Read content of txt files into lists to find duplicates

I'm new to Python.

My code should read 2 different .txt files into lists and compare them to find and delete duplicates.

Code

import os
dir = os.listdir
T = "Albums"
if T not in dir():
    os.mkdir("Albums")
with open('list.txt','w+') as f:
    linesA = f.readlines()
    print(linesA)   # output empty
with open('completed.txt','w+') as t:
    linesB = t.readlines()
    print(linesB)  # output empty
for i in linesA[:]:
    if i in linesB:
        linesA.remove(i)
print(linesA)
print(linesB)

I tried the code above with following inputs:

• in list.txt I wrote (on separate lines) A, B and C.
• in completed.txt I wrote (also on separate lines) A and B.

It should have first output the content of the lists, which were empty for some reasons.

Why are the read lists empty?

Does this help:

• I suggest using not os.path.exists(entry) instead of not entry in os.listdir(), it's not relevant for the problem, but I point it out anyway. (Also, you overwrote the built-in dir function)
• I've split up the file using split("\n")
• I've changed the way the files are opened to r+, this doesn't clear the file unlike w+.

Please note that if you want to use readlines you have to remove the new line for each entry.

import os

with open('list.txt','w+') as file:
    file.write("Foo\n")
    file.write("Bar")
with open('completed.txt','w+') as file:
    file.write("Bar\n")
    file.write("Python")

T = "Albums"
if not os.path.exists(T):
    os.mkdir("Albums")
with open('list.txt','r+') as f:
    linesA = f.read().split("\n")
    print(linesA)
with open('completed.txt','r+') as t:
    linesB = t.read().split("\n")
    print(linesB)
for entry in list(linesA):
    if entry in linesB:
        linesA.remove(entry)
print(linesA)
print(linesB)

Output:

['Foo', 'Bar']
['Bar', 'Python']
['Foo']
['Bar', 'Python']