Search if all string in .txt

I am trying to run a program to look at a .txt file and if, else depending on the content

I thought would be

Searhterms = [A, B]
with('output.txt') as f:

    if ('A' and 'B') in f.read():
        print('mix')
    eLif ('A') in f.read:
        if ('B') not in f.read:
            print('ONLY A')
    elif ('B') in f.read():
        if ('A') not in f.read:
            print('ONLY B') 
    else:
        if ('A' and 'B') not in f.read:
            print('NO AB)

But if A and B present it works, but if only one it skips to the else. I am getting more confused about the longer I look at this.

You'd better use this:

Searhterms = [A, B]  # not sure why you need this

with('output.txt') as fin :  # nice name for the INPUT file, btw
    lines = fin.readlines()

for line in lines :
    if ('A' in line) and ('B' in line):
        print('mix')
    eLif 'A' in line:  # nice uppercase 'L', will puzzle the python
        #if 'B' not in line:    # don't need this
        print('ONLY A')
    elif 'B' in line:
        #if 'A' not in line:    # don't need this
        print('ONLY B') 
    else:
        #if ('A' and 'B') not in f.read:   # this condition is not required
        print('NO AB')

if len(lines) == 0 :
    print('empty file')