arctan2(x1, x2, /, out=None, *, where=True, casting='same_kind', order='K', dtype=None, subok=True[, signature, extobj]) = <ufunc 'arctan2'>¶
Element-wise arc tangent of
x1/x2choosing the quadrant correctly.
The quadrant (i.e., branch) is chosen so that
arctan2(x1, x2)is the signed angle in radians between the ray ending at the origin and passing through the point (1,0), and the ray ending at the origin and passing through the point (x2, x1). (Note the role reversal: the “y-coordinate” is the first function parameter, the “x-coordinate” is the second.) By IEEE convention, this function is defined for x2 = +/-0 and for either or both of x1 and x2 = +/-inf (see Notes for specific values).
This function is not defined for complex-valued arguments; for the so-called argument of complex values, use
- x1 : array_like, real-valued
- x2 : array_like, real-valued
x-coordinates. x2 must be broadcastable to match the shape of x1 or vice versa.
- 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 freshly-allocated array is returned. A tuple (possible only as a keyword argument) must have length equal to the number of outputs.
- where : array_like, optional
Values of True indicate to calculate the ufunc at that position, values of False indicate to leave the value in the output alone.
For other keyword-only arguments, see the ufunc docs.
- angle : ndarray
Array of angles in radians, in the range
[-pi, pi]. This is a scalar if both x1 and x2 are scalars.
arctan2 is identical to the atan2 function of the underlying C library. The following special values are defined in the C standard: 
x1 x2 arctan2(x1,x2) +/- 0 +0 +/- 0 +/- 0 -0 +/- pi > 0 +/-inf +0 / +pi < 0 +/-inf -0 / -pi +/-inf +inf +/- (pi/4) +/-inf -inf +/- (3*pi/4)
Note that +0 and -0 are distinct floating point numbers, as are +inf and -inf.
 (1, 2) ISO/IEC standard 9899:1999, “Programming language C.”
Consider four points in different quadrants:
>>> x = np.array([-1, +1, +1, -1]) >>> y = np.array([-1, -1, +1, +1]) >>> np.arctan2(y, x) * 180 / np.pi array([-135., -45., 45., 135.])
Note the order of the parameters.
arctan2is defined also when x2 = 0 and at several other special points, obtaining values in the range
>>> np.arctan2([1., -1.], [0., 0.]) array([ 1.57079633, -1.57079633]) >>> np.arctan2([0., 0., np.inf], [+0., -0., np.inf]) array([ 0. , 3.14159265, 0.78539816])