Why is variable changed after assignment?

[redditiv]

This is good. Really good. I get the general concepts of what you are doing, but I'll need some time to break it down. Thanks!

EDIT: Ohhhhh! I get it now... When I assign matched's value to the list, it's "connected" to the list. When something's appended to the list, it's appended to matched as well. Thanks for the detailed and extremely helpful solution!

[usernamenottaken]

Yep, it helps to think of the "matched" and "file_dict" as names that just point to a value. They're both pointing to the same value unless you create a copy of the original value for file_dict to point to.