Augmented augmented assignment?

David Bolen db3l at fitlinxx.com
Tue Oct 23 00:43:37 CEST 2001


Dale Strickland-Clark <dale at riverhall.NOTHANKS.co.uk> writes:

> I assume it's something to do with the in-place operation of augmented
> assignment but I can't seem to reproduce it using normal assignment.
> 
> >>> x = 'spam'
> >>> y = []
> >>> y = y + x
> Traceback (most recent call last):
>   File "<interactive input>", line 1, in ?
> TypeError: can only concatenate list (not "string") to list
> >>> y += x
> >>> y
> ['s', 'p', 'a', 'm']

It looks like between 1.5.2 and 2.0, the extend() method was adjusted
to support any sequence.  Doing y.extend('spam') accomplishes the same
thing as the augmented assignment - probably what's happening behind
the scenes - whereas in 1.5.2, y.extend('spam') generated a TypeError,
and wants a list.

It does seem like an undesirable inconsistency though, since it's only
with a mutable list that it happens (you get the same concatenation
TypeError in either case if y is a tuple).

--
-- David
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