You are opening an existing file for output. How do you open the file without erasing it, and at the same time make sure that new data that is written to the file is appended to the end of the file's existing data?

In file handling in Python, the append mode is an essential concept for adding data to the end of a file without deleting its existing content. When the 'a' mode is used, the file is opened and the file pointer is positioned at the end of the file. This is a crucial feature when you need to keep a log or add entries to an existing dataset.

Consider a scenario where you have a text file with log entries from a previous operation. You want to add more entries without losing the earlier ones. Using the append mode is like telling Python, 'Let's add to the story without ripping out the previous pages.' In essence, this mode protects the narrative that's already in place while allowing you to continue where you left off.

Compared to the 'w' mode, which starts a fresh slate every time you open the file, the append mode is more like a patient historian, always ready to record new events without erasing the past.

The open() function is the gateway to file manipulation in Python. It's the tool you reach for when you want to start reading from or writing to a file. When you call open(), you pass it the name of the file you want to work with, and equally important, the mode in which you want to open the file.

For instance, file = open('example.txt', 'a') opens 'example.txt' in append mode. Different modes determine how the function will interact with the file. These are specified by single or multiple character strings like 'r' for read, 'w' for write, and 'a' for append, which we've already encountered. There's also 'b' for binary files, and you can combine them, like 'rb' or 'wb' for reading or writing binary files, respectively.

It's also important to note that the open() function returns a file object. This object gives you a handle to push or pull data to and from the file as needed. Always remember, responsibility accompanies this power: always close the file after you're done using file.close() to avoid potential data loss or corruption.

The write() method is the tool you use to send data into a file. Once you've opened a file with open() and have your file object, for example file, you can start adding text to the file by invoking file.write(). This method takes one argument—the string you want to be written into the file.

Continuing the use case from the append mode, when you've opened the file in 'a' mode, calling file.write('This is a new entry') will add 'This is a new entry' to the end of the file, followed by a newline ''. Always ensure that what you pass to write() is a string; if you try to write other data types, such as integers or lists, you will encounter an error. If you have other data types, convert them to string first using functions like str().

To summarize, the write() method is the pen with which you inscribe your data into the digital parchment of the file. Whether you're jotting down single words or entire tales, the write() method is how your data get from your script and into your file for storage.