A question on modification of a list via a function invocation

[Ben Finney]

Mok-Kong Shen <mok-kong.shen at t-online.de> writes:

> I don't yet understand. Why (by which rule of the language reference)
> should "alist=[30,60,90]" mean discarding the name's reference to the
> [1,2,3] list?

I think I understand that question, but I find it surprising.

What is your expectation of the following code?

    foo = "spam"
    foo = "beans"
    print(foo)

What should ‘foo’ be bound to after the second assignment?

Your question seems to imply you expect that the ‘foo’ reference would
be bound to *both* of “"spam"” and “"beans"”, somehow. Is that right?
How would that work?

To answer the question: The semantics of an assignment statement are at
in the Python language reference.

    Assignment statements are used to (re)bind names to values […]

    If the target is an identifier (name): […] The name is rebound if it
    was already bound.

    <URL:https://docs.python.org/3/reference/simple_stmts.html#grammar-token-assignment_stmt>

A reference (a name is one kind of reference) can be bound to exactly
one object at any time.

> But then, since now the name alist is known to be global, why then in
> the next line of test2 the name is suddenly interpreted to be local?

The same section of the language reference discusses that.

In brief: The fact that the first use of a name, in the current scope,
is an assignment, means that the name is implicitly within that scope.

-- 
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Ben Finney