Python File IO

Opening a file

file = open("path", "<mode>")

Mode	Description
r	read
w	write (creates or overwrites)
a	append (creates or adds to the end)

Reading

file.readline() returns one line (including its trailing \n, except possibly the last line of the file), then '' forever once exhausted. A for-loop iterates line by line and is the idiomatic way to consume a whole file:

with open("path", "r") as file:
    for line in file:
        ...

Reading always gives back strings — cast (int(...), float(...), …) as needed.

A file-pointer only moves forward: once a for loop (or repeated readline() calls) has consumed a file, iterating again yields nothing further, unless the file is reopened (or the pointer is seeked back to the start).

Closing

file.close() releases the file; forgetting to close leaves it vulnerable to side effects (missing or extra data). with open(...) as file: closes it automatically — even if the block raises an exception — so it’s preferred over manual open/close.

Writing and appending

with open("path", "w") as file:
    file.write("a single string\n")
    file.writelines(["one string\n", "per element\n"])

"w" overwrites any existing file; "a" instead appends to the end without touching existing content.