# [python] How to judge scalar

When you want to easily judge whether the input argument is scalar, there is a function called isscalar in matlab, but in python, it is. I didn't know what to do, so I looked it up.

# version information

Since these stories are version-dependent, I will leave the version at the time of writing this article.

• python 3.7.3
• numpy 1.17.0

# Method 1: Determine if it inherits the Number class

When I read, it says something like this.

class numbers.Number The root of the number hierarchy. If you just want to check that the argument x is a number of any kind, you can use isinstance (x, Number).

The code looks like this. It can be simple.

``````import numbers

def is_scalar(x):
return isinstance(x, numbers.Number)
``````

# Method 2: Use numpy's decision function

When I looked it up, numpy had a function called `numpy.isscalar`. The specifications are easy to see around here.

The definition on the source code was here. As far as the code is concerned, the range of judgment as scalar is definitely wider than that of method 1. As far as the function comment is seen, it is a specification that str is also a scalar (I personally feel a little strange. Can I make a vector space where str is a coefficient?)

# Scalar judgment range

I tried it with an object that seems to be used often, so I will post it for the time being. In the trials, if `str` and` bytes` were method 1, it was False, and if it was method 2 (numpy.isscalar), it was True. Other than that, the judgment results were in agreement. It seems better to be aware of this difference when implementing.

#### `scalar_test.py`

``````
import numpy
import numbers
import decimal

def is_scalar(x):
return isinstance(x, numbers.Number)

def check(input_var):
print("=========================")
print(input_var)
print(type(input_var))
print("is_sclar_1: " + str(is_scalar(input_var)))
print("is_sclar_2: " + str(numpy.isscalar(input_var)))

if __name__ == "__main__":
#Both methods 1 and 2 are judged as scalar
var_int = 1
var_bool = True
var_float = 1.0
var_complex = 1 + 1j
var_npint32 = numpy.int32(1)
var_decimal = decimal.Decimal('12.345')

#Method 1 is judged to be scalar and method 2 is judged to be non-scalar
var_bytes_1 = bytes(1)
var_bytes_hoge = b'hoge'
var_str1 = "1"
var_str4 = "hoge"

#Judgment that both methods 1 and 2 are non-scalar
var_nan = None
var_nparray = numpy.array(1)
var_list = [1]
var_taple = (1,2)  #(1)If so, it will not be a tuple, so it will be judged as a scalar
var_dict = {"a": 1}
var_set = {1}

#Verification
check(var_int)
check(var_bool)
check(var_float)
check(var_complex)
check(var_npint32)
check(var_decimal)

check(var_bytes_1)
check(var_bytes_hoge)
check(var_str1)
check(var_str4)

check(var_nan)
check(var_nparray)
check(var_list)
check(var_taple)
check(var_dict)
check(var_set)
``````

Output result

``````=========================
1
<class 'int'>
is_sclar_1: True
is_sclar_2: True
=========================
True
<class 'bool'>
is_sclar_1: True
is_sclar_2: True
=========================
1.0
<class 'float'>
is_sclar_1: True
is_sclar_2: True
=========================
(1+1j)
<class 'complex'>
is_sclar_1: True
is_sclar_2: True
=========================
1
<class 'numpy.int32'>
is_sclar_1: True
is_sclar_2: True
=========================
12.345
<class 'decimal.Decimal'>
is_sclar_1: True
is_sclar_2: True
=========================
b'\x00'
<class 'bytes'>
is_sclar_1: False
is_sclar_2: True
=========================
b'hoge'
<class 'bytes'>
is_sclar_1: False
is_sclar_2: True
=========================
1
<class 'str'>
is_sclar_1: False
is_sclar_2: True
=========================
hoge
<class 'str'>
is_sclar_1: False
is_sclar_2: True
=========================
None
<class 'NoneType'>
is_sclar_1: False
is_sclar_2: False
=========================
1
<class 'numpy.ndarray'>
is_sclar_1: False
is_sclar_2: False
=========================
[1]
<class 'list'>
is_sclar_1: False
is_sclar_2: False
=========================
(1, 2)
<class 'tuple'>
is_sclar_1: False
is_sclar_2: False
=========================
{'a': 1}
<class 'dict'>
is_sclar_1: False
is_sclar_2: False
=========================
{1}
<class 'set'>
is_sclar_1: False
is_sclar_2: False
``````