · Answer

gt  gt  gt  isinstance  0  10  20  30   list  True  gt  gt  gt  isinstance 50  list  False   To support any type of sequence  check collections Sequence instead of list   note  isinstance also supports a tuple of classes  check type x  in            should be avoided and is unnecessary   You may also wanna check not isinstance x   str  unicode

[python] python: how to identify if a variable is an array or a scalar

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