On Tue, Jan 29, 2019 at 09:21:48PM +0000, Jonathan Fine wrote:
> I've not been following closely, so please forgive me if I'm repeating
> something already said in this thread.
>
> Summary: str.join allows us to easily avoid, when assembling strings,
> 1. Quadratic running time.
> 2. Redundant trailing comma syntax error.
The lack of a syntax error for trailing commas is a language-wide
feature that has nothing to do with str.join.
> The inbuilt help(str.join) gives:
> S.join(iterable) -> str
> Return a string which is the concatenation of the strings in the
> iterable. The separator between elements is S.
>
> This is different from sum in two ways.
Three ways. sum() intentionally doesn't support strings at all:
py> sum(['a', 'b', 'c'], '')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]
unless you cleverly defeat this intentional limitation. (How to do this
is left as an exercise for the reader.)
> The first is the separator S.
> The second is performance related. Consider
> s = 0
> for i in range(100):
> s += 1
> and
> s = ''
> for i in range(100):
> s += 'a'
>
> The first has linear running time (in the parameter represented by
> 100). The second has quadratic running time (unless string addition is
> doing something clever, like being lazy in evaluation).
In CPython, string addition does often do something clever. But not by
being lazy -- it optimizes the string concatenation by appending to the
strings in place if and only if it is safe to do so.
--
Steve
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