9.4 Updating Text Files

• Formatted data written to a text file cannot be modified without the risk of destroying other data
• If the name 'White' needs to be changed to 'Williams' in accounts.txt, the old name cannot simply be overwritten
• The original record for White is stored as

  300 White 0.00
  

• If you overwrite the name 'White' with the name 'Williams', the record becomes

  300 Williams00
  

• The characters beyond the second “i” in 'Williams' overwrite other characters in the line
• The problem is that records and their fields can vary in size

9.4 Updating Text Files (cont.)

• To make the preceding name change, we can:
  • copy the records before 300 White 0.00 into a temporary file,
  • write the updated and correctly formatted record for account 300 to this file,
  • copy the records after 300 White 0.00 to the temporary file,
  • delete the old file and
  • rename the temporary file to use the original file’s name.
• Requires processing every record in the file, even if you need to update only one record
  • More efficient when an application needs to update many records in one pass of the file

Updating accounts.txt

• Update the accounts.txt file to change account 300’s name from 'White' to 'Williams' as described above:

accounts = open('accounts.txt', 'r')

temp_file = open('temp_file.txt', 'w')

with accounts, temp_file:
    for record in accounts:
        account, name, balance = record.split()
        if account != '300':
            temp_file.write(record)
        else:
            new_record = ' '.join([account, 'Williams', balance])
            temp_file.write(new_record + '\n')

Updating accounts.txt (cont.)

• This with statement manages two resource objects, specified in a comma-separated list after with
  • If the account is not '300', we write record (which contains a newline) to temp_file
  • Otherwise, we assemble the new record containing 'Williams' in place of 'White' and write it to the file

# macOS/Linux Users: View file contents
!cat temp_file.txt

100 Jones 24.98
200 Doe 345.67
300 Williams 0.00
400 Stone -42.16
500 Rich 224.62

# Windows Users: View file contents
!more temp_file.txt

os Module File-Processing Functions

• To complete the update, delete the old accounts.txt file, then rename temp_file.txt as accounts.txt

import os

os.remove('accounts.txt')

• Use the rename function to rename the temporary file as 'accounts.txt'

os.rename('temp_file.txt', 'accounts.txt')

# macOS/Linux Users: View file contents
!cat accounts.txt

100 Jones 24.98
200 Doe 345.67
300 Williams 0.00
400 Stone -42.16
500 Rich 224.62

# Windows Users: View file contents
!more accounts.txt

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