CS50 - Introduction to Programming with Python

Week on File I/O (Input and Output of Files)

Key Concepts

• File I/O: Writing code to read from (load information) and write to (save information) files.
• Storage in Memory vs. Storage in Files: Data stored in memory (non-persistent) vs. stored in files (persistent).

Working With Lists

• Lists and Memory: Lists store multiple values in memory. Upon program exit, the contents in memory are lost.
• Program Example: Collecting names using a list.

  names = []  # Empty list
  for _ in range(3):
      name = input("What's your name?")
      names.append(name)
  for name in sorted(names):
      print(f"Hello, {name}") 
  

• Problem: After program exits, collected names are lost.

Reading and Writing to Files

• Writing to Files: Using the open function with mode 'w' (write).

  with open("names.txt", "w") as file:
      name = input("What's your name?")
      file.write(name + "\n")  # Ensuring new line
  

• Appending to Files: Using mode 'a' (append) to add to the file without overwriting.
• Common Issue: Writing without new lines results in concatenated text.
• Best Practice: Use with statement to handle file opening and closing automatically.

  with open("names.txt", "a") as file:
      name = input("What's your name?")
      file.write(f"{name}\n")
  

Reading from Files

• Reading Line by Line: Use readlines() to read all lines at once.

  with open("names.txt") as file:
      lines = file.readlines()
  for line in lines:
      print(f"Hello, {line.strip()}")  # Stripping new lines
  

• Using Loops for Efficiency: Optimizing reading and manipulation:

  with open("names.txt") as file:
      for line in file:
          print(f"Hello, {line.strip()}")
  

• Sorting Names: Load, sort, and then print for ordered output.

  names = []
  with open("names.txt") as file:
      for line in file:
          names.append(line.strip())
  for name in sorted(names):
      print(f"Hello, {name}")
  

• Reverse Order Sorting: Use reverse=True parameter in sorted function.

CSV Files

• CSV (Comma-Separated Values): Storing multiple related data points.
• CSV Example:

  name,house
  Hermione,Gryffindor
  Harry,Gryffindor
  Ron,Gryffindor
  Draco,Slytherin
  

• Reading CSV: Using csv.reader for simple parsing and csv.DictReader for column keys.

  import csv
  with open("students.csv") as file:
      reader = csv.DictReader(file)
      for row in reader:
          print(f"{row['name']} is in {row['house']}")
  

• Writing CSV: Using csv.writer with field names.

  import csv
  name = input("What's your name?")
  house = input("What's your house?")
  with open("students.csv", "a") as file:
      writer = csv.writer(file)
      writer.writerow([name, house])
  

Advanced CSV Operations

• Using Lambda Functions: For custom sorting keys.

  sorted(students, key=lambda student: student['name'])
  

• Handling CSV Headers: Storing the header in CSV files to ensure data is correctly parsed.

  with open("students.csv") as file:
      reader = csv.DictReader(file)
      for row in reader:
          print(f"{row['name']} is from {row['home']}")
  

Handling Binary Files (Images)

• Pillow Library: For image manipulation and creation of animated gifs.

  from PIL import Image
  images = []
  for arg in sys.argv[1:]:
      image = Image.open(arg)
      images.append(image)
  images[0].save("animation.gif", save_all=True, append_images=images[1:], loop=0)
  

Summary

• Files are essential for persistent data storage.
• Employ libraries (e.g., CSV, Pillow) to handle complex file operations and formats.
• Reading, writing, appending to files and ensuring clean data handling with practical examples.

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This concludes the lecture on file I/O and handling different file types in Python.