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Thu May 18 20:20:58 EDT 2017 · https://mail.python.org/pipermail/python-dev/2017-May/147955.html

Yea, *must* probably should have been *should (unless indicated by the
method name)*. I forgot about the many methods that don't. But the names of
these methods, like `split()`, imply the types that they return.

Your example reminded me of the convention (outside python core) to name
getters and setters so that they imply their returned type. Similarly for
conversion methods like `to_*()`). But the name `format`, in my mind,
implies that it only changes the *contents* of the str, rather than
morphing its type.

The % example surprised me too, but it clicked for me when I realized % is
a binary operator and not a method.

--Hobson
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On Thu, May 18, 2017 at 7:25 AM, Random832 <random832 at fastmail.com> wrote:

> On Thu, May 18, 2017, at 01:14, Hobson Lane wrote:
> > Because `.format()` is a method on an instantiated `str` object in e and
> > so
> > must return the same type
>
> That it *must* return the same type is overstating the matter. Split
> returns a list (and, rather like %, the list is of unicode or str
> objects depending on the argument). Join will return a unicode object if
> any of the elements of the sequence are unicode. I was honestly
> surprised though to see that % returns unicode when formatting a unicode
> value, since my mental model of %s was more like {!s} - call str() on
> whatever object is at the given position in the right-hand argument.
> This kind of ad hoc implementation decision (format always returns str,
> other methods can return unicode,  ljust/rjust refuse to accept a
> unicode character argument) is what Python 3 moved away from.
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[Python-Dev] Format strings, Unicode, and Py2.7: need clarification