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comp.lang.python

Making string-formatting smarter by handling generators?

Tim Chase

2/27/2008 4:24:00 PM

Is there an easy way to make string-formatting smart enough to
gracefully handle iterators/generators? E.g.

transform = lambda s: s.upper()
pair = ('hello', 'world')
print "%s, %s" % pair # works
print "%s, %s" % map(transform, pair) # fails

with a """
TypeError: not enough arguments for format string
"""

I can force it by wrapping the results of my generator in a call
to tuple() or list()

print "%s, %s" % tuple(map(transform, pair))

but it feels a bit hackish to me.

I find I hit it mostly with calls to map() where I want to apply
some transform (as above) to all the items in a list of
parameters such as

"%s=%s&%s=%s" % map(urllib.quote, params)

Any suggestions? (even if it's just "get over your hangup with
wrapping the results in list()/tuple()" :)

Thanks,

-tkc
2 Answers

Gerard Flanagan

2/27/2008 4:58:00 PM

0

On Feb 27, 5:23 pm, Tim Chase <python.l...@tim.thechases.com> wrote:
> Is there an easy way to make string-formatting smart enough to
> gracefully handle iterators/generators? E.g.
>
> transform = lambda s: s.upper()
> pair = ('hello', 'world')
> print "%s, %s" % pair # works
> print "%s, %s" % map(transform, pair) # fails
>
> with a """
> TypeError: not enough arguments for format string
> """
>
> I can force it by wrapping the results of my generator in a call
> to tuple() or list()
>
> print "%s, %s" % tuple(map(transform, pair))
>
> but it feels a bit hackish to me.
>
> I find I hit it mostly with calls to map() where I want to apply
> some transform (as above) to all the items in a list of
> parameters such as
>
> "%s=%s&%s=%s" % map(urllib.quote, params)
>
> Any suggestions? (even if it's just "get over your hangup with
> wrapping the results in list()/tuple()" :)
>
> Thanks,
>
> -tkc

FWIW, I had a similar problem and came up with a custom scheme:

http://gflanagan.net/site/python/utils/t...

(still a work in progress).

From the doctest:

Map operator. Expects a list (iterator) whose each item is either a
tuple
or a dict. If the variable exists (and optionally is not null), then
for each tuple/dict yield the formatted default.

>>> t = Template("""
.... {{names|
.... Hello %s %s!
.... }}
.... """)
>>> print t.replace()
Traceback (most recent call last):
...
KeyError: 'names'
>>> print t.replace(names=[('John', 'Doe'), ('Jane', 'Doe')])
Hello John Doe!
Hello Jane Doe!

HTH

Gerard

Arnaud Delobelle

2/27/2008 5:27:00 PM

0

On Feb 27, 4:23 pm, Tim Chase <python.l...@tim.thechases.com> wrote:
> Is there an easy way to make string-formatting smart enough to
> gracefully handle iterators/generators?  E.g.
>
>    transform = lambda s: s.upper()
>    pair = ('hello', 'world')
>    print "%s, %s" % pair # works
>    print "%s, %s" % map(transform, pair) # fails
>
> with a """
> TypeError:  not enough arguments for format string
> """
>
> I can force it by wrapping the results of my generator in a call
> to tuple() or list()

(Are you using python 3.0 ? For python < 3, map returns a list)

list() wouldn't work as % expects a tuple (otherwise it considers that
only one argument is needed). The problem would arise with any non-
tuple iterable, not just generators.

See http://docs.python.org/lib/typesseq-st...

>
>    print "%s, %s" % tuple(map(transform, pair))
>
> but it feels a bit hackish to me.

I think it is the way to do it though.

> I find I hit it mostly with calls to map() where I want to apply
> some transform (as above) to all the items in a list of
> parameters such as
>
>    "%s=%s&%s=%s" % map(urllib.quote, params)
>
> Any suggestions?  (even if it's just "get over your hangup with
> wrapping the results in list()/tuple()" :)

get over your hangup with wrapping the results in tuple()! (not list()
though, as explained above).

Or you could always define

def tmap(*args):
return tuple(map(*args))

--
Arnaud