File handling

Reading and writing files without leaking handles.

open and with

open(path, mode) returns a file object. Wrapping it in with closes the file even if the code inside raises, which is why it is the only form worth writing.

Modes: "r" read, "w" overwrite, "a" append, "x" create and fail if it exists.
Add encoding="utf-8" explicitly, because the default depends on the machine.

Reading

read() gives the whole file as one string and readlines() gives a list. Looping the file object itself reads one line at a time, which is what you want for anything large.