Python — Read Files

The whole file at once

with open("notes.txt", encoding="utf-8") as f:
    text = f.read()          # one big string

print(len(text))             # character count

Fine for small files (config, templates). A 2GB log would eat your RAM.

Line by line — the memory-safe default

The file object IS iterable:

with open("big.log", encoding="utf-8") as f:
    for line in f:           # streams one line at a time
        if "ERROR" in line:
            print(line.rstrip())   # rstrip removes the trailing \n

Handles gigabyte files with constant memory.

readlines vs readline

f.readlines()    # → ["line1\n", "line2\n"]  list of all (memory!)
f.readline()     # → next single line each call; "" when done

Cleaning newlines

Every line keeps its \n:

for line in f:
    line = line.strip()          # kills \n AND surrounding spaces
    # or keep interior spacing with .rstrip("\n")

Parsing structured lines

scores = {}
with open("scores.csv") as f:
    header = f.readline()                    # skip header row
    for line in f:
        name, score = line.strip().split(",")
        scores[name] = int(score)

Chunked binary reads

with open("movie.mp4", "rb") as src:
    while chunk := src.read(1024 * 1024):    # 1MB at a time (walrus!)
        process(chunk)

The walrus operator assigns and tests in one expression — clean copy loops.

Reading JSON/CSV the smart way

Structured formats have dedicated readers:

import json
config = json.load(open("config.json"))      # parses for you

import csv
with open("users.csv", newline="") as f:
    for row in csv.DictReader(f):            # dicts per row!
        print(row["email"])

Which reader when

Need	Method
Config/template, small file	read()
Huge/streaming logs	for line in f
List of lines to mutate	readlines()
Binary media	read(chunk) loop
CSV/JSON data	csv / json modules

Mini Practice

1. Count words across a whole text file.
2. Print only lines containing "TODO".
3. Parse a two-column file into a dict.
4. Copy a file 1MB-chunk-at-a-time.
5. Load a JSON config with defaults on FileNotFoundError.

Next: write/create files →