if python assignments don't return a value how can we do a = b = c = 42

Question

After looking at these two questions,

• Why does Python assignment not return a value?
• Python assigning multiple variables to same list value?

I had this question:

How does a = b = c = 42 propagate 42 to the left without returning the value at each step?

Answer 1

Because of a special exception in the syntax, carved out for that exact use case. See the BNF:

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

Note the (target_list "=")+.

Answer 2

Chained assignment in this manner does not require assignment to return a value. It is a special form of the assignment statement which binds the object to multiple names.

Answer 3

It's a python feature. From Python Tutorial:

A value can be assigned to several variables simultaneously:

>>> x = y = z = 0  # Zero x, y and z
>>> x
0
>>> y
0
>>> z
0

Note that in fact, an assignment doesn't return any value. You cannot do this

a = b = (c = 2)