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07-01: File I/O — Reading and Writing Files

Opening a File

file = open(path, mode, encoding=...)
Mode Meaning
"r" Read (default). Error if file doesn't exist
"w" Write. Creates file or overwrites if exists
"a" Append. Creates or appends to existing file
"x" Exclusive create. Error if file already exists
"r+" Read and write
"b" Binary mode (add to others: "rb", "wb")
"t" Text mode (default, add to others: "rt", "wt")

The with statement ensures the file is always closed, even if an exception occurs:

with open("myfile.txt", "rt", encoding="utf-8") as f:
    content = f.read()
# File is automatically closed here

# Without with (error-prone)
f = open("myfile.txt", "rt")
content = f.read()
f.close()   # easy to forget!

Reading Files

read() — Read entire file as one string

with open("poem.txt", "rt", encoding="utf-8") as f:
    content = f.read()
    print(content)
    print(f"Length: {len(content)} characters")

readlines() — Read all lines into a list

with open("data.txt", "rt") as f:
    lines = f.readlines()
    # Each line includes the trailing \n
    print(lines)   # ['line1\n', 'line2\n', 'line3\n']
    print(len(lines), "lines")

readline() — Read one line at a time

with open("data.txt", "rt") as f:
    line1 = f.readline()   # reads first line
    line2 = f.readline()   # reads second line
    print(repr(line1))
    print(repr(line2))

Iterating line by line (most efficient)

with open("data.txt", "rt", encoding="utf-8") as f:
    for line in f:
        line = line.rstrip("\n")   # remove trailing newline
        print(line)

Reading into a list with comprehension

with open("data.txt", "rt") as f:
    lines = [line.rstrip("\n") for line in f]
    # Or strip all whitespace
    lines = [line.strip() for line in f if line.strip()]  # skip empty lines

Writing Files

write() — Write a string

with open("output.txt", "wt", encoding="utf-8") as f:
    f.write("Hello, World!\n")
    f.write("Second line\n")
    count = f.write("Third line\n")   # returns number of chars written
    print(count)   # 11

writelines() — Write a list of strings

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "wt") as f:
    f.writelines(lines)  # note: does NOT add \n automatically

Using print() to write to file

with open("output.txt", "wt") as f:
    print("Hello!", file=f)          # adds \n by default
    print("Value:", 42, file=f)
    print("A", "B", "C", sep=",", file=f)

Appending to a File

with open("log.txt", "at", encoding="utf-8") as f:
    f.write("New log entry\n")
    f.write(f"Timestamp: {__import__('datetime').datetime.now()}\n")

Copying a File

with open("source.txt", "rt", encoding="utf-8") as fin, \
     open("destination.txt", "wt", encoding="utf-8") as fout:
    for line in fin:
        fout.write(line)

Or copy all at once: 

with open("source.txt", "rt") as fin:
    content = fin.read()
with open("destination.txt", "wt") as fout:
    fout.write(content)

Working with CSV Files

Reading CSV manually

with open("students.csv", "rt", encoding="utf-8") as f:
    for line in f:
        cols = line.strip().split(",")
        name, age, grade = cols
        print(f"{name} is {age} years old, grade: {grade}")

import csv

# Reading
with open("students.csv", "rt", encoding="utf-8", newline="") as f:
    reader = csv.reader(f)
    header = next(reader)     # skip header
    for row in reader:
        print(row)            # ['Alice', '22', 'A']

# Reading as dicts
with open("students.csv", "rt", encoding="utf-8", newline="") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["grade"])

# Writing
data = [["Alice", 22, "A"], ["Bob", 21, "B"]]
with open("out.csv", "wt", encoding="utf-8", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["Name", "Age", "Grade"])   # header
    writer.writerows(data)

Working with JSON Files

import json

# Write JSON
data = {"name": "Alice", "age": 30, "scores": [85, 92, 78]}
with open("data.json", "wt", encoding="utf-8") as f:
    json.dump(data, f, indent=2)

