What mode allows writing only if the file already exists?

The correct mode for writing only if the file already exists is known as "append mode," which is represented by the 'a' setting in Python. When you open a file in append mode, it allows you to add content to the end of the file without overwriting the existing data. However, if the file does not exist, it will indeed raise an error or create a new file, depending on the implementation, but generally, it is understood that this mode is used with an existing file’s data in mind.

This distinguishes it from "write mode" which creates a new file if it does not already exist and truncates any existing file, and "read mode," which only permits reading the data from a file and does not allow any modifications or appending. Thus, "append mode" effectively allows for writing to an already-existing file while preserving its current contents.