Reading and Writing Files

# The "with" statement ensures the file is properly closed
with open("example.txt", "r") as file:
    content = file.read()
    print(content)

# Read line by line
with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() removes the newline

# Read all lines into a list
with open("example.txt", "r") as file:
    lines = file.readlines()
    print(lines)  # ['Line 1\n', 'Line 2\n', ...]

# Write to a file (creates new or overwrites)
with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is line 2\n")

# Append to existing file
with open("output.txt", "a") as file:
    file.write("This line is appended\n")

# Write multiple lines
lines = ["Line 1", "Line 2", "Line 3"]
with open("output.txt", "w") as file:
    for line in lines:
        file.write(line + "\n")

# Or use writelines (doesn't add newlines automatically)
with open("output.txt", "w") as file:
    file.writelines(line + "\n" for line in lines)

# "r"  - Read (default). Error if file doesn't exist
# "w"  - Write. Creates new file or overwrites existing
# "a"  - Append. Creates new file or adds to existing
# "x"  - Create. Error if file already exists
# "r+" - Read and write

# For binary files (images, etc.), add "b"
# "rb", "wb", "ab"

import csv

# Reading CSV
with open("data.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)  # Each row is a list

# Reading CSV as dictionaries (using header row as keys)
with open("data.csv", "r") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row["name"], row["age"])

# Writing CSV
data = [
    ["name", "age", "city"],
    ["Alice", "25", "New York"],
    ["Bob", "30", "Boston"]
]

with open("output.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerows(data)

# Check if file exists before reading
import os

if os.path.exists("data.txt"):
    with open("data.txt", "r") as file:
        content = file.read()
else:
    print("File not found")

# Or use try/except
try:
    with open("data.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("File not found")
except PermissionError:
    print("Permission denied")

def count_errors(log_file):
    """Count error messages in a log file."""
    error_count = 0
    error_lines = []

    with open(log_file, "r") as file:
        for line_number, line in enumerate(file, 1):
            if "ERROR" in line.upper():
                error_count += 1
                error_lines.append((line_number, line.strip()))

    return error_count, error_lines

# Use it
count, errors = count_errors("app.log")
print(f"Found {count} errors")
for num, text in errors[:5]:  # Show first 5
    print(f"  Line {num}: {text}")