# Read JSON
with open("data.json", "rt", encoding="utf-8") as f:
    loaded = json.load(f)
print(loaded["name"])   # Alice
print(loaded["scores"]) # [85, 92, 78]

# JSON string ↔ Python object
json_str = json.dumps(data, indent=2)    # Python → JSON string
data2 = json.loads(json_str)             # JSON string → Python

File Position

with open("test.txt", "rt") as f:
    print(f.tell())        # 0 — at beginning
    chunk = f.read(5)      # read 5 chars
    print(f.tell())        # 5

    f.seek(0)              # go back to beginning
    content = f.read()     # read from start

    f.seek(0, 2)           # seek to end (0=begin, 1=current, 2=end)
    print(f.tell())        # file size in bytes

Error Handling with Files

def read_file(path):
    try:
        with open(path, "rt", encoding="utf-8") as f:
            return f.read()
    except FileNotFoundError:
        print(f"File not found: {path}")
        return None
    except PermissionError:
        print(f"Permission denied: {path}")
        return None
    except UnicodeDecodeError:
        print(f"Encoding error — try a different encoding")
        return None
    except OSError as e:
        print(f"OS error: {e}")
        return None

Checking File Existence

import os
import os.path

path = "myfile.txt"

# Check if path exists
print(os.path.exists(path))    # True or False

# Check if it's a file (not directory)
print(os.path.isfile(path))    # True

# Check if it's a directory
print(os.path.isdir("mydir"))  # True

# Get file size
print(os.path.getsize(path))   # size in bytes

Using pathlib (modern, recommended):

from pathlib import Path

p = Path("myfile.txt")
print(p.exists())      # True/False
print(p.is_file())     # True/False
print(p.stat().st_size)  # file size

# Create parent directories
p.parent.mkdir(parents=True, exist_ok=True)

# Read/write with pathlib
content = p.read_text(encoding="utf-8")
p.write_text("new content", encoding="utf-8")

Working with Binary Files

# Read binary file
with open("image.png", "rb") as f:
    header = f.read(8)    # read first 8 bytes
    print(header.hex())

# Write binary file
data = bytes([0x89, 0x50, 0x4E, 0x47])
with open("test.bin", "wb") as f:
    f.write(data)

Practical Examples

Log file writer

from datetime import datetime

def log(message, level="INFO", logfile="app.log"):
    timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    entry = f"[{timestamp}] [{level}] {message}\n"
    with open(logfile, "at", encoding="utf-8") as f:
        f.write(entry)

log("Application started")
log("User logged in: Alice")
log("Database connection failed", level="ERROR")

Word frequency counter (from file)

from collections import Counter

with open("document.txt", "rt", encoding="utf-8") as f:
    text = f.read().lower()

# Clean and count
words = text.split()
words = [w.strip(".,!?;:\"'()-") for w in words]
freq = Counter(words)

print("Top 10 words:")
for word, count in freq.most_common(10):
    print(f"  {word}: {count}")

Simple config file reader

def read_config(path):
    config = {}
    with open(path, "rt") as f:
        for line in f:
            line = line.strip()
            if not line or line.startswith("#"):  # skip empty/comments
                continue
            key, _, value = line.partition("=")
            config[key.strip()] = value.strip()
    return config

# config.txt:
# host = localhost
# port = 8080
# debug = true
cfg = read_config("config.txt")
print(cfg["host"])   # localhost

Quick Summary

Operation Code
Open for reading open(path, "rt", encoding="utf-8")
Open for writing open(path, "wt", encoding="utf-8")
Open for appending open(path, "at", encoding="utf-8")
Read all f.read()
Read lines into list f.readlines()
Read line-by-line for line in f:
Write string f.write(text)
Write list of lines f.writelines(lines)
Write with print print(text, file=f)
Auto-close with open(...) as f:

Exercises: 07-01: Exercises — File I/O

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