Different types of passing parameters to a recursive function-CodePudding

The below code worked perfectly fine.

        def perm(cur):
            avail = [i for i in nums if not i in cur]
            if len(avail) == 0:
                res.append(cur.copy())
                return
            
            for i in avail:
                cur.append(i)
                perm(cur)
                cur.pop()
        
        perm([])
        return res

However, the below one returned error

         def perm(cur):
            avail = [i for i in nums if not i in cur]
            if len(avail) == 0:
                res.append(cur.copy())
                return
            
            for i in avail:
                perm(cur.append(i))
                cur.pop()
        
        perm([])
        return res

TypeError: argument of type 'NoneType' is not iterable
    avail = [i for i in nums if not i in cur]
Line 5 in <listcomp> (Solution.py)
    avail = [i for i in nums if not i in cur]
Line 5 in perm (Solution.py)
    perm(cur.append(i))
Line 12 in perm (Solution.py)
    perm([])
Line 15 in permute (Solution.py)
    ret = Solution().permute(param_1)
Line 38 in _driver (Solution.py)
    _driver()
Line 49 in <module> (Solution.py)

I'm trying to understand the logic behind this.

I'm not sure of the difference between both ways of parameter passing.

CodePudding user response：

.append returns None.

def perm(cur):
        avail = [i for i in nums if not i in cur]
        if len(avail) == 0:
            res.append(cur.copy())
            return
        
        for i in avail:
            // Here .append(i) does not return anything so the cur you are passing in the perm funcion is None
            perm(cur.append(i))
            cur.pop()
    
    perm([])
    return res

CodePudding user response：

Function cur.append(i) doesn't return anything. It only appends i to cur list.

Then when you call perm with it as an argument, you basically calling perm(None), which is not iterable. Therefore [i for i in nums if not i in cur] fails.