COUNTER_CLOCKWISE = 1
CLOCKWISE = -1
ONLINE_BACK = 2
ONLINE_FRONT = -2
ON_SEGMENT = 0
#occw = {"COUNTER_CLOCKWISE":1, "CLOCKWISE":-1, "ONLINE_BACK":2, "ONLINE_FRONT":-2, "ON_SEGMENT":0}
def ccw(p0, p1, p2):
    a = np.array(p1)-np.array(p0)
    b = np.array(p2)-np.array(p0)
    #print("==p0,p1,p2,a,b==")
    #print(p0,p1,p2,a,b)
    #print("==corss,dot==")
    #print(np.cross(a,b),np.dot(a,b))
    if (np.cross(a,b) > np.finfo(np.float).eps):
        return COUNTER_CLOCKWISE
    if (np.cross(a,b) < - np.finfo(np.float).eps):
        return CLOCKWISE
    if (np.dot(a,b) < - np.finfo(np.float).eps):
        return ONLINE_BACK
    if (np.linalg.norm(a)**2 < np.linalg.norm(b)**2):
        return ONLINE_FRONT
    return ON_SEGMENT

def andrewScan(s):
  u = []
  l = []
  if (len(s) < 3):
      return s
  #print(s)
  #s = np.sort([s])
  s = sorted(s)
  #print(s,len(s))
  u.append(s[0])
  u.append(s[1])
  l.append(s[len(s)-1])
  l.append(s[len(s)-2])
  for i in range(2,len(s)):
      for n in range(len(u),2-1,-1):
          if (ccw(u[n-2], u[n-1], s[i]) in [ONLINE_FRONT,CLOCKWISE]):
            break
          u.pop()
      u.append(s[i])
  #print(u)
  for i in range(len(s)-3,-1,-1):
      for n in range(len(l),2-1,-1):
          ret = ccw(l[n-2],l[n-1],s[i])
          if (ret in [CLOCKWISE,ONLINE_FRONT]):
              break
          l.pop()
      l.append(s[i])
  l = l[::-1]
  for i in range(len(u)-2,0,-1):
      l.append(u[i])
  return l

s = []
n = input()
for i in range(n):
    a = []
    a.append(map(int, raw_input().split()))
    for aa in a:
        x = aa[0]
        y = aa[1]
        s.append([x,y])

ret = andrewScan(s)
print(str(len(ret)))
out = ""
for x in ret:
    if (out):
        out = out + "\n"
    _out = ""
    for xx in x:
        if (_out):
            _out = _out + " "
        _out = _out + str(xx)
    out = out + _out
print(out)

In getting convex hull, you must compare the return of ccw with ONLINE_FRONT also, I think.

Getting convex hull in python

Refs.