Keeping a value the same despite changing the variable it was equated to

[Yoriz]

The list that is referenced by currentPath that is appended to the list referenced by completedPaths is not a copy of the list it is the same list.

currentPath = ["EWR", "JFK", "SFO"]
completedPaths = []
 
completedPaths.append(currentPath)
print(id(currentPath))
print(id(completedPaths[0]))

Output:
2794339074624
2794339074624

You can view this here this visualizer will show you that the same list object is being referenced by currentPath and index 0 of completedPaths.

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If you don't want this behaviour rather than passing the reference to the list pass a copy by using a slice of all items

currentPath = ["EWR", "JFK", "SFO"]
completedPaths = []
 
completedPaths.append(currentPath[:])
print(completedPaths)
 
currentPath.clear()
print(completedPaths)

Output:
[['EWR', 'JFK', 'SFO']]
[['EWR', 'JFK', 'SFO']]

or by using the copy module

import copy

currentPath = ["EWR", "JFK", "SFO"]
completedPaths = []
 
completedPaths.append(copy.copy(currentPath))
print(completedPaths)
 
currentPath.clear()
print(completedPaths)

Output:
[['EWR', 'JFK', 'SFO']]
[['EWR', 'JFK', 'SFO']]

or the list itself's copy method

completedPaths.append(currentPath.copy())

Both of these are shallow copies, the copy module also has copy.deepcopy() for list within list.