A NumPy scalar is any object which is an instance of np.generic or whose type is in np.ScalarType:

In [12]: np.ScalarType
Out[13]:
(int,
 float,
 complex,
 long,
 bool,
 str,
 unicode,
 buffer,
 numpy.int16,
 numpy.float16,
 numpy.int8,
 numpy.uint64,
 numpy.complex192,
 numpy.void,
 numpy.uint32,
 numpy.complex128,
 numpy.unicode_,
 numpy.uint32,
 numpy.complex64,
 numpy.string_,
 numpy.uint16,
 numpy.timedelta64,
 numpy.bool_,
 numpy.uint8,
 numpy.datetime64,
 numpy.object_,
 numpy.int64,
 numpy.float96,
 numpy.int32,
 numpy.float64,
 numpy.int32,
 numpy.float32)

This definition comes from looking at the source code for np.isscalar:

def isscalar(num):
    if isinstance(num, generic):
        return True
    else:
        return type(num) in ScalarType

Note that you can test if something is a scalar by using np.isscalar:

>>> np.isscalar(3.1)
True

>>> np.isscalar([3.1])
False

>>> np.isscalar(False)
True

How do we know what we know?

I like learning how people know what they know—more than the answers themselves. So let me try to explain where the above answer comes from.

Having the right tools can help you figure out things like this for yourself.

I found this out by using IPython. Using its TAB-completion feature, typing

In [19]: import numpy as np
In [20]: np.[TAB]

causes IPython to display all variables in the np module namespace. A search for the string "scalar" will lead you to np.ScalarType and np.isscalar. Typing

In [20]: np.isscalar?

(note the question mark at the end) prompts IPython to show you where np.isscalar is defined:

File:  /data1/unutbu/.virtualenvs/dev/lib/python2.7/site-packages/numpy/core/numeric.py

which is how I got to the definition of isscalar. Alternatively, the NumPy documentation for isscalar has a link to the source code as well.

The term "scalar" comes from linear algebra, where it is used to differentiate a single number from a vector or matrix. The meaning in computing is similar. It distinguishes a single value like an integer or float from a data structure like an array. This distinction is very prominent in Perl, where the $ sigil (which resembles an 's') is used to denote a scalar variable and an @ sigil (which resembles an 'a') denotes an array. It doesn't have anything to do with the type of the element itself. It could be a number, character, string, or object. What matters to be called a scalar is that there is one of them.