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Newbie: tuple list confusion.

Alex Martelli aleaxit at yahoo.com
Mon Sep 13 03:05:04 EDT 2004 
Elaine Jackson <elainejackson7355 at home.com> wrote:

> In addition to what you've found out already, there is another difference
> that, to my mind, is of paramount importance; namely, the fact that an
> augmented assignment (a+=b) is actually not an assignment at all. An
> actual assignment (a=a+b) binds the name "a" to a new object, while a
> so-called augmented assignment mutates the object a itself.

Ah, careful there: augmented assignment (nothing "so-called" about it;-)
mutates the object and THEN assigns ("re-binds" if you want to get
technical) the name or other slot holding the object.

For example...:

>>> L=[]
>>> T=(L, L)
>>> T[0] += [1]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object does not support item assignment

If it were true that the += "is actually not an assignment" this
behavior and error message would be impossible.  It _IS_ an assignment,
_augmented_ by the fact that the object in question (if mutable) is also
supposed to mutate.


Regarding the fact that the mutation takes place before the assignment,
this is the kind of behavior details you can check by writing a simple
dummy class with the appropriate special method doing a print. For
example:

>>> class X:
...   def __iadd__(self, other):
...       print 'iadd running'
...       return self
... 
>>> T = (X(), X())
>>> T[0] += 23
iadd running
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object does not support item assignment
>>> 

See?  First we see the message "iadd running", so __iadd__ is executing;
THEN, the error message about assignment to item not being supported.

This is important because the object DOES get mutated even though the
assignment fails.  If you check the L in the previous fragment, indeed,
you'll see it's now [1], not [] any more.


Yes, I do agree these subtleties are potentially confusing -- a pity,
because Python's foremost strength is mostly to AVOID being confusing.
But I don't believe we could have designed +='s behavior to be
substantially clearer and yet just as useful as it is today, given all
the rest of the details about the way Python works; and I do believe
+='s usefulness is sufficient to warrant having it in Python.


Alex
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