Help with Python UnboundLocalError: local variable referenced before assignment

Question

I have post the similar question before,however,I think I may have misinterpreted my question,so may I just post my origin code here,and looking for someone can help me,I am really stuck now..thanks alot.

from numpy import *
import math as M

#initial condition  All in SI unit
G=6.673*10**-11   #Gravitational constant
ms=1.9889*10**30 #mass of the sun
me=5.9742*10**24 #mass of the earth
dt=10           #time step
#Creat arrays
vs=array([[0,0,0]])     #1st element stand for x component of V of earth
ve=array([[29770,0,0]])
rs=array([[0,0,0]])           
re=array([[0,1.4960*10**11,0]])

#First update velocity in order to start leapfrog approximation
fs=-G*ms*me*((rs-re)/(M.sqrt((rs-re)[0][0]**2+(rs-re)[0][1]**2+(rs-re)[0][2]**2))**3)
fe=-fs
vs=vs+fs*dt/ms 
ve=ve+fe*dt/me

n=input('please enter the number of timestep you want it evolve:')
#update force
def force(n,ms,me,rs,re,G):
    rs,re=update_r(rs,re,n,dt)
    fs=-G*ms*me*((rs-re)/(M.sqrt((rs-re)[0][0]**2+(rs-re)[0][1]**2+(rs-re)[0][2]**2))**3)
    fe=-fs
    return fs,fe

#update velocities
def update_v(n,vs,ve,ms,me,dt,fs,fe):
    fs,fe=force(n,ms,me,rs,re,G)
    i=arange(n)
    vs=vs+fs[:]*i[:,newaxis]*dt/ms
    ve=ve+fe[:]*i[:,newaxis]*dt/me
    return vs,ve

#update position
def update_r(rs,re,n,dt):
    vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)
    i=arange(n)
    rs=rs+vs[:]*i[:,newaxis]*dt
    re=re+ve[:]*i[:,newaxis]*dt
    return rs,re
#there is start position,v,r,f all have initial arrays(when n=0).
#then it should calculate f(n=1) then use this to update v(n=0)
#to v(n=1),then use v(n=1) update r(n=0) to r(n=1),then use r(n=1)
#update f(n=1) to f(n=2)....and so on until finish n.but this code seems doesnt do this,,how can I make it? – 

when i call force python gives:

please enter the number of timestep you want it evolve:4Traceback (most recent call last):
  File "<pyshell#391>", line 1, in <module>
    force(n,ms,me,rs,re,G)
  File "/Users/Code.py", line 24, in force
    rs,re=update_r(rs,re,n,dt)
  File "/Users/Code.py", line 39, in update_r
    vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)
UnboundLocalError: local variable 'vs' referenced before assignment

can anyone give me some tips?thanks......

Answer 1

where do you call force in this code?

In any event, the problem is in update_r. You reference vs in the first line of update_r even though vs is not defined in this function. Python is not looking at the vs defined above. Try adding

global vs

as the first line of update_r or adding vs to the parameter list for update_r

Answer 2

Put an additional global statement containing all your globals after each def statement. Otherwise, all globals are transformed into locals within your def without it.

def update_v(n,vs,ve,ms,me,dt,fs,fe):
    global vs, ve, ...  

Answer 3

On line 39 you do

vs,ve=update_v(n,vs,ve,ms,me,dt,fs,fe)

while you are inside a function. Since you defined a global variable called vs, you would expect this to work. It would have worked if you had:

vs_new,ve_new = update_v(n,vs,ve,ms,me,dt,fs,fe)

because then the interpreter knows vs in the function arguments is the global one. But since you had vs in the left hand side, you created an uninitialized local variable.

But dude, you have a much bigger problem in your code: update_r calls update_v, update_v calls force, and force calls update_r - you will get a stack overflow :)

Answer 4

I got that error when my class name was assigned to a variable that is called exactly like its name. example ClassName = ClassName. You may do this if you come from .Net