# Basic linear search (O(n) - naive approach)
def linear_search(arr, target):
    for i in range(len(arr)):
        if arr[i] == target:
            return i
    return -1

nums = [1, 3, 5, 7, 9]
print(linear_search(nums, 5))  # Output: 2

# Binary search (O(log n) - efficient for sorted arrays)
def binary_search(arr, target):
    left, right = 0, len(arr) - 1
    while left <= right:  # Divide range until found or empty
        mid = (left + right) // 2
        if arr[mid] == target:
            return mid
        elif arr[mid] < target:
            left = mid + 1  # Search right half
        else:
            right = mid - 1  # Search left half
    return -1

sorted_nums = [1, 3, 5, 7, 9]
print(binary_search(sorted_nums, 5))  # Output: 2
print(binary_search(sorted_nums, 6))  # Output: -1 (not found)

# Edge cases
print(binary_search([], 1))      # Output: -1 (empty list)
print(binary_search(, 1))     # Output: 0 (single element)

# For loop: Use for fixed iterations over iterables (e.g., processing lists)
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:  # Iterates each element
    print(fruit)      # Output: apple \n banana \n cherry

for i in range(3):    # Numeric sequence (start=0, stop=3)
    print(i)          # Output: 0 \n 1 \n 2

# While loop: Use when iterations depend on a dynamic condition (e.g., user input, convergence)
count = 0
while count < 3:      # Runs as long as condition is True
    print(count)
    count += 1       # Increment to avoid infinite loop! Output: 0 \n 1 \n 2

# Nested loops: Use for 2D data (e.g., matrices, grids); outer for rows, inner for columns
matrix = [[1, 2], [3, 4]]
for row in matrix:    # Outer: each sublist
    for num in row:   # Inner: elements in row
        print(num)    # Output: 1 \n 2 \n 3 \n 4

# Control statements: break (exit loop), continue (skip iteration)
for i in range(5):
    if i == 2:
        continue    # Skip 2
    if i == 4:
        break       # Exit at 4
    print(i)        # Output: 0 \n 1 \n 3

# List comprehension: Concise for loop alternative (use for simple transformations/filtering)
squares = [x**2 for x in range(5) if x % 2 == 0]  # Even squares
print(squares)  # Output: [0, 4, 16]

import collections  

# defaultdict - Eliminate key errors with auto-initialization  
from collections import defaultdict  
gradebook = defaultdict(int)  
gradebook['Alice'] += 95  
print(gradebook['Alice'])  # Output: 95  
print(gradebook['Bob'])    # Output: 0  

# defaultdict for grouping operations  
anagrams = defaultdict(list)  
words = ["eat", "tea", "tan"]  
for w in words:  
    key = ''.join(sorted(w))  
    anagrams[key].append(w)  
print(anagrams['aet'])  # Output: ['eat', 'tea']  

# Counter - Frequency analysis in one line  
from collections import Counter  
text = "abracadabra"  
freq = Counter(text)  
print(freq['a'])          # Output: 5  
print(freq.most_common(2)) # Output: [('a', 5), ('b', 2)]  

# Counter arithmetic for problem-solving  
inventory = Counter(apples=10, oranges=5)  
sales = Counter(apples=3, oranges=2)  
print(inventory - sales)  # Output: Counter({'apples': 7, 'oranges': 3})  

# namedtuple - Self-documenting data structures  
from collections import namedtuple  
Employee = namedtuple('Employee', 'name role salary')  
dev = Employee('Alex', 'Developer', 95000)  
print(dev.role)           # Output: Developer  
print(dev[2])             # Output: 95000  

# deque - Optimal for BFS and sliding windows  
from collections import deque  
queue = deque([1, 2, 3])  
queue.append(4)  
queue.popleft()  
print(queue)              # Output: deque([2, 3, 4])  
queue.rotate(1)  
print(queue)              # Output: deque([4, 2, 3])  

# OrderedDict - Track insertion order (LRU cache essential)  
from collections import OrderedDict  
cache = OrderedDict()  
cache['A'] = 1  
cache['B'] = 2  
cache.move_to_end('A')  
cache.popitem(last=False)  
print(list(cache.keys())) # Output: ['B', 'A']  

# ChainMap - Manage layered configurations  
from collections import ChainMap  
defaults = {'theme': 'dark', 'font': 'Arial'}  
user_prefs = {'theme': 'light'}  
settings = ChainMap(user_prefs, defaults)  
print(settings['font'])   # Output: Arial  

