Unlike the C programming language, Python assignments are statements, not expressions. Therefore the common C idiom of:

while (line = fgets(f)) {
    /* do something with the line */
}

is not possible to implement directly.

However, effbot.org provides several alternatives using iterators or the while True idiom. I recently discovered the built-in iter() function as well:

for line in iter(f.readline, "END"):
    ... do something with line ...