how to calculate depreciation rate for straight line method python

1def sl(val,n):
2    deprec=val/n
3    print "in ",n
4    
5    for year in range(1,n+1):
6        val=val-deprec
7        writeoutput(year,deprec,val)
8
9    return
10
11def ddb(val,n):
12    for year in range(1,n+1):
13        deprec=2*val/n
14        val=val-deprec
15        writeoutput(year,deprec,val)
16
17    return
18
19def syd(val,n):
20    tag=val
21    for year in range(1,n+1):
22        deprec=(n-year+1)*tag/(n*(n+1)/2)
23        val=val-deprec
24        writeoutput(year,deprec,val)
25
26    return
27
28def writeoutput(year,depreciation,value):
29    print "End of the year %2d  Depreciation: %7.2f  Current Value: %8.2f" %(year,depreciation,value)
30
31    return
32
33
34
35def main(choice,val,n):
36    
37    print "Original value : ",val
38    val=float(val)
39    print "Number of years : ",n
40
41    if choice==1:
42        print "Straight-Line Method\n\n"
43        sl(val,n)
44    elif choice==2:
45        print "Double-Declining-Balance Method \n\n"
46        ddb(val,n)
47    elif choice==3:
48        print "Sum-Of-The-Years'-Digits Method"
49        syd(val,n)
50
51    return
52
53
54print "\n\nMethod: (1-SL 2-DDB 3-SYD)"
55main(1,8000,10)
56print "\n\nMethod: (1-SL 2-DDB 3-SYD)"
57main(2,8000,10)
58print "\n\nMethod: (1-SL 2-DDB 3-SYD)"
59main(3,8000,10)
60print "\n\nMethod: (1-SL 2-DDB 3-SYD)"
61main(1,5000,4)
62
63
64    
65    
66