bash-like heredoc for data input

Sometimes it would be handy to test a script that reads data from a file using inline data (so that both the data and the code would be in the same file). In bash this can be done with a heredoc:

while read l;do
    echo $l
done << EOF
test
test2
test3
EOF

In some real code there would of course happen something more apart from writing out the lines. Suppose I would do something similar in python:

def read_file(f):
    for line in f.readlines():
        print(line.replace('\n',''))

with open('input.txt') as f:
    read_file(f)

What would be the best way to provide the content of input.txt to read_file() inline?