Python Multiple Assignment Statements In One Line

Question

(Don't worry, this isn't another question about unpacking tuples.)

In python, a statement like foo = bar = baz = 5 assigns the variables foo, bar, and baz to 5. It assigns these variables from left to right, as can be proved by nastier examples like

>>> foo[0] = foo = [0] Traceback (most recent call last):   File "<stdin>", line 1, in <module> NameError: name 'foo' is not defined >>> foo = foo[0] = [0] >>> foo [[...]] >>> foo[0] [[...]] >>> foo is foo[0] True 

But the python language reference states that assignment statements have the form

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

and on assignment the expression_list is evaluated first and then the assigning happens.

So how can the line foo = bar = 5 be valid, given that bar = 5 isn't an expression_list? How are these multiple assignments on one line getting parsed and evaluated? Am I reading the language reference wrong?

Answer 1

All credit goes to @MarkDickinson, who answered this in a comment:

Notice the + in (target_list "=")+, which means one or more copies. In foo = bar = 5, there are two (target_list "=") productions, and the expression_list part is just 5

All target_list productions (i.e. things that look like foo =) in an assignment statement get assigned, from left to right, to the expression_list on the right end of the statement, after the expression_list gets evaluated.

And of course the usual 'tuple-unpacking' assignment syntax works within this syntax, letting you do things like

>>> foo, boo, moo = boo[0], moo[0], foo[0] = moo[0], foo[0], boo[0] = [0], [0], [0] >>> foo [[[[...]]]] >>> foo[0] is boo True >>> foo[0][0] is moo True >>> foo[0][0][0] is foo True