Variable value is changing between print and append-CodePudding

The variable is changing and prints different value and it saves another If i run this code

def swap(string,x,y):
  string[y], string[x] = string[x], string[y]

def permutations(string ,i=0):
    if i == len(string):
        yield string

    for x in range(i, len(string)):
        perm = string
        swap(perm,x,i)
        yield from permutations(perm, i 1)
        swap(perm,i,x)

result = []
test = permutations(['a','b','c'])
for x in test:
    print(x)
    result.append(x)

print(result)

It prints this and i dont know why:

['a', 'b', 'c']
['a', 'c', 'b']
['b', 'a', 'c']
['b', 'c', 'a']
['c', 'b', 'a']
['c', 'a', 'b']
[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]

CodePudding user response：

You're mutating the same x in place, so only the final version of it is printed after the loop.

result.append(x) does not copy the object (x in this case), it just places a reference to it into the result list.

Do e.g. result.append(x[:]) or result.append(list(x)) to put copies of x into the result list.

CodePudding user response：

That's why the yielded list has the same references, so whenever you change it, the previous referenced value will be changed, too. The quick fix is to return a copy instance of the list.

def swap(string,x,y):
  string[y], string[x] = string[x], string[y]

def permutations(string ,i=0):
    if i == len(string):
        yield string.copy()

    for x in range(i, len(string)):
        perm = string
        swap(perm,x,i)
        yield from permutations(perm, i 1)
        swap(perm,i,x)

result = []
test = permutations(['a','b','c'])
for x in test:
    print(x)
    result.append(x)

print(result)