# Local variable referenced before assignment python error

## Question:

I have this code which needs to be plotted perfectly. I’m new to python. The error is as mentioned above. Any kind of help would be appreciated.

``````    #Variable Initialization
#%pylab inline
# (a) Curie Temperature for h = 15 W/m^2
# (b) Value of h for cure temp = 50 deg C

import math
import numpy
from numpy import roots
import matplotlib
from matplotlib import pyplot
Tsurr = 30+273;                                     #[K] - Temperature of surrounding
Tf = 20+273;                                        #[K] - Temperature of Fluid Flow
e=.5;                                               #  Emissivity of Surface
a = .8;                                             # Absorptivity of Surface
G = 2000;                                           #[W/m^2] - Irradiation falling on surface
h=15;                                               #[W/m^2.k] - Thermal Convectivity from plate to air
stfncnstt=5.67*math.pow(10,(-8));                   # [W/m^2.K^4] - Stefan Boltzmann Constant
T=375;                                              #[K] Value initially assumed for trial-error approach
#Using Eq 1.3a & 1.7 and trial-and error approach of Newton Raphson
#calculations and results
while(1>0):
f=((a*G)-(h*(T-Tf)+e*stfncnstt*(T*T*T*T - Tsurr*Tsurr*Tsurr*Tsurr)));
fd=(-h*T-4*e*stfncnstt*T*T*T);
Tn=T-f/fd;
if(((a*G)-(h*(Tn-Tf)+e*stfncnstt*(Tn*Tn*Tn*Tn - Tsurr*Tsurr*Tsurr*Tsurr)))<.01):
break;
T=Tn;

print '%s %.2f %s' %("n (a) Cure Temperature of Plate =",T-273.,"degCn");
#solution (b)
Treq=50+273;
#def T(h):
#    t=375;
#    while(1>0):
#        f=((a*G)-(h*(t-Tf)+e*stfncnstt*(t*t*t*t - Tsurr*Tsurr*Tsurr*Tsurr)));
#        fd=(-h*t-4*e*stfncnstt*t*t*t);
#        Tn=t-f/fd;
#        if((a*G)-(h*(Tn-Tf)+e*stfncnstt*(Tn*Tn*Tn*Tn - Tsurr*Tsurr*Tsurr*Tsurr))<.01):
#           break;
#    tnew=Tn;
#    return tnew;
rot=numpy.zeros(4);
rt=0;

def T(h):
coeff = ([-e*stfncnstt, 0,0, -h, a*G+h*Tf+e*stfncnstt*Tsurr*Tsurr*Tsurr*Tsurr]);
rot=numpy.roots(coeff);
for i in range (0,3):
if rot[i]<250 and rot[i]>0:
rt=rot[i];
return rt;
Error at this part. The function is not working.! What can be the error. Please suggest.
h = range(0,100)
tn=range(0,100)
for i in range (0,100):
tn[i] = T(i) -273;

Ti=50+273;
hnew=((a*G)-(e*stfncnstt*(Ti*Ti*Ti*Ti - Tsurr*Tsurr*Tsurr*Tsurr)))/(Ti-Tf);

pyplot.plot(h,tn);
pyplot.xlabel("h (W m^2/K)");
pyplot.ylabel("T (C)");
pyplot.show();
print '%s %.2f %s' %("n (b) Air flow must provide a convection of =",hnew," W/m^2.K");
#print '%s' %("n The code for the graph requires more than 10 min to run. ")
#print '%s' %("n To run it, please remove comments. It is perfectly correct. The reason it takes such a long time")
#print '%s' %("n  is that it needs to calculate using Newton raphson method at 100 points. Each point itself takes a minute.")
#END
``````

it will say

``````UnboundLocalError: local variable 'rt' referenced before assignment
``````

since you have assigned something to rt `rt=rot[i]`, `rt` is recognized as a local variable then. If you want to make it global, make it global explicitly using `global rt`:

``````def T(h):
global rt
#do sth....
``````

note that even if the assignment is not executed, the intepreter will still see assigned var as local:

``````In [136]: var=2
...: def foo():
...:   if False:
...:       var=4
...:   print var
...:

In [137]: foo()
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-137-624891b0d01a> in <module>()
----> 1 foo()

<ipython-input-136-ab29dd655074> in foo()
3         if False:
4                 var=4
----> 5         print var
6

UnboundLocalError: local variable 'var' referenced before assignment
``````
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