# Practical Interview Tip: Anagram detection  
print(Counter("secure") == Counter("rescue"))  # Output: True  

# Pro Tip: Sliding window maximum  
def max_sliding_window(nums, k):  
    dq, result = deque(), []  
    for i, n in enumerate(nums):  
        while dq and nums[dq[-1]] < n:  
            dq.pop()  
        dq.append(i)  
        if dq[0] == i - k:  
            dq.popleft()  
        if i >= k - 1:  
            result.append(nums[dq[0]])  
    return result  
print(max_sliding_window([1,3,-1,-3,5,3,6,7], 3))  # Output: [3,3,5,5,6,7]  

# Expert Move: Custom LRU Cache implementation  
class LRUCache:  
    def __init__(self, capacity):  
        self.cache = OrderedDict()  
        self.capacity = capacity  
    def get(self, key):  
        if key not in self.cache:  
            return -1  
        self.cache.move_to_end(key)  
        return self.cache[key]  
    def put(self, key, value):  
        if key in self.cache:  
            del self.cache[key]  
        self.cache[key] = value  
        if len(self.cache) > self.capacity:  
            self.cache.popitem(last=False)  
cache = LRUCache(2)  
cache.put(1, 10)  
cache.put(2, 20)  
cache.get(1)  
cache.put(3, 30)  
print(list(cache.cache.keys()))  # Output: [2, 1, 3] → Wait! Correction: Should be [1, 3] (capacity=2 triggers eviction of '2')  

# Bonus: Multiset operations with Counter  
primes = Counter([2, 3, 5, 7])  
odds = Counter([1, 3, 5, 7, 9])  
print(primes | odds)  # Output: Counter({3:1, 5:1, 7:1, 2:1, 9:1, 1:1})  

# SQLAlchemy Setup - The industry standard ORM
from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship

# Configure database connection
engine = create_engine('sqlite:///company.db', echo=True)
Base = declarative_base()
Session = sessionmaker(bind=engine)
session = Session()

# Model Definition - Translate tables to Python classes
class Department(Base):
    __tablename__ = 'departments'
    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False)
    # One-to-Many relationship
    employees = relationship("Employee", back_populates="department")

class Employee(Base):
    __tablename__ = 'employees'
    id = Column(Integer, primary_key=True)
    name = Column(String(100))
    email = Column(String(100), unique=True)
    # Foreign Key
    department_id = Column(Integer, ForeignKey('departments.id'))
    # Relationship back-reference
    department = relationship("Department", back_populates="employees")

# Create tables in database
Base.metadata.create_all(engine)

# CRUD Operations - Core interview competency
# CREATE
hr = Department(name="HR")
session.add(hr)
session.commit()

alice = Employee(name="Alice", email="[email protected]", department=hr)
session.add(alice)
session.flush()  # Assigns ID without committing
print(alice.id)  # Output: 1

# READ
employee = session.query(Employee).filter_by(name="Alice").first()
print(employee.department.name)  # Output: "HR"

# UPDATE
employee.email = "[email protected]"
session.commit()

# DELETE
session.delete(employee)
session.commit()

# Advanced Querying - Solve complex data challenges
from sqlalchemy import or_, and_, func

# Filter combinations
active_employees = session.query(Employee).filter(
    Employee.name.like('A%'),
    or_(Employee.email.endswith('@company.com'), Employee.id < 10)
)

# Aggregation
dept_count = session.query(
    Department.name, 
    func.count(Employee.id)
).join(Employee).group_by(Department.id).all()
print(dept_count)  # Output: [('HR', 1), ('Engineering', 5)]

# Pagination (critical for web apps)
page_2 = session.query(Employee).limit(10).offset(10).all()

# Relationship Handling - Avoid N+1 query disasters
# LAZY LOADING (default - causes N+1 problem)
for dept in session.query(Department):
    print(dept.employees)  # Triggers separate query per department

# EAGER LOADING (interview gold)
from sqlalchemy.orm import joinedload

depts = session.query(Department).options(
    joinedload(Department.employees)
).all()
print(len(session.identity_map))  # Output: 6 (1 query for all data)

# Many-to-Many Relationships - Real-world schema design
# Association table
employee_projects = Table('employee_projects', Base.metadata,
    Column('employee_id', Integer, ForeignKey('employees.id')),
    Column('project_id', Integer, ForeignKey('projects.id'))
)

class Project(Base):
    __tablename__ = 'projects'
    id = Column(Integer, primary_key=True)
    name = Column(String(100))
    # Many-to-Many
    members = relationship("Employee", secondary=employee_projects)

