GUI: Function for the discharge equation by Addition in 1949

I have upgraded my Python code to include a graphical user interface. Here is the code:

Run the program online here: http://www.codeskulptor.org/#user21_ikyPgf2883_1.py

import simplegui

C,L,H,n=0,0,0,0
C_str, L_str,H_str,n_str='','','',''
result=0
result_str=''

# Handler for mouse click

# Handler to draw on canvas

def enter_C(t):
    global C, C_str
    C = float(t)
    C_str='You have entered C: '+''+ str(C)
    print C  

def enter_L(t):
    global L, L_str
    L = float(t)
    L_str='You have entered L: '+''+ str(L)
    print L    

def enter_H(t):
    global H, H_str
    H = float(t)
    H_str='You have entered H: '+''+ str(H)
    print H 

def enter_n(t):
    global n, n_str
    n = float(t)
    n_str='You have entered n: '+''+ str(n)
    print n     

def discharge_weir():
    #Weirs allow hydrologists and engineers a simple method of measuring
    #the volumetric flow rate in small to medium-sized streams
    # Function for the discharge equation by Addition in 1949, for TRAPEZOIDAL WEIR
    '''
    Q is flow rate
    C is a constant for structure, when flow in cfs, C=3.37
    L is the width of the crest in FEET
    H is the height of head of water over the crest in FEET
    n varies with structure (e.g. 3/2 for horizontal weir, 5/2 for v-notch weir)
    '''
    global C,L,H,n, result, result_str
    result=round(C*L*(H**n),2)
    print result
    result_str=str(result)+ ' is the flow in CFS using Addition (1949) formula'

def draw_handler(canvas):
        canvas.draw_text(C_str, [50,22], 20, "White")
        canvas.draw_text(L_str, [50,52], 20, "White")
        canvas.draw_text(H_str, [50,82], 20, "White")
        canvas.draw_text(n_str, [50,112], 20, "White")
        canvas.draw_text(result_str, [50,142], 20, "White")
    #canvas.draw_text(result_str, [50,152], 24, "White")    

# Create a frame and assign callbacks to event handlers
frame = simplegui.create_frame("Calculate Flow in CFS based on Addition Formula", 500, 400,400)
label = frame.add_label('Calculate Flow in CFS from Weirs:')
labe2 = frame.add_label('By Ankan Basu, CPG')
labe3 = frame.add_label('contact: admin@coalgeology.com')
inp1=frame.add_input('Enter C in CFS, hit enter', enter_C, 50)
inp2=frame.add_input('Enter L in Feet, hit enter', enter_L, 50)
inp3=frame.add_input('Enter H, head in Feet, hit enter', enter_H, 50)
inp3=frame.add_input('Enter n for the structure, hit enter', enter_n, 50)
flow= frame.add_button('Calculate flow in CFS', discharge_weir,200)
frame.set_draw_handler(draw_handler)

# Start the frame animation
frame.start()