Python File Handling

# open(file, mode='r', encoding=None)
# Always specify encoding for text files

# 'r' - read (default): file must exist
with open("data.txt", "r", encoding="utf-8") as f:
    content = f.read()

# 'w' - write: creates or overwrites the file
with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello, World!\n")

# 'a' - append: creates or adds to end of file
with open("log.txt", "a", encoding="utf-8") as f:
    f.write("New entry\n")

# 'x' - exclusive creation: fails if file exists
try:
    with open("new_file.txt", "x", encoding="utf-8") as f:
        f.write("Created fresh")
except FileExistsError:
    print("File already exists")

# 'r+' - read and write (file must exist)
with open("data.txt", "r+", encoding="utf-8") as f:
    content = f.read()
    f.seek(0)           # go back to start
    f.write("NEW: " + content)

# Binary modes: add 'b' to any mode
# 'rb', 'wb', 'ab', 'rb+'
with open("image.png", "rb") as f:
    header = f.read(8)  # read first 8 bytes

from pathlib import Path

# Create test file
Path("sample.txt").write_text("Line 1\nLine 2\nLine 3\n", encoding="utf-8")

# read() - entire file as one string
with open("sample.txt", encoding="utf-8") as f:
    content = f.read()
    print(repr(content))    # 'Line 1\nLine 2\nLine 3\n'

# readlines() - list of lines (includes \n)
with open("sample.txt", encoding="utf-8") as f:
    lines = f.readlines()
    print(lines)    # ['Line 1\n', 'Line 2\n', 'Line 3\n']

# readline() - one line at a time
with open("sample.txt", encoding="utf-8") as f:
    first  = f.readline()   # 'Line 1\n'
    second = f.readline()   # 'Line 2\n'

# Iterate directly - most memory-efficient for large files
with open("sample.txt", encoding="utf-8") as f:
    for line in f:
        print(line.strip())     # removes trailing \n

# Read and strip lines
with open("sample.txt", encoding="utf-8") as f:
    lines = [line.strip() for line in f if line.strip()]

# Read with error handling
try:
    with open("missing.txt", encoding="utf-8") as f:
        data = f.read()
except FileNotFoundError:
    data = ""
except PermissionError as e:
    print(f"No permission: {e}")
    data = ""

# Read with seek / tell (file position)
with open("sample.txt", encoding="utf-8") as f:
    print(f.tell())     # 0 - position at start
    chunk = f.read(7)   # read 7 characters
    print(chunk)        # 'Line 1\n'
    print(f.tell())     # 7
    f.seek(0)           # back to start
    print(f.read(6))    # 'Line 1'

# write() - write a string
with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello, World!\n")
    f.write("Second line\n")

# writelines() - write a list of strings (no auto newlines)
lines = ["Alice\n", "Bob\n", "Charlie\n"]
with open("names.txt", "w", encoding="utf-8") as f:
    f.writelines(lines)

# Append to file
with open("log.txt", "a", encoding="utf-8") as f:
    import datetime
    timestamp = datetime.datetime.now().isoformat()
    f.write(f"[{timestamp}] Application started\n")

# Write formatted data
data = [
    {"name": "Alice", "score": 95},
    {"name": "Bob",   "score": 87},
]
with open("results.txt", "w", encoding="utf-8") as f:
    f.write("Name     Score\n")
    f.write("-" * 16 + "\n")
    for row in data:
        f.write(f"{row['name']:<8} {row['score']}\n")

# print() can write to a file
with open("debug.txt", "w", encoding="utf-8") as f:
    print("Debug output:", file=f)
    print("Value:", 42, file=f)

# Atomic write - write to temp file, then rename (safe)
import os
import tempfile

def atomic_write(filepath, content, encoding="utf-8"):
    """Write content to filepath atomically."""
    dir_name = os.path.dirname(os.path.abspath(filepath))
    with tempfile.NamedTemporaryFile(
        mode="w", encoding=encoding,
        dir=dir_name, delete=False, suffix=".tmp"
    ) as tmp:
        tmp.write(content)
        tmp_name = tmp.name
    os.replace(tmp_name, filepath)  # atomic on most OS

# Binary mode - no encoding, bytes not str
import struct

# Write binary data
with open("data.bin", "wb") as f:
    f.write(b"\x89PNG\r\n\x1a\n")      # write bytes literal
    f.write(bytes([72, 101, 108, 108, 111]))  # bytes from list of ints

# Read binary data
with open("data.bin", "rb") as f:
    header = f.read(4)                  # first 4 bytes
    print(header)                       # b'\x89PNG'
    rest   = f.read()                   # rest of file

# Check file signature (magic bytes)
def is_png(filepath):
    with open(filepath, "rb") as f:
        return f.read(8) == b"\x89PNG\r\n\x1a\n"

# Read in chunks (large files)
def read_chunks(filepath, chunk_size=8192):
    with open(filepath, "rb") as f:
        while chunk := f.read(chunk_size):
            yield chunk

# Copy a file in binary mode
def copy_file(src, dst):
    with open(src, "rb") as src_f, open(dst, "wb") as dst_f:
        for chunk in read_chunks(src):
            dst_f.write(chunk)

# struct - pack/unpack binary data (like C structs)
# Format: < = little-endian, I = unsigned int, f = float
packed = struct.pack("