Semantics of 'x = y = z' assignment in Python

Question

I had the following line in a loop occurring during the initialization of a singly linked list class:

previous = previous.pointer = Node(item, None)

The intended semantics was what I can obtain with:

previous.pointer = Node(item, None)
previous = previous.pointer

What I found out using pdb is that previous gets reassigned to the new Node object. And the (former) previous's pointer attribute is unchanged.

I couldn't find documentation about the expected behaviour of this kind of assignment.

Answer 1

This is well explained in the documentation:

An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.

^{(emphasis mine)}

Where the term target_list is used in the grammar as follows:

assignment_stmt ::=  (target_list "=")+ (expression_list | yield_expression)

^{(Note the + sign after the first parenthesis - this allows chain assignments)}

Thus, the resulting semantics:

target_list1 = target_list2 = expression

is equivalent to:

target_list1 = expression
target_list2 = expression

There is no way of confusing what is being assigned (evaluates the expression list) with what is the target of the assignment, because assignment is a statement, not an expression. Hence, all with = in it, will not be treated as an expression - only the right most part. Next, all assignment statements will be processed left to right (i.e. their target lists will have the expression's value assigned).

Answer 2

It should assign both, previous.pointer and previous to the newly created Node, effectively at the same time¹.

^{1I'm not sure which one gets assigned first (or if that's even defined by the specification -- though it should only matter in the case of descriptors like the builtin property).}