[Python-Dev] converting the stdlib to str.format (original) (raw)

Steven D'Aprano steve at pearwood.info
Thu Jun 5 17:21:22 CEST 2008

On Thu, 5 Jun 2008 10:43:20 pm Nick Coghlan wrote:

I'm really starting to wonder if supporting positional arguments to str.format() at all is a mistake. Maybe we should ditch support for positional arguments and just accept a single dictionary as the sole parameter to format().

For dictionary formatting, str.format() is a clear winner over str.mod(). For positional formatting I'm not so sure - if someone decided to convert from %-formatting to str.format, would it be such a burden to ask them to name their substitution variables in the process?

If positional arguments are dropped, I would expect to see a proliferation of meaningless names used in the dicts:

"items {a} {b}".format(dict(a=spam, b=eggs))

In other words, the names will be just aliases for the position.

I would also expect a performance hit. On my computer, dict(a=spam, b=eggs) is two times slower than {'a':spam, 'b':eggs} and nearly three times slower than (spam, eggs). I don't imagine building a dict will ever be faster than building a tuple. Given that format() is already significantly slower than %, why make it slower still?

-1 on dropping positional arguments.

-- Steven

More information about the Python-Dev mailing list