Reading and writing files

open, read, close — without the close

open() gives you a file object. Used with with, Python closes it for you even if something raises:

with open("notes.txt", "w") as f:
    f.write("first line\n")

with open("notes.txt") as f:
    content = f.read()

The mode is the second argument: "r" read (the default), "w" write — which truncates the file to nothing first — and "a" append.

Three ways to read:

• f.read() — the whole thing as one string
• f.readlines() — a list of lines, newlines included
• for line in f: — one line at a time, which is the only option that works on a file bigger than memory

line.rstrip("\n") is the usual way to drop the trailing newline.

In PyLearn these files live in Pyodide's in-memory filesystem, so nothing touches your real disk.

Your turn: write "alpha", "beta" and "gamma" to words.txt, one per line, then read them back into lines with no trailing newlines.

Processing a file

The everyday shape: read the file, ignore the blank lines, do something with the rest.

Your turn: write count_lines(path) returning how many non-blank lines a file contains. A line of nothing but spaces counts as blank. Return 0 if the file does not exist.