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For your point about Python string concatenation, I believe that's what io.StringIO is for (https://docs.python.org/3/library/io.html#io.StringIO).

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If you are using CPython 2.4 or 2.5 or later, use the clearest code. In earlier versions using join on a list was faster, but the inplace add operation was optimized, turning the stuff about string.join being faster mostly into mythlore.

https://bugs.python.org/issue980695

PyPy and other implementations may do better with the join idiom though.

Of course someone with code spending a lot of time on joining strings can measure which is best for their situation, but += is fine for most things.

http://pypy.org/performance.html says "String concatenation is expensive" and suggests using join() in loops.

Its an old problem https://bitbucket.org/pypy/pypy/issues/1925/very-slow-string...

So whilst pypy is otherwise much faster than CPython, its missing of this kind of optimisation is why actually CPython can be faster for parsing my logs.

I know about this because I've been bitten by it :)

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