Need help with .replace()

[jasonikakis]

Greetings,im really new to python and im trying to create a simple encryption program (basically ill input a text and it'll get converted to numbers, nothing complicated for now). Heres my code so far:

a = 1
b = 2
c = 3
d = 4
e = 5
f = 6
g = 7
h = 8
#just random letters and their respective numbers

fr = open('tryy.txt', 'r')
fw = open('tryy.txt', 'a')

for elem in fr:
    fw.write(elem.replace('a', str(a))) & fw.write(elem.replace('b', str(b)))

fr.close()
fw.close()

the problem is that when i try using this, (my basic example text is just "ab"), instead of converting both characters to "12" as it should, the program returns a text file like this:

ab

1b

a2

any advice on how to correct the code here?