问这种递归函数是如何产生所有排列的呢？

EN

def permute(data, i, length): 
    # If "i" is at the end, we're done with one permutation: print the result.
    if i==length: 
        print(''.join(data) )

    else:
        # for each remaining character (later in the string)
        for j in range(i,length):

            # Swap that character with the "ith" character
            data[i], data[j] = data[j], data[i]

            # Keeping data[0:i] fixed,
            # generate all permutations of the remainder of the string
            permute(data, i+1, length)

            # Swap the characters back
            data[i], data[j] = data[j], data[i]

indent = ""

def permute(data, i, length): 
    global indent
    print(indent, "ENTER", ''.join(data), i)
    indent += "  "

    if i==length: 
        print("PERMUATATION:", ''.join(data) )
    else: 
        for j in range(i,length): 
            print(indent, "SWAP", i, j)
            #swap
            data[i], data[j] = data[j], data[i] 
            permute(data, i+1, length) 
            data[i], data[j] = data[j], data[i]  

    indent = indent[2:]


string = "ABC"
n = len(string) 
data = list(string) 
permute(data, 0, n)

 ENTER ABC 0
   SWAP 0 0
   ENTER ABC 1
     SWAP 1 1
     ENTER ABC 2
       SWAP 2 2
       ENTER ABC 3
PERMUATATION: ABC
     SWAP 1 2
     ENTER ACB 2
       SWAP 2 2
       ENTER ACB 3
PERMUATATION: ACB
   SWAP 0 1
   ENTER BAC 1
     SWAP 1 1
     ENTER BAC 2
       SWAP 2 2
       ENTER BAC 3
PERMUATATION: BAC
     SWAP 1 2
     ENTER BCA 2
       SWAP 2 2
       ENTER BCA 3
PERMUATATION: BCA
   SWAP 0 2
   ENTER CBA 1
     SWAP 1 1
     ENTER CBA 2
       SWAP 2 2
       ENTER CBA 3
PERMUATATION: CBA
     SWAP 1 2
     ENTER CAB 2
       SWAP 2 2
       ENTER CAB 3
PERMUATATION: CAB