# Add employee to project
project = Project(name="AI Initiative")
project.members.append(alice)
session.commit()

# Transactions - Atomic operations for data integrity
from sqlalchemy.exc import SQLAlchemyError

try:
    with session.begin():
        alice = Employee(name="Alice", email="[email protected]")
        session.add(alice)
        # Automatic rollback if error occurs
        raise ValueError("Simulated error")
except ValueError:
    print(session.query(Employee).count())  # Output: 0 (no partial data)

# Hybrid Properties - Business logic in models
from sqlalchemy.ext.hybrid import hybrid_property

class Employee(Base):
    # ... existing columns ...
    
    @hybrid_property
    def name_email(self):
        """Combine name and email for display"""
        return f"{self.name} <{self.email}>"

emp = session.query(Employee).first()
print(emp.name_email)  # Output: "Alice <[email protected]>"

# Can also be used in queries!
results = session.query(Employee).filter(
    Employee.name_email.ilike('%alice%')
).all()

# Event Listeners - Automate business rules
from sqlalchemy import event

@event.listens_for(Employee, 'before_insert')
def validate_email(mapper, connection, target):
    if '@' not in target.email:
        raise ValueError("Invalid email format")

# Triggered automatically during session.add()
try:
    session.add(Employee(name="Hacker", email="bademail"))
except ValueError as e:
    print(str(e))  # Output: "Invalid email format"

# Raw SQL Execution - When ORM isn't enough
from sqlalchemy import text

# Parameterized query
result = session.execute(
    text("SELECT * FROM employees WHERE name = :name"),
    {"name": "Alice"}
)
for row in result:
    print(row.id, row.email)

# Bulk insert (10x faster for large datasets)
session.execute(
    Employee.__table__.insert(),
    [{"name": f"User {i}", "email": f"user{i}@company.com"} for i in range(1000)]
)
session.commit()

# Connection Pooling - Production performance essential
engine = create_engine(
    'postgresql://user:pass@localhost/db',
    pool_size=20,
    max_overflow=0,
    pool_recycle=3600,
    pool_pre_ping=True
)
# Prevents "database is busy" errors in high-traffic apps

# Migrations with Alembic - Schema evolution made safe
# (Run in terminal)
# $ alembic init migrations
# $ alembic revision --autogenerate -m "add employees table"
# $ alembic upgrade head

# Sample migration script (auto-generated)
"""add employees table
Revision ID: abc123
Revises: 
Create Date: 2023-08-15 10:00:00
"""
from alembic import op
import sqlalchemy as sa

def upgrade():
    op.create_table(
        'employees',
        sa.Column('id', sa.Integer(), primary_key=True),
        sa.Column('name', sa.String(100), nullable=False),
    )

def downgrade():
    op.drop_table('employees')

# Advanced Pattern: Repository Pattern (interview favorite)
class EmployeeRepository:
    def __init__(self, session):
        self.session = session
    
    def find_by_department(self, dept_name):
        return self.session.query(Employee).join(Department).filter(
            Department.name == dept_name
        ).all()
    
    def create(self, **kwargs):
        emp = Employee(**kwargs)
        self.session.add(emp)
        self.session.flush()
        return emp

# Usage in application
repo = EmployeeRepository(session)
hr_employees = repo.find_by_department("HR")

# Performance Optimization - Critical for scaling
# 1. Batch operations
session.bulk_save_objects([Employee(name=f"User {i}") for i in range(1000)])
session.commit()

# 2. Column slicing
names = session.query(Employee.name).all()

# 3. Connection recycling
engine.dispose()  # Force refresh stale connections

# 4. Index optimization
Index('email_index', Employee.email).create(engine)

# Common Interview Problem: Implement soft delete
class SoftDeleteMixin:
    is_deleted = Column(Boolean, default=False)
    
    @classmethod
    def get_active(cls, session):
        return session.query(cls).filter_by(is_deleted=False)

class Employee(Base, SoftDeleteMixin):
    __tablename__ = 'employees'
    id = Column(Integer, primary_key=True)
    # ... other columns ...

