copy on write
steve+comp.lang.python at pearwood.info
Fri Jan 13 08:04:16 EST 2012
On Fri, 13 Jan 2012 23:30:56 +1100, Chris Angelico wrote:
> It seems there's a distinct difference between a+=b (in-place
> addition/concatenation) and a=a+b (always rebinding),
Actually, both are always rebinding. It just happens that sometimes a+=b
rebinds to the same object that it was originally bound to.
In the case of ints, a+=b creates a new object (a+b) and rebinds a to it.
In the case of lists, a+=b nominally creates a list a+b, but in fact it
implements that as an in-place operation a.extend(b), and then rebinds
the name a to the list already bound to a.
It does that because the Python VM doesn't know at compile time whether
a+=b will be in-place or not, and so it has to do the rebinding in order
to support the fall-back case of a+=b => a=a+b. Or something -- go read
the PEP if you really care :)
Normally this is harmless, but there is one interesting little glitch you
>>> t = ('a', )
>>> t += 
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
('a', [23, 42])
> which is sorely
> confusing to C programmers. But then, there's a lot about Python
> that's sorely confusing to C programmers.
I prefer to think of it as "there's a lot about C that is sorely
confusing to anyone who isn't a C programmer" <wink>
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