# b_round_float.py

# b_round_float.py

n = [round(123456.789, -(i-3)) for i in xrange(10)]
for i in n: print i

'''
output:

123456.789
123456.79
123456.8
123457.0
123460.0
123500.0
123000.0
120000.0
100000.0
0.0
'''

print

def f(z): return str(abs(i))
x = '-123456789'
t = [float(x[i:]) for i in xrange(len(x))] ### as floats just for testing
n = [float(('-' if i<0 else '')+f(i)[0]+'0'*(len(f(i))-3)) for i in t]
for i in n: print i

'''
output:

-100000000.0
100000000.0
20000000.0
3000000.0
400000.0
50000.0
6000.0
700.0
80.0
9.0
'''