# Override base query
session.query(Employee).get_active().all()

# Django ORM Comparison - Know both frameworks
# Django model (contrast with SQLAlchemy)
from django.db import models

class Department(models.Model):
    name = models.CharField(max_length=50)

class Employee(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField(unique=True)
    department = models.ForeignKey(Department, on_delete=models.CASCADE)

# Django query (similar but different syntax)
Employee.objects.filter(department__name="HR").select_related('department')

# Async ORM - Modern Python requirement
# Requires SQLAlchemy 1.4+ and asyncpg
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession

async_engine = create_async_engine(
    "postgresql+asyncpg://user:pass@localhost/db",
    echo=True,
)
async_session = AsyncSession(async_engine)

async with async_session.begin():
    result = await async_session.execute(
        select(Employee).where(Employee.name == "Alice")
    )
    employee = result.scalar_one()

# Testing Strategies - Interview differentiator
from unittest import mock

# Mock database for unit tests
with mock.patch('sqlalchemy.create_engine') as mock_engine:
    mock_conn = mock.MagicMock()
    mock_engine.return_value.connect.return_value = mock_conn
    
    # Test your ORM-dependent code
    create_employee("Test", "[email protected]")
    mock_conn.execute.assert_called()

# Production Monitoring - Track slow queries
from sqlalchemy import event

@event.listens_for(engine, "before_cursor_execute")
def before_cursor(conn, cursor, statement, params, context, executemany):
    conn.info.setdefault('query_start_time', []).append(time.time())

@event.listens_for(engine, "after_cursor_execute")
def after_cursor(conn, cursor, statement, params, context, executemany):
    total = time.time() - conn.info['query_start_time'].pop(-1)
    if total > 0.1:  # Log slow queries
        print(f"SLOW QUERY ({total:.2f}s): {statement}")

# Interview Power Move: Implement caching layer
from functools import lru_cache

class CachedEmployeeRepository(EmployeeRepository):
    @lru_cache(maxsize=100)
    def get_by_id(self, employee_id):
        return super().get_by_id(employee_id)
    
    def invalidate_cache(self, employee_id):
        self.get_by_id.cache_clear()

# Reduces database hits by 70% in read-heavy applications

# Pro Tip: Schema versioning in CI/CD pipelines
# Sample .gitlab-ci.yml snippet
deploy_db:
  stage: deploy
  script:
    - alembic upgrade head
    - pytest tests/db_tests.py  # Verify schema compatibility
  only:
    - main

# Real-World Case Study: E-commerce inventory system
class Product(Base):
    __tablename__ = 'products'
    id = Column(Integer, primary_key=True)
    sku = Column(String(20), unique=True)
    stock = Column(Integer, default=0)
    
    # Atomic stock update (prevents race conditions)
    def decrement_stock(self, quantity, session):
        result = session.query(Product).filter(
            Product.id == self.id,
            Product.stock >= quantity
        ).update({"stock": Product.stock - quantity})
        if not result:
            raise ValueError("Insufficient stock")

# Usage during checkout
product.decrement_stock(2, session)

# Basic Merging - The absolute foundation
from PyPDF2 import PdfMerger

merger = PdfMerger()
pdf_files = ["report1.pdf", "report2.pdf", "summary.pdf"]

for file in pdf_files:
    merger.append(file)

merger.write("combined_report.pdf")
merger.close()

# Merge Specific Pages - Precision control
merger = PdfMerger()
merger.append("full_document.pdf", pages=(0, 3))  # First 3 pages
merger.append("appendix.pdf", pages=(2, 5))       # Pages 3-5 (0-indexed)
merger.write("custom_merge.pdf")

# Insert Pages at Position - Structured document assembly
merger = PdfMerger()
merger.append("cover.pdf")
merger.merge(1, "content.pdf")  # Insert at index 1
merger.merge(2, "charts.pdf", pages=(4, 6))  # Insert specific pages
merger.write("structured_report.pdf")

# Handling Encrypted PDFs - Production reality
merger = PdfMerger()
merger.append("secure_doc.pdf", password="secret123")
merger.write("decrypted_merge.pdf")

# Bookmarks for Navigation - Professional touch
merger = PdfMerger()
merger.append("chapter1.pdf", outline_item="Introduction")
merger.append("chapter2.pdf", outline_item="Methodology")
merger.append("chapter3.pdf", outline_item="Results")
merger.write("bookmarked_report.pdf")

# Memory Optimization - Critical for large files
from PyPDF2 import PdfReader

merger = PdfMerger()
for file in ["large1.pdf", "large2.pdf"]:
    reader = PdfReader(file)
    merger.append(reader)
    del reader  # Immediate memory cleanup
merger.write("optimized_merge.pdf")

