A Summary: Expression-Assignments. (Very Long)

[guido at python.org]

Christian Tismer <tismer at appliedbiometrics.com> writes:

> The meaning of multiple assignments is:
> - Evaluate the right hand side and hold it
> - for each left hand side target expression
>   - evaluate to assignable object
>   - store value
> 
> Example how this works 
> (not recommended to use this side effect, tho:)
> 
> a=b=c[a](5).x=1+2
> 
> Evaluation of 1+2, temp=3
> Assign temp to a
> Assign temp to b
> Index c by current value which is 3
> Call the resulting function with argument 5
> Lookup the result's attribute "x"
> Assign temp to attribute "x".
> 
> It is very consequent, and has nothing to do with a=(b=1) .

Historical note: Algol-60 (my first programming language) had multiple
assignment spelled as a:=b:=c, but no assignment expressions.  I never
thought this was strange.  When I first learned C, I thought it was
pretty neat how they had defined the binding for assignment
expressions so that a=b=c still meant what was familiar to me at the
time.  So all I can do is smile when I hear that people are now coming 
at Python's use of a=b=c from the opposite direction...

--Guido van Rossum (home page: http://www.python.org/~guido/)