a = b = 1 just syntactic sugar?

[sismex01 at hebmex.com]

> 
> Batista, Facundo wrote:
> > "b = 1" does not return 1, just returns b. a and b are the 
> > same object now (they are not just equal, they are the very same).
> 
> if b = 1 returns b, then why can't you say  a = (b = 1)  ?
>

Don't "think in another language", think in Python.

"b=1" returns nothing, because it's a statement, not
an expression.  It's easy to see with eval(), which
is used to evaluate expressions and it returns it's
result:

Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32
Type "copyright", "credits" or "license" for more information.
IDLE 0.8 -- press F1 for help
>>> eval("1")
1
>>> eval("b=1")
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in ?
    eval("b=1")
  File "<string>", line 1
    b=1
     ^
SyntaxError: invalid syntax


So, as you can see, "b=1" is not an expression that
"returns" b, it's a statement that assigns to the
name "b" a reference to the int() object "1".

When the compiler sees "a = b = 1", what it does is
multiple assignments of the same object (after the
last "=") to all the names it finds.


HTH.

-gca
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