Basics (Opening & Closing, File Modes)

Files are opened with open(filename, mode), which gives you back a file object to read or write through. The mode tells Python what you intend to do:

• "r" — read (the default); raises FileNotFoundError if the file doesn't exist
• "w" — write; creates the file if needed, and erases any existing content first
• "a" — append; creates the file if needed, and adds new content after whatever's already there

f = open("notes.txt", "r")
data = f.read()
f.close()
print(data)

Closing a file with close() matters — Python buffers what you write and doesn't always save it to disk immediately, so forgetting to close a file can mean losing data.

With Clause

with open(...) as f: closes the file automatically once the block finishes — even if an error happens partway through — so you don't have to remember to call close() yourself.

with open("notes.txt", "r") as f:
    data = f.read()

print(data)

This is the recommended way to work with files for exactly that reason: it's safer, and one line shorter.

Text Files

For reading, read() grabs the whole file as one string, readline() grabs just the next line, and readlines() returns every line as a list of strings.

with open("notes.txt", "r") as f:
    print(f.readlines())

For writing, write() writes a single string, and writelines() writes a list of strings all at once. Remember that "w" mode erases the file first — use "a" if you want to add on to what's already there instead.

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

File Pointers

Every open file has a pointer marking where the next read or write will happen. tell() reports its current position, and seek(offset) moves it.

with open("notes.txt", "r") as f:
    f.read(5)
    print(f.tell())
    f.seek(0)
    print(f.read(5))

Binary Files & Pickle

Binary files store raw bytes instead of text, so the regular read()/write() functions don't work on Python objects directly. The pickle module bridges that gap — it serializes objects like lists and dictionaries into bytes to write, and deserializes them back when reading. File modes need a "b", e.g. "wb" or "rb".

import pickle

data = ["ByteWise", "on", "top"]
with open("data.dat", "wb") as f:
    pickle.dump(data, f)

import pickle

with open("data.dat", "rb") as f:
    loaded = pickle.load(f)

print(loaded)

Trying to load() past the last object saved raises an EOFError, so files with multiple saved objects are usually read in a loop wrapped in a try/except.

CSV Files

CSV (Comma-Separated Values) files store tabular data as plain text, with a comma separating each field by default. The csv module provides a writer and a reader object to handle the formatting for you.

import csv

with open("people.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["Name", "Age"])
    writer.writerow(["ByteWise", 5])

import csv

with open("people.csv", "r") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Notice everything comes back as a string, even the age — convert it with int() or float() if you need to do math with it.