> One of the biggest early hurdles in our porting effort was how to overcome the string literals type mismatch between Python 2 and 3. In Python 2, a '' string literal is a sequence of bytes. In Python 3, a '' string literal is a sequence of Unicode code points. These are fundamentally different types. And in Mercurial's code base, most of our string types are binary by design: use of a Unicode based str for representing data is flat out wrong for our use case. We knew that Mercurial would need to eventually switch many string literals from '' to b'' to preserve type compatibility. But doing so would be problematic.

> In the early days of Mercurial's Python 3 port in 2015, Mercurial's project maintainer (Matt Mackall) set a ground rule that the Python 3 port shouldn't overly disrupt others: he wanted the Python 3 port to more or less happen in the background and not require every developer to be aware of Python 3's low-level behavior in order to get work done on the existing Python 2 code base. This may seem like a questionable decision (and I probably disagreed with him to some extent at the time because I was doing Python 3 porting work and the decision constrained this work). But it was the correct decision. Matt knew that it would be years before the Python 3 port was either necessary or resulted in a meaningful return on investment (the value proposition of Python 3 has always been weak to Mercurial because Python 3 doesn't demonstrate a compelling advantage over Python 2 for our use case).

As a general rule, this seems like good practice, but surely b-strings, print_function, etc are a trivial upfront cost, and one that would have to be paid sooner or later anyway?