shell.py

import lexer

txt = ''
print("Example: (test.tst)")
file = input('Type in the file name > ')

with open(file, 'r') as file:
    txt = file.read()

result, error = lexer.run(txt)

file = open('output.tst', 'w+')
file.write(str(result))
file.close()

if error: print(error.astr())
else: print("Created file 'output.tst' in file directory")