On Thu, Jan 31, 2013 at 8:20 PM, Danny Yoo <d...@hashcollision.org> wrote:
> On Thu, Jan 31, 2013 at 11:36 AM, heathen <godsoflibe...@lavabit.com> > wrote: > > why is this: > > > >>>> d *= 3 + 4 > > > The gory details about how Python understands this expression can be found > in: > > > http://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements > > Technically, "3 + 4" is the "expression list" part of the statement, > so it gets evaluated. Then it does the multiplication of the target > (d) and the value of the expression list (7). > > The docs really hit it with that "similar but not exactly equal" phrase. Augmented assignment is one of those things that is sort-of-but-not-really equivalent, so it can really bite you in the ass. I recently encountered this little gem on: >>> a = ([], []) >>> a[0] += [1] Traceback (most recent call last): File "<pyshell#1>", line 1, in <module> a[0] += [1] TypeError: 'tuple' object does not support item assignment >>> a ([1], []) >>> tuples are immutable, but lists *do* support in-place modification. And thus we get an operation that both fails and succeeds at the same time..
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