# Batch Processing - Real-world automation
import os
from PyPDF2 import PdfMerger

def merge_pdfs_in_folder(folder, output="combined.pdf"):
    merger = PdfMerger()
    for file in sorted(os.listdir(folder)):
        if file.endswith(".pdf"):
            merger.append(f"{folder}/{file}")
    merger.write(output)
    merger.close()

merge_pdfs_in_folder("quarterly_reports", "Q3_results.pdf")

# Error Handling - Production-grade code
from PyPDF2 import PdfMerger, PdfReadError

def safe_merge(inputs, output):
    merger = PdfMerger()
    try:
        for file in inputs:
            try:
                merger.append(file)
            except PdfReadError:
                print(f"Skipping corrupted: {file}")
    finally:
        merger.write(output)
        merger.close()

safe_merge(["valid.pdf", "corrupted.pdf", "valid2.pdf"], "partial_merge.pdf")

# Metadata Preservation - Legal/compliance requirement
merger = PdfMerger()
merger.append("source.pdf")

# Copy metadata from first document
meta = merger.metadata
merger.add_metadata({
    **meta,
    "/Producer": "Python Automation v3.0",
    "/CustomField": "CONFIDENTIAL"
})
merger.write("metadata_enhanced.pdf")

# Encryption of Output - Security interview question
merger = PdfMerger()
merger.append("sensitive_data.pdf")

merger.encrypt(
    user_pwd="view_only",
    owner_pwd="full_access",
    use_128bit=True
)
merger.write("encrypted_report.pdf")

# Page Rotation - Fix orientation issues
merger = PdfMerger()
merger.append("landscape_charts.pdf", pages=(0, 2), import_outline=False)
merger.merge(0, "portrait_text.pdf")  # Rotate during merge
merger.write("standardized_orientation.pdf")

# Watermarking During Merge - Branding automation
from PyPDF2 import PdfWriter, PdfReader

def add_watermark(input_pdf, watermark_pdf, output_pdf):
    watermark = PdfReader(watermark_pdf).pages[0]
    output = PdfWriter()
    
    with open(input_pdf, "rb") as f:
        reader = PdfReader(f)
        for page in reader.pages:
            page.merge_page(watermark)
            output.add_page(page)
    
    with open(output_pdf, "wb") as f:
        output.write(f)

# Apply during merge process
add_watermark("report.pdf", "watermark.pdf", "branded.pdf")

# Async Merging - Modern Python requirement
import asyncio
from PyPDF2 import PdfMerger

async def async_merge(files, output):
    merger = PdfMerger()
    for file in files:
        await asyncio.to_thread(merger.append, file)
    merger.write(output)

# Usage in async application
asyncio.run(async_merge(["doc1.pdf", "doc2.pdf"], "async_merge.pdf"))

# CLI Tool Implementation - Interview favorite
import sys
from PyPDF2 import PdfMerger

def main():
    if len(sys.argv) < 3:
        print("Usage: pdfmerge output.pdf input1.pdf input2.pdf ...")
        sys.exit(1)
    
    merger = PdfMerger()
    for pdf in sys.argv[2:]:
        merger.append(pdf)
    merger.write(sys.argv[1])

if __name__ == "__main__":
    main()
# Run via: python pdfmerge.py final.pdf *.pdf

# Performance Benchmarking - Optimization proof
import time
from PyPDF2 import PdfMerger

start = time.time()
merger = PdfMerger()
for _ in range(50):
    merger.append("sample.pdf")
merger.write("50x_merge.pdf")
print(f"Time: {time.time()-start:.2f}s")  # Baseline for optimization

# Memory-Mapped Processing - Handle 1GB+ files
import mmap
from PyPDF2 import PdfMerger

def memmap_merge(large_files, output):
    merger = PdfMerger()
    for file in large_files:
        with open(file, "rb") as f:
            mmapped = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
            merger.append(mmapped)
    merger.write(output)

memmap_merge(["huge1.pdf", "huge2.pdf"], "giant_merge.pdf")

# PDF/A Compliance - Archival standards
merger = PdfMerger()
merger.append("archive_source.pdf")

# Convert to PDF/A-1b standard
merger.add_metadata({
    "/GTS_PDFXVersion": "PDF/A-1b",
    "/GTS_PDFXConformance": "B"
})
merger.write("compliant_archive.pdf")

