Carl Banks wrote:
On Oct 16, 12:40 pm, Tim Chase <python.l...@tim.thechases.com> wrote:
Then I'm fine with sum() being smart enough to recognize this
horrid case and do the "right" thing by returning ''.join()
instead.
You don't want Python to get into this business. Trust me. Just
don't go there.
Well python is already in this business of special cases -- it
trys to be smart about a dumb operation by raising an error.
Just call __add__ ... if it's slow, that's my problem as a
programmer. Python doesn't complain about lists, which Steven
points out
Steven D'Aprano wrote:
>And indeed, if you pass a list-of-lists to sum(), it
>does:
>
>>>> >>> sum([[1,2], ['a',None], [1,'b']], [])
>[1, 2, 'a', None, 1, 'b']
>
>(For the record, summing lists is O(N**2), and unlike
>strings, there's no optimization in CPython to avoid the
>slow behaviour.)
which is also slow. By your own words (from a subsequent email)
If, say, you were to accept that Python is going to guard against a
small number of especially bad cases, this has got to be one of the
top candidates.
In guarding, you can do the intended thing (for strings, that's
concatenation as the "+" operator does which can be optimized
with ''.join()), or you can raise an error. I don't see how
using ''.join() is much different from being smart enough to
raise an error, except it doesn't break user expectations.
If you want sum to call ''.join transparently, then "".join would have
to produce identical results to what sum() would have produced in all
cases. That does not happen. If an object within the list defines
both __str__ and __add__ methods, then "".join will call __str__,
whereas sum would call __add__, leading to potentially different
results. Therefore, transparently substituting a call to "".join is
not an option.
AFAICT, "".join() does not call __str__ on its elements:
>>> ''.join(['hello', 42, 'world'])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sequence item 1: expected string, int found
>>> '__str__' in dir(42)
True
which is exactly what I'd expect from
>>> 'hello' + 42 + "world"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects
This is under 2.x (I don't have 3.x on hand to see if that
changed unexpectedly)
It'd be better to just remove the special case.
I'd be happy with either solution for summing strings. Be slow
or be fast, but don't be erroneous.
-tkc
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