When opening a file with open('filename.txt', 'a'), what does 'a' stand for?

Using 'a' as the mode in the open function specifically stands for "append." When a file is opened with this mode, any data that is written to the file is added to the end, rather than overwriting the existing content. This is particularly useful when you want to add logs or records without losing previous entries.

In this mode, if the file does not already exist, it will be created. File pointers are also positioned at the end of the file, so that subsequent write operations will append new data after the existing data. This behavior makes it easy to expand a file's content without having to read its current contents or manually manage the position within the file.

The other options signify different functionalities: reading, writing with overwriting behavior, or operating in binary mode, which are not applicable when using the 'a' mode. Hence, the functionality associated with the 'a' mode aligns perfectly with appending data to a file.