Chapter 7: Files in Python Programming

Need for File Analysis: Frequently, data analysis requires the examination of text files that are stored in secondary memory.
Text File Definition: A text file is a sequence of lines, which can be contrasted with a string that represents a sequence of characters.

End of Line Marker: The newline character (\n) indicates the end of a line in text files. It is treated as a single character.
- Example:
  python stuff = 'Hello\nWorld!' print(stuff)
- Output:
  Hello World!
- The length of stuff including newline character:
  python len(stuff) # Returns 3

Using open() Function:
- To manipulate a file, use the open() function:
  handle = open(filename, mode)
- Example: fhandle = open('mbox.txt', 'r') returns a handle for read operations.
- Mode Options:
- 'r': Read only.
- 'w': Read and write; creates a file if it doesn't exist.

File Handle as a Sequence: The file handle, when opened for reading, acts as a sequence of strings, with each line representing a string.
- Example:
  python xfile = open('mbox.txt') for cheese in xfile: print(cheese)

File Open Errors: Occurs if a file does not exist or is located in a different directory. Example of error:

  fhand = open('stuff.txt')

Counting Lines in a File:
- Open a file and iterate through each line to count:
  python fhand = open('mbox.txt') count = 0 for line in fhand: count += 1 # or count = count + 1 print('Line Count:', count)
- Output:
  Line Count: 132045

Reading All Content: You can read an entire file into a single string including newline characters:

  fhand = open('mbox-short.txt')
  inp = fhand.read()
  print(len(inp))  # 94626
  print(inp[:20])

Searching Lines: Use an if statement to only print lines meeting specific criteria:
python fhand = open('mbox-short.txt') for line in fhand: if line.startswith('From:'): print(line)
Stripping Newline Characters: You can use rstrip() to remove unnecessary newline characters before printing:
python for line in fhand: line = line.rstrip() # Removes trailing whitespace if line.startswith('From:'): print(line)

Skipping with continue:

for line in fhand:
    line = line.rstrip()
    if not line.startswith('From:'):
        continue
    print(line)

Using in for String Search: Check for the presence of a substring anywhere in a line:
python for line in fhand: line = line.rstrip() if not '@uct.ac.za' in line: continue print(line)

Using Try/Except for error management while opening files:
python fname = input('Enter the file name: ') try: fhand = open(fname) except: print('File cannot be opened:', fname) quit()
Count specific lines after error handling:
python count = 0 for line in fhand: if line.startswith('Subject:'): count += 1 print('There were', count, 'subject lines in', fname)

write() Method: To write to a writable file:
python filehandle = open('example.txt', 'w') line1 = 'INSY5336 Intro to Python' filehandle.write(line1) filehandle.close()
Inserting Lines in a File:
1. Read the content into a list.
2. Modify the list as needed.
3. Write the updated list back to the file.
  python filehandle = open('marytext.txt', 'r') lines = filehandle.readlines() filehandle.close() lines[5] = 'INSY5336\n' filehandle = open('marytext.txt', 'w') filehandle.writelines(lines) filehandle.close()