I believe the original intention behind using mode="a" was to append to the output file while reading the input files at the same time. This way, there is no need for an ever-growing string array.

But there are still many other issues like using default platform string encoding instead of detecting it properly or at least using utf-8, checking for ".txt" anywhere in the path instead of at the end, and not closing the input files with a context manager like the output file, which suggests that this code is just pierced together from various sources.

A robust solution would require many more lines.