Python Compunding Investment Calculator

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#==============MODULES================#
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#==============VARIABLES==============#
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#==============FUNCTIONS==============#
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#defines investment algorithm
def invest(principal, rate, period, years):
    for i in range(years):
        apr = (rate / period)
        principal = (principal * (1 + apr))
        if i == (years - 1):
            print "The value in %s years is %s" % (years, principal)

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#=============MAIN PROGRAM============#
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def main():
    principal = input("Please enter the initial principal: ")
    rate = input("Please enter the annual interest rate: ")
    period = input("Please enter the number of times the interest rate is compounded: ")
    years = input("Please enter the number of years: ")
    invest(principal, rate, period, years)