How to generate permutations of a list without “reverse duplicates” in Python using generators

Question

This is related to question How to generate all permutations of a list in Python

How to generate all permutations that match following criteria: if

Answer 1

Here is code that does the trick. To get rid of the dups I noticed that for your list if the value of the first location is greater than the value of the last location then it will be a dup. I create a map to keep track of where each item was in the list to start with and then use that map to do the test. The code also does not use recursion so it keeps its memory footprint small. Also the list can be of any type items not just numbers see the last two test cases.

Here is the code.

class Permutation:
    def __init__(self, justalist):
        self._data = justalist[:]
        self._len=len(self._data)
        self._s=[]
        self._nfact=1
        self._map ={}
        i=0
        for elem in self._data:
            self._s.append(elem)
            self._map[str(elem)]=i
            i+=1
            self._nfact*=i
        if i != 0:
            self._nfact2=self._nfact//i

    def __iter__(self):
        for k in range(self._nfact):
            for i in range(self._len):
                self._s[i]=self._data[i]
            s=self._s
            factorial=self._nfact2
            for i in range(self._len-1):
                tempi = (k // factorial) % (self._len - i)
                temp = s[i + tempi]
                for j in range(i + tempi,i,-1):
                    s[j] = s[j-1]
                s[i] = temp
                factorial //= (self._len - (i + 1))

            if self._map[str(s[0])] < self._map[str(s[-1])]:
                yield s




s=[1,2]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

s=[1,2,3]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

s=[1,2,3,4]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

s=[3,2,1]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

s=["Apple","Pear","Orange"]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

s=[[1,4,5],"Pear",(1,6,9),Permutation([])]
print("_"*25)
print("input list:",s)
for sx in Permutation(s):
    print(sx)

and here is the output for my test cases.

_________________________
input list: [1, 2]
[1, 2]
_________________________
input list: [1, 2, 3]
[1, 2, 3]
[1, 3, 2]
[2, 1, 3]
_________________________
input list: [1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 4, 3]
[1, 3, 2, 4]
[1, 3, 4, 2]
[1, 4, 2, 3]
[1, 4, 3, 2]
[2, 1, 3, 4]
[2, 1, 4, 3]
[2, 3, 1, 4]
[2, 4, 1, 3]
[3, 1, 2, 4]
[3, 2, 1, 4]
_________________________
input list: [3, 2, 1]
[3, 2, 1]
[3, 1, 2]
[2, 3, 1]
_________________________
input list: ['Apple', 'Pear', 'Orange']
['Apple', 'Pear', 'Orange']
['Apple', 'Orange', 'Pear']
['Pear', 'Apple', 'Orange']
_________________________
input list: [[1, 4, 5], 'Pear', (1, 6, 9), <__main__.Permutation object at 0x0142DEF0>]
[[1, 4, 5], 'Pear', (1, 6, 9), <__main__.Permutation object at 0x0142DEF0>]
[[1, 4, 5], 'Pear', <__main__.Permutation object at 0x0142DEF0>, (1, 6, 9)]
[[1, 4, 5], (1, 6, 9), 'Pear', <__main__.Permutation object at 0x0142DEF0>]
[[1, 4, 5], (1, 6, 9), <__main__.Permutation object at 0x0142DEF0>, 'Pear']
[[1, 4, 5], <__main__.Permutation object at 0x0142DEF0>, 'Pear', (1, 6, 9)]
[[1, 4, 5], <__main__.Permutation object at 0x0142DEF0>, (1, 6, 9), 'Pear']
['Pear', [1, 4, 5], (1, 6, 9), <__main__.Permutation object at 0x0142DEF0>]
['Pear', [1, 4, 5], <__main__.Permutation object at 0x0142DEF0>, (1, 6, 9)]
['Pear', (1, 6, 9), [1, 4, 5], <__main__.Permutation object at 0x0142DEF0>]
['Pear', <__main__.Permutation object at 0x0142DEF0>, [1, 4, 5], (1, 6, 9)]
[(1, 6, 9), [1, 4, 5], 'Pear', <__main__.Permutation object at 0x0142DEF0>]
[(1, 6, 9), 'Pear', [1, 4, 5], <__main__.Permutation object at 0x0142DEF0>]