我真的很需要这个python代码解释

Question

I have a code in python that uses amount and coins change array as parameters and based on a possible number of combinations of the coins to be used to provide the money or amount to be given as complete indepth. but the issue here is that I seem not to have a clue how this recursion works

I already found the working code but I need some explanation about how thos works since I got this with my code called countChange(4, [1,2]) . The issue here is how does it work because I noticed with the code below, the amount started reducing and the the array was zero

def countChange(amount, coins):
    if (amount == 0):
        print("amount is 0", amount, coins)
        return 1
    elif (amount < 0 or coins == []):

        print("amount is less than 0 or empty coins array", amount, coins)
        return 0
    else:
        countChange(amount, coins[:-1])
        print("first called")
        countChange(amount - coins[-1], coins)
        print("second called")

countChange(4, [1,2])

I really dont undertand how come this correct result happens.

from python3 myprogram_name.py
amount is less than 0 or empty coins array 4 []
first called
amount is less than 0 or empty coins array 3 []
first called
amount is less than 0 or empty coins array 2 []
first called
amount is less than 0 or empty coins array 1 []
first called
amount is 0 0 [1]
second called
second called
second called
second called
first called
amount is less than 0 or empty coins array 2 []
first called
amount is less than 0 or empty coins array 1 []
first called
amount is 0 0 [1]
second called
second called
first called
amount is 0 0 [1, 2]
second called
second called

as it goes back and forth

Answer 1

At first time,

countChange(4, [1,2])

the amount is not zero, and the coins array was not empty, else statement gets worked. Inside the else statement, there is a recursion call with,

countChange(4, [1])-------->opened

then again comes into else statement, and again recursion called,

countChange(4, [])----->opened

Now comes to the elif statement, prints the string and return zero called.so the final call of recursion going to be ended.

countChange(4, [])------>ended..

still we have remains three statement on that else block.

print("first called")
countChange(amount - coins[-1], coins)
print("second called")

So according to that, the "first called" string gets printed.And again we have a recursive call, Now we subtract the amount value by counts[-1], so amount = 4 - 1 = 3.

countChange(3, [1])----->opened

Similar to again, else statement gets activated, and again having a recursive call,

countChange(3, [])----->opened

This way, if you trace recursion, you can get cleared., Hope this helps you:)