Arrays Merging Instead of Appending in Python

Question

I'm attempting to append these two arrays as they're built but am getting unexpected results in Python:

This = []
That = []

This.append('A')
That.append(This)

This.append('B')
That.append(This)

print(That)

Expected results: [['A'],['A','B']]

Actual results: [['A','B'],['A','B']]

Whereas this works as expected:

That = []

This = ['A']
That.append(This)

This2 = ['A', 'B']
That.append(This2)

print(That)

Expected results: [['A'],['A','B']]

Actual results: [['A'],['A','B']]

Answer 1

You have to think of the "this" as just one object, if you want 2 different objects in you have to copy a copy of this like That.append(This.copy())

https://docs.python.org/3/library/copy.html

Answer 2

The problem arises on line 7 of your code. When I am updating This array the second time with This.append('B'), it is automatically updated in That array.

Similar issue:

Python list updating elements when appending

Hope this helps.

Answer 3

The reason you're getting unexpected results is because both This and That are referring to the same list object. When you append This to That, you're appending a reference to the same list object. So when you modify This, it reflects in both places where it's referenced.

To achieve the expected results, you need to make a copy of This when appending it to That, so that modifications to This don't affect the elements already appended to That.

This = []
That = []

This.append('A')
That.append(This.copy())

This.append('B')
That.append(This.copy())

print(That)

Answer 4

To understand this, add print statement after each line and see the output. Explanation given

This = []
That = []

This.append('A')
print(This)  #Output : ['A'] 
#Now This has become ['A]


# appending This to that. (i.e) appending ['A'] to [] should give [['A']]
That.append(This)
print(That) # Output : [['A']]


# This is ['A']. If 'B' is appended, this becomes ['A', 'B']. But this is already inside that. that becomes [['A', 'B']]
This.append('B')
print(This) # Output : ['A', 'B']
print(That)

#Now  that is [['A', 'B']]. this is ['A', 'B']. appending this to that, that becomes [['A', 'B'], ['A', 'B']]
That.append(This)
print(That)

So if at all you want to use the same object after appending but without change in appended list, create a copy of this

This = []
That = []
This.append('A')
That.append(This)
There = This.copy()
There.append('B')
That.append(There) 
print(That) #Output: [['A'], ['A', 'B']]