Assignment semantics x = y = z in Python

Question

I had the following line in a loop that occurs during initialization of a separately linked list class:

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

Alleged semantics are what I can get with:

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

What I learned using pdb is that the previous reassigned to the new Node object. And the attribute of the pointer (former) previous does not change.

I could not find documentation of the expected behavior of this type of assignment.

Answer 1

This is well explained in the documentation :

The assignment operator evaluates the list of expressions (remember that this can be a single expression or a comma-delimited list, the last of which has a tuple), and assigns a single result object for each target list, from left to right .

^{(my emphasis)}

If the term target_list used in the grammar as follows:

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

^{(Pay attention to the + sign after the first bracket - this allows you to assign a chain)}

Thus, the resulting semantics:

 target_list1 = target_list2 = expression 

is equivalent to:

 target_list1 = expression target_list2 = expression 

It is not possible to mislead what is assigned (evaluates the list of expressions) with what the purpose of the assignment is, because assignment is an expression, not an expression. Therefore, all c = in it will not be considered as an expression - only the right side. Then all assignment operators will be processed from left to right (i.e., their target lists will have an assigned expression value).

Answer 2

It should assign both previous.pointer and previous newly created Node , effectively at the same time ¹ .

^{1 I'm not sure which one is assigned first (or if it is even specified by the specification), although this should only matter in the case of descriptors such as a built-in property ).}