How to know the angle between two vectors?

I am making small game with pygame and I have made a gun that rotates around its center. My problem is that I want the gun to rotate by itself to the enemy direction, but I couldn't do that because I can't find the angle between the gun and the enemy to make the gun rotate to it I have searched and I found that I have to use the atan2 but I didn't find any working code so I hope someone could help me.

Here is my code:

import pygame
from pygame.locals import*
pygame.init()
height=650
width=650
screen=pygame.display.set_mode((height,width))
clock=pygame.time.Clock()
gun=pygame.image.load("m2.png").convert_alpha() 
gun=pygame.transform.smoothscale(gun,(200,200)).convert_alpha()
angle=0
angle_change=0
RED=(255,0,0)
x=525
y=155
while True :
    screen.fill((150,150,150))
    for event in pygame.event.get():
        if event.type==QUIT:
            pygame.quit()
            quit()
        if event.type==KEYDOWN:
            if event.key==K_a:
                angle_change=+1
            if event.key==K_d:
                angle_change=-1
        elif event.type==KEYUP:
            angle_change=0
    angle+=angle_change
    if angle>360:
        angle=0
    if angle<0:
        angle=360
    pygame.draw.rect(screen,RED,(x,y,64,64))
    position = (height/2,width/2)
    gun_rotate=pygame.transform.rotate(gun,angle) 
    rotate_rect = gun_rotate.get_rect()
    rotate_rect.center = position
    screen.blit(gun_rotate, rotate_rect)
    pygame.display.update()
    clock.tick(60) 

And here is a picture trying to make it clear:

How do I solve the problem?

The tangent of the angle between two points is defined as delta y / delta x That is (y2 - y1)/(x2-x1). This means that math.atan2(dy, dx) give the angle between the two points assuming that you know the base axis that defines the co-ordinates.

Your gun is assumed to be the (0, 0) point of the axes in order to calculate the angle in radians. Once you have that angle, then you can use the angle for the remainder of your calculations.

Note that since the angle is in radians, you need to use the math.pi instead of 180 degrees within your code. Also your test for more than 360 degrees (2*math.pi) is not needed. The test for negative (< 0) is incorrect as you then force it to 0, which forces the target to be on the x axis in the positive direction.

Your code to calculate the angle between the gun and the target is thus

myradians = math.atan2(targetY-gunY, targetX-gunX)

If you want to convert radians to degrees

mydegrees = math.degrees(myradians)

To convert from degrees to radians

myradians = math.radians(mydegrees)

Python ATAN2

The Python ATAN2 function is one of the Python Math function which is used to returns the angle (in radians) from the X -Axis to the specified point (y, x).

math.atan2()

Definition Returns the tangent(y,x) in radius.

Syntax
math.atan2(y,x)

Parameters
y,x=numbers

Examples
The return is:

>>> import math  
>>> math.atan2(88,34)  
1.202100424136847  
>>>

Specifically for working with shapely linestring objects, assuming your object (two points) is of the form (min long, min lat, max long, max lat)

from math import atan2,degrees
line = #Your-LineString-Object
lineList = list(line.coords)

def AngleBtw2Points(pointA, pointB):
  changeInX = pointB[0] - pointA[0]
  changeInY = pointB[1] - pointA[1]
  return degrees(atan2(changeInY,changeInX)) #remove degrees if you want your answer in radians

AngleBtw2Points(lineList[0],lineList[1])