spec: allow parentheses on LHS of augmented assignment?

[josharian]

Consider:

a = 1
(a) += 2

Python 2 and 3 both accept this, yielding a == 3.

Starlark rejects it. The spec (over in the Go implementation repo) says of augmented assignments: "The left-hand side must be a simple target: a name, an index expression, or a dot expression."

Should Starlark accept this code, to better match Python?

I originally filed this at https://github.com/google/starlark-go/issues/25, in response to the Go implementation panicking when evaluating this code.

[laurentlb]

Good catch!

The Go implementation doesn't seem very consistent:

(a) = 5  # okay
(a) += 1 # fails

Both statements are allowed by the Java implementation (used in Bazel).

I suspect it's a bug introduced by my change (https://github.com/google/starlark-go/commit/28ceca7fdc8d88c4b6a4a64453b2251e00f73f56).

I suggest that:

• We allow both statements
• We clarify the spec

[josharian]

The Go implementation now accepts both statements, so the only thing left here is the spec change. I’ll leave that for Alan or Laurent.