Python's **`numpy.isnan()`** tests if an element is #key# NaN: Not a Number #key# or not. It tests an array element-wise and returns Boolean values. 

# Syntax
`numpy.isnan()` is declared as follows:
```python
numpy.isnan(x, /, out=None, *, where=True, casting='same_kind', order='K', dtype=None, subok=True[, signature, extobj]) = <ufunc 'isnan'>
```
In the syntax above, `x` is the non-optional parameter and the rest are optional parameters. 

> A **universal function** (ufunc) is a function that operates on ndarrays in an element-by-element fashion. The `isnan()` method is a universal function. 


# Parameters
The `numpy.isnan()` method takes the following compulsory parameter:

* **`x`** [*array-like*] - input array.

The `numpy.isnan()` method takes the following optional parameters:

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# Return value
`numpy.isnan()` returns a Boolean array. 

* It returns `True` if an element is `NaN`. 

* It returns `False` otherwise. 

> If `x` is scalar, the return type is also scalar. 


# Examples
The example below shows the use of `numpy.isnan()` on the elements `a` and `b`:

import numpy as np

a = np.nan
b = 20

print (np.isnan(a))
print (np.isnan(b))

The following example shows the use of `numpy.isnan()` on the array `arr1`:

import numpy as np

arr1 = np.array([1,2,np.nan, np.log(-3)])
print (np.isnan(arr1))


What is numpy.isnan() in Python?

Checks if an element in an array is NaN, returning a Boolean array for each element.

Parameter	Description
out	represents the location into which the output of the method is stored.
where	True value indicates that a universal function should be calculated at this position.
casting	controls the type of datacasting that should occur. The same_kind option indicates that safe casting or casting within the same kind should take place.
order	controls the memory layout order of the output function. The option K means reading the elements in the order they occur in memory.
dtype	represents the desired data type of the array.
subok	decides if subclasses should be made or not. If True, subclasses will be passed through.