Chris Barker writes:
But I still think the original:
[g(y) for x in range(5) for y in [f(x)]]
Is always going to be confusing to read.
But the point I was making with "def f(x=[0]):" was this: you have a situation where your desired semantics is "value of some type"[1], but the language's syntax doesn't permit a value of that type there, while "singleton sequence of that type" works fine. In fact, "singleton as value" is baked into Python in the form of str.__getitem__ and bytes.__getitem__. So we now have four use cases for singleton as value: two stringish actual types, and the two idioms "mutable default argument" and "local variable in comprehension". The horse is long since out of the barn. Steve Footnotes: [1] Both "value" and "type" are used rather loosely here.