python - How to write recursive function properly? -

im new python , therefore apologize if question ist basic:

i write function calculates minimum monthly payment needed in order pay off credit card balance within 12 months using recursion.

given following:

initialbalance = 1000 # thats initial balance on card / debt annualinterest = .2 startperiod = 1 # when start begin pament minmonthpayment = 5 # begin monthly payment of 5 $  def repayment(initialbalance,annualinterest,minmonthpayment,startperiod):      while startperiod < 12 , initialbalance > 0:         monthrate = annualinterest/12         interest = monthrate * initialbalance         initialbalance -= minmonthpayment         initialbalance += interest         startperiod += 1         # here check if still havent paid off balance initial 5 dollars         # if not call function again , increase minmonthpayment      if initialbalance > 0:         repayment(initialbalance,                   annualinterest,                   minmonthpayment + 5,                   startperiod)     return round(initialbalance,1)  print(repayment(1000,.2,5,1))   

however doesnt seem call function appropriately , return:

recursionerror: maximum recursion depth exceeded in comparison 

when coding recursive functions have first answer question when function stop going deeper , return value. check first few lines of recursive function.

so first should check if function should stop, , if doesn't need stop, is, if should go deeper have change parameters pass down make sure satisfies condition.

for example simple recursive function print elements of list this:

def print_list_recursive(list):     # condition answers question 'do need go deeper?'     if list empty: # don't go deeper         return     else: # have go deeper         first_element = remove_first_element(list)         print first_element         print_list_recursive(list) # list parameter changed 

note how parameter list changes every time go deeper. each time 1 element removed list empty , return (stop going deeper).

recursion seem easy once define problem recursively , define stopping condition.

in "print list" problem should see print list can print first element , call print_list on rest of list, again prints first element , calls print_list on rest of list... when stop? when stopping condition, is, when list empty.

this same logic can apply when coding recursive function.

i hope helps.