The point Serhiy is trying to make is that not everyone needs or cares about setting specific file permissions. Python's built-in open() function has not supported this for 26 years and so there's obviously not that much of a need if this has not consistently come up as a shortcoming.

Two, you say to "just pass" something like "0o644" as the mode. OK, but what does that even mean? As a Linux user you may know thanks to chmod usage, but a Windows user or someone who doesn't use a Linux shell won't have any idea what that octal constant represents. So how do you propose to help people set the proper mode? If you say "use os.O_CREAT" then you just made open() have functionality depend on the os module which no other built-in directly does for their API.

In other words you might call this simple, but I call it an advanced feature that's supported by os.fdopen(os.open()) already. So when I say a "clear design" I mean addressing the fact that it's non-obvious for beginners on how to use and something not everyone needs.