在for循环中追加一个列表？

Question

There are 2 given lists, suppose:

list1 = ['node1','node2','node3','node4']
list2 = ['goal1','goal2','goal3']

I require a list which returns:

result = [['node1','node2','node3','node4','goal1'],
       ['node1','node2','node3','node4','goal2'],
       ['node1','node2','node3','node4','goal3']]

Here is what I have:

result = []
for i in range (len(list2)):
    list1.append(list2[i])
    result.append(list1)
    list1.pop()

The problem is, result is not being appended with the desired value. It prints,

[['node1', 'node2', 'node3', 'node4'],
 ['node1', 'node2', 'node3', 'node4'],
 ['node1', 'node2', 'node3', 'node4']] 

after the for loop is completed.

What am I doing wrong?

Answer 1

You can add list1 each goal to the end to list1 using a list comp:

list1 = ['node1', 'node2', 'node3', 'node4']
list2 = ['goal1', 'goal2', 'goal3']
print([list1 + [gl] for gl in list2])

Output:

[['node1', 'node2', 'node3', 'node4', 'goal1'], 
['node1', 'node2', 'node3', 'node4', 'goal2'], 
['node1', 'node2', 'node3', 'node4', 'goal3']]

Answer 2

What your loop is doing is repeatedly appending the same list object ( list1 ) to result . You can verify that by doing something like list1.append(True) after running the loop and checking result again:

[['node1', 'node2', 'node3', 'node4', True], 
 ['node1', 'node2', 'node3', 'node4', True], 
 ['node1', 'node2', 'node3', 'node4', True]]

What you want to do is instead make a copy of list1 to append each time, such as:

for i in range (len(list2)):
    list1.append(list2[i])
    result.append(list(list1))
    list1.pop()

Although I would probably instead make use of list concatenation, which implicitly makes a new list:

for item in list2:
    result.append(list1 + [item])

Answer 3

You can fix this using extend :

>>> for i in range(len(list2)):
    tmp = []
    tmp.extend(list1)
    tmp.append(list2[i])
    all_paths.append(tmp)


>>> all_paths
[['node1', 'node2', 'node3', 'node4', 'goal1'], ['node1', 'node2', 'node3', 'node4', 'goal2'], ['node1', 'node2', 'node3', 'node4', 'goal3']]