Notes: Appending To Files

Mr. Mace's Demo Code from the Sandbox

In Python, when you want to use a text file in your program you must ALWAYS perform the following tasks in order:

Open the file
Process the file (Read, Write, Append, etc.)
Close the file

Opening Files for Appending

Remember when we wanted to use a file to read the data from it we opened it using the 'r' (read) mode. When we want to open a file for writing we must also choose the appropriate mode. Python provides three (3) ways to open a file for writing to it. Pay CLOSE attention to what each one does!

'a' (append) - will open an existing file named the filename used for adding data to the end of the file. This is a write only mode.
'a+' (append plus read) - behaves like the append mode above except that it allows read functionality.

GENERAL SYNTAX: fileObjectName = open(strFileName, strMode)

ALL Of THE OPEN MODES SUMMARIZED

MODE	Description
'r'	read only mode
'w'	write - overwrites file with same name or creates a new file
'w+'	write and read mode - overwrites file with same name or creates a new file
'r+'	read and write mode (no overwrite)
'a'	opens for appending to end of file (no overwrite)
'a+'	opens for appending to end of file (no overwrite) plus read

Using the seek(offset, whence ) Method to Move the File Pointer in Files

The seek( ) method can set the pointer to a desired index from the beginning of the file. The example below moves to the character to offset 10 (the 11th character)

newFile.seek(10)

which is also written with an optional second argument, known as whence ("from where")

newFile.seek(10, 0)

using 0 for whence starts the offset from the beginning of the file

Note: the offset must be a positive integer in Python 3, so we cannot offset backwards from the end of the file

GENERAL SYNTAX: fileObjectName.seek(offset, whence)

file.seek(offset, whence)

whence mode	description
0	points to the beginning of the file
1	points to the current location
2	points to the end of the file & offset is always 0

using whence the index can be offset from either the beginning, current location or to the end of the file (where there is no offset applied