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Wed Oct 18 20:01:35 CEST 2006 · https://mail.python.org/pipermail/python-dev/2006-October/069461.html

My statement wasn't clear enough.

Rendering occurs if the string being concatenated is already a concatenation
object created by an earlier assignment.

In s = a + b + c + d + e + f , there would be rendering of the source string
if it is already a concatenation.

Here is an example that would make it clear:
a = "Value a ="
a += "anything"  # creates a concatenation
c = a + b             #This would cause rendering of a and then c will
become concatenation between a and b.
c += "Something"
# This will not append to the concatenation object, but cause rendering of c
and then it will create a concatenation between c and "Something", which
will be assigned to c.

Now if there are a series of assignments,
(1) s = c + "something" # causes rendering of c
(2) s += a   # causes rendering of s and creates a new concatenation
(3) s += b  # causes rendering of s and creates a new concatenation
(4) s += c  # causes rendering of s and creates a new concatenation
(5) print s   # causes rendering of s

If there is list of strings created and then they are concatenated with +=,
I would expect it to be slower because of the additional allocations
involved in rendering.

-Chetan
On 10/18/06, Kristján V. Jónsson <kristjan at ccpgames.com> wrote:
>
>  Doesn't it end up in a call to PyString_Concat()?  That should return a
> PyStringConcatenationObject too, right?
> K
>
>  ------------------------------
>
>
> Construct like s = a + b + c + d + e , where a, b etc. have been assigned
> string values earlier will not benefit from the patch.
>
>
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Speed up + for string concatenation, now as fast as "".join(x) idiom