numpy.signbit

numpy.signbit(x, /, out=None, *, where=True, casting='same_kind', order='K', dtype=None, subok=True[, signature, extobj]) = <ufunc 'signbit'>

Returns elementwise True where signbit is set (less than zero).
Parameters: 

x : array_like 
The input value(s). 
out : ndarray, None, or tuple of ndarray and None, optional 
A location into which the result is stored. If provided, it must have a shape that the inputs broadcast to. If not provided or None , a freshlyallocated array is returned. A tuple (possible only as a keyword argument) must have length equal to the number of outputs. 
where : array_like, optional 
This condition is broadcast over the input. At locations where the condition is True, the out array will be set to the ufunc result. Elsewhere, the out array will retain its original value. Note that if an uninitialized out array is created via the default out=None , locations within it where the condition is False will remain uninitialized.  **kwargs

For other keywordonly arguments, see the ufunc docs. 
Returns: 

result : ndarray of bool 
Output array, or reference to out if that was supplied. This is a scalar if x is a scalar. 
Examples
>>> np.signbit(1.2)
True
>>> np.signbit(np.array([1, 2.3, 2.1]))
array([False, True, False])