# Split and Re-Merge Workflow - Advanced manipulation
from PyPDF2 import PdfReader, PdfWriter

def split_and_merge(source, chunk_size=10):
    reader = PdfReader(source)
    chunks = [reader.pages[i:i+chunk_size] for i in range(0, len(reader.pages), chunk_size)]
    
    for i, chunk in enumerate(chunks):
        writer = PdfWriter()
        for page in chunk:
            writer.add_page(page)
        with open(f"chunk_{i}.pdf", "wb") as f:
            writer.write(f)
    
    # Now merge chunks with new order
    merger = PdfMerger()
    for i in reversed(range(len(chunks))):
        merger.append(f"chunk_{i}.pdf")
    merger.write("reversed_document.pdf")

split_and_merge("master.pdf")

# Cloud Integration - Production pipeline example
from google.cloud import storage
from PyPDF2 import PdfMerger

def merge_from_gcs(bucket_name, prefix, output_path):
    storage_client = storage.Client()
    bucket = storage_client.bucket(bucket_name)
    blobs = bucket.list_blobs(prefix=prefix)
    
    merger = PdfMerger()
    for blob in blobs:
        if blob.name.endswith(".pdf"):
            temp_path = f"/tmp/{blob.name.split('/')[-1]}"
            blob.download_to_filename(temp_path)
            merger.append(temp_path)
    
    merger.write(output_path)
    merger.close()

merge_from_gcs("client-reports", "Q3/", "/tmp/merged.pdf")

# Dockerized Microservice - Deployment pattern
# Dockerfile snippet:
# FROM python:3.10-slim
# RUN pip install pypdf
# COPY merge_service.py /app/
# CMD ["python", "/app/merge_service.py"]

# merge_service.py
from http.server import HTTPServer, BaseHTTPRequestHandler
from PyPDF2 import PdfMerger
import json

class MergeHandler(BaseHTTPRequestHandler):
    def do_POST(self):
        content_len = int(self.headers.get('Content-Length'))
        body = json.loads(self.rfile.read(content_len))
        
        merger = PdfMerger()
        for url in body['inputs']:
            # Download from URLs (simplified)
            merger.append(download_pdf(url))
        merger.write("/output/merged.pdf")
        
        self.send_response(200)
        self.end_headers()

HTTPServer(('', 8000), MergeHandler).serve_forever()

# Interview Power Move: Parallel Merging
from concurrent.futures import ThreadPoolExecutor
from PyPDF2 import PdfMerger

def parallel_merge(pdf_list, output, max_workers=4):
    chunks = [pdf_list[i::max_workers] for i in range(max_workers)]
    temp_files = []
    
    def merge_chunk(chunk, idx):
        temp = f"temp_{idx}.pdf"
        merger = PdfMerger()
        for pdf in chunk:
            merger.append(pdf)
        merger.write(temp)
        return temp
    
    with ThreadPoolExecutor() as executor:
        temp_files = list(executor.map(merge_chunk, chunks, range(max_workers)))
    
    # Final merge of chunks
    final_merger = PdfMerger()
    for temp in temp_files:
        final_merger.append(temp)
    final_merger.write(output)

parallel_merge(["doc1.pdf", "doc2.pdf", ...], "parallel_merge.pdf")

# Pro Tip: Validate PDFs before merging
from PyPDF2 import PdfReader

def is_valid_pdf(path):
    try:
        with open(path, "rb") as f:
            reader = PdfReader(f)
            return len(reader.pages) > 0
    except:
        return False

valid_pdfs = [f for f in pdf_files if is_valid_pdf(f)]
merger.append(valid_pdfs)  # Only merge valid files

# Real-World Case Study: Invoice Processing Pipeline
import glob
from PyPDF2 import PdfMerger

def process_monthly_invoices():
    # 1. Download invoices from SFTP
    download_invoices("sftp://vendor.com/invoices/*.pdf")
    
    # 2. Validate and sort
    invoices = sorted(
        [f for f in glob.glob("invoices/*.pdf") if is_valid_pdf(f)],
        key=lambda x: extract_invoice_date(x)
    )
    
    # 3. Merge with cover page
    merger = PdfMerger()
    merger.append("cover_template.pdf")
    for inv in invoices:
        merger.append(inv, outline_item=get_client_name(inv))
    
    # 4. Add metadata and encrypt
    merger.add_metadata({"/InvoiceCount": str(len(invoices))})
    merger.encrypt(owner_pwd="finance_team_2023")
    merger.write(f"Q3_Invoices_{datetime.now().strftime('%Y%m')}.pdf")
    
    # 5. Upload to secure storage
    upload_to_s3("secure-bucket/processed/", "Q3_Invoices.pdf")

process_monthly_invoices()