> We should extract small functions when it helps understanding the program.

This. If I may reiterate your point:

`is_foolike(x)` may not be much better than `x.startswith("foo")` but, say, `is_superuser(x)` is much easier to understand than `x.username.starts_with("su_")`. And note how the mental burden is decreased in this case as well, as a first-time reader would not be wondering why we're checking usernames in the middle of a function way past login.