Python static variable in function global name not defined -

i have written function calculate heading between 2 points if vehicle reports it's moving , vehicle has moved 20cm between points.

the function uses static variables - or @ least if worked - keep track of previous positions , heading values.

here code:

def withcan(pos):      eastdist = pos[0]-previous_pos[0]     northdist = pos[1]-previous_pos[1]     canflag = pos[2]      if (canflag == 1 or canflag==2):          if (previous_canflag == 1 , canflag == 2):             previous_heading += 180.0             previous_canflag = canflag         elif (previous_canflag == 2 , canflag == 1):             previous_heading += 180.0             previous_canflag = canflag         else:             previous_canflag = canflag      if ( (canflag == 1 or canflag == 2) , math.sqrt(northdist*northdist+eastdist*eastdist) > canstep ):          previous_heading = math.degrees(math.atan2(eastdist, northdist))         previous_pos[0] = pos[0]         previous_pos[1] = pos[1]      return previous_heading  withcan.previous_pos = [0.0,0.0] withcan.previous_heading = 0.0 withcan.previous_canflag = 0 withcan.canstep = 0.2  positions = backandforth([100,100]) #populates array of form [x,y,canflag]  p in positions:     print withcan(p) 

i getting error says eastdist = pos[0]-previous_pos[0] nameerror: global name 'previous_pos' not defined. please explain cause of error?

when this:

def foo():     pass  foo.name = 1 

you not creating global name name. instead adding property foo function! can access with:

def foo():     return foo.name  foo.name = 1 

but quite weird. if need global name, it:

def foo():     global name     name += 1     return name  name = 1 

remember if want modify global name function, have declare global. if fail this, can use cannot assign it.

your confusion static names may come using classes. note in code withcan not class, plain function!