Files: open, read, write, close

You will be able to

• ✓ See open, read, write and close (or with) as the laptop edge around a file
• ✓ Treat a CSV in memory as a list of labelled rows
• ✓ Filter a table without destroying the only copy of this morning’s export

A file is text until you parse it

The attendance CSV on the lab PC is bytes on disk. open opens a handle, read pulls text, write sends text back, close (or with) releases the handle so Windows does not keep a lock. None of that runs in this browser box. You still need the idea: the disk is the edge; the program’s honest middle is a list of dictionaries — each row already split into roll, name, present.

with open(path) as f: is the laptop spelling that closes even if a later line fails. A forgotten close is how a nightly job leaves a 0-byte file for the clerk. Practise the table here; copy the open sample onto a laptop inside a folder you control.

path = "attendance.csv"
with open(path, encoding="utf-8") as f:
    text = f.read()
print(len(text))

with open("keepers.csv", "w", encoding="utf-8") as out:
    out.write("roll,ok\n12,1\n")

The in-memory twin is a list of rows

Header plus rows becomes keys on each dict. You already know how to read one dict. A table is many of them. Filter by building a new list of keepers with append. Leave the raw list alone — it is the only copy of this morning’s file until a laptop write replaces the disk.

“Save” in this sandbox is a status print: Wrote, plus the keepers you would have sent. Do not pretend print is a USB stick. Do remember that a function should return the table; the edge decides the path.

Words that matter

Handle

The open file object — close it, or use with.

Row

One labelled record; a dict in this course, a CSV line on disk.

Table

A list of rows — the in-memory shape of a small CSV.

Common mistakes

On a full Python install — open / pathlib

Disk I/O lives at the edge. Parse into rows, then the rest of the program never sees a path.

stdlib — no pip

Read, parse mentally, write keepers.

from pathlib import Path

raw = Path("upi.csv").read_text(encoding="utf-8")
lines = raw.splitlines()
# parse header + rows on a laptop, then:
# Path("keepers.csv").write_text(out, encoding="utf-8")
print("chars", len(raw))

Example program — Present students as a new table

Source stays. present is what you would write back on a laptop.

Your turn — Mark the load

rows already “arrived”. Print Loaded and the first roll 12 (not the whole list).

Self-assessment

Check your understanding before you mark the lesson complete.