Convert list or numpy array of single element to float in python

Question

I have a function which can accept either a list or a numpy array   In either case  the list array has a single element  always   I just need to return a float   So  e g   I could receive   list     4    or the numpy array   array    array  4     And I should return   4 0   So  naturally  I would say   I employ float      on list  and get   TypeError  float   argument must be a string or a number   I do the same to array  and this time it works by responding with  4 0   From this  I learn that Python s list cannot be converted to float this way   Based on the success with the numpy array conversion to float this lead me to the approach   float np asarray list      And this works when list  is both a Python list and when it is a numpy array   Question  But it seems like this approach has an overhead first converting the list to a numpy array and then to float  Basically  Is there a better way of doing this

User · Answer

I would simply use   np asarray input  dtype np float  0     If input is an ndarray of the right dtype  there is no overhead  since np asarray does nothing in this case  if input is a list  np asarray makes sure the output is of the right type

User · Answer

Use numpy asscalar to convert a numpy array   matrix a scalar value    gt  gt  gt  a numpy array     42       gt  gt  gt  numpy asscalar a  42      The output data type is the same type returned by the input   s item method    It has built in error-checking if there is more than an single element    gt  gt  gt  a numpy array  1  2    gt  gt  gt  numpy asscalar a    gives   ValueError  can only convert an array of size 1 to a Python scalar     Note  the object passed to asscalar must respond to item  so passing a list or tuple won t work

User · Answer

Just access the first item of the list array  using the index access and the index 0    gt  gt  gt  list     4   gt  gt  gt  list  0  4  gt  gt  gt  array    np array  4    gt  gt  gt  array  0  4   This will be an int since that was what you inserted in the first place  If you need it to be a float for some reason  you can call float   on it then    gt  gt  gt  float list  0   4 0

User · Answer

You may want to use the ndarray item method  as in a item    This is also equivalent to  the now deprecated  np asscalar a   This has the benefit of working in situations with views and superfluous axes  while the above solutions will currently break  For example   gt  gt  gt  a   np asarray 1  view    gt  gt  gt  a item      correct 1   gt  gt  gt  a 0     breaks Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  IndexError  too many indices for array    gt  gt  gt  a   np asarray   2     gt  gt  gt  a item      correct 2   gt  gt  gt  a 0     bad result array  2    This also has the benefit of throwing an exception if the array is not a singleton  while the a 0  approach will silently proceed  which may lead to bugs sneaking through undetected    gt  gt  gt  a   np asarray  1  2    gt  gt  gt  a 0     silently proceeds 1  gt  gt  gt  a item      detects incorrect size Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  ValueError  can only convert an array of size 1 to a Python scalar

User · Answer

np asscalar a  is deprecated since NumPy v1 16  use a item   instead  For example  a   np array   0 6813    print a item     gives  0 6813

[python] Convert list or numpy array of single element to float in python

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