Very technically speaking, in "def f(a=[0]): ..." or in "a = [[0]] * 4" there is only one textual occurrence of "[0]", and the function definition is literally

Very technically speaking, in "def f(a=[0]): ..." or in "a = [[0]] * 4" there is only one textual occurrence of "[0]", and the function definition is literally evaluated to define a function (you can try def "f(a=sys.stdout.write('Hi!')): return" in REPL to see when the default value gets constructed). On the other hand, if we abstract from the technical minutiae, yes, it would have been entirely possible for Python to treat default arguments in the function definitions as expressions, by using machinery similar to what gets used for comprehensions: after all, as you point out, function's arguments (and local variables) do get re-assigned on every function call.