Python — File Open

open() — the gateway

f = open("notes.txt")          # default mode "r" = read text
content = f.read()
f.close()                      # must remember!

with — the only way you should open files

The context manager closes automatically, even on errors:

with open("notes.txt") as f:
    content = f.read()
# file closed here — no matter what happened inside

Never write manual open/close in real code; with is the idiom.

The mode argument

Mode	Meaning
"r"	read (default) — fails if missing
"w"	write — erases existing content!
"a"	append to the end
"x"	create new; fail if exists
"r+"	read + write
"rb" / "wb"	raw bytes (images, binaries)

with open("log.txt", "a") as f:    # append, don't clobber
    f.write("new entry\n")

Encoding — set it explicitly

with open("data.txt", encoding="utf-8") as f:
    ...

Windows defaults differ from other OSes; declaring utf-8 makes files portable and keeps é/中/emoji intact.

Missing files raise

try:
    with open("ghost.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("No such file")

Listing a folder

import os

os.listdir(".")                       # names only

from pathlib import Path              # the modern path API
for p in Path(".").glob("*.txt"):
    print(p.name)

pathlib.Path is preferred for anything beyond quick scripts:

path = Path("data") / "users.csv"     # OS-safe joining
path.exists()

Paths & locations

Path("notes.txt").resolve()      # absolute location
Path.home()                      # user's home dir

Gotchas: "w" truncates instantly on open · relative paths depend on WHERE you ran Python, not where the script lives (Path(__file__).parent anchors it) · forgetting encoding → mojibake on Windows.

Mini Practice

1. Read any .py file; print its line count.
2. Append three lines; reopen and verify.
3. Trigger FileNotFoundError; catch it gracefully.
4. List all .md files in a folder via pathlib glob.
5. Prove "w" erases: write, reopen with w, check empty.

Next: read files →