method to intercept string formatting % operations

[bradallen]

Hello,

For container class derived from namedtuple, but which also behaves
like a dictionary by implementing __getitem__ for non-integer index
values, is there a special reserved method which allows intercepting %
string formatting operations? I would like for my container type to
behave appropriately depending on whether the string formatting
operation has a string like this :

"whatever %s yadayada" % mycontainer # needs to act like a tuple

"whatever %(key)s yadayada" % mycontainer # needs to act like a
dictionary

I looked through the Python data model docs at <http://docs.python.org/
reference/datamodel.html#emulating-container-types> but only found
this:

"If the left operand of a % operator is a string or Unicode object, no
coercion takes place and the string formatting operation is invoked
instead."

Thanks!

[Jean-Michel Pichavant]

class toto:
def __getitem__(self, key):
return 'paf'
def __str__(self):
return 'pif'
def toTuple(self):
return (1,2,3)

1/ print '%s %s %s' % toto().toTuple()
'1 2 3'
2/ print '%(key)s ' % toto()
'paf'
3/ print '%s' % toto()
'pif'

1/ I don't know how to spare the explicit toTuple conversion, supporting
tuple() would be tedious
2/ thanks to __getitem__ (duck typing)
3/ thanks to __str__

Anyway why would you want to use the tuple form ? it's beaten in every
aspect by the dictionary form.

JM

"If you need something call me, I'll tell you how to live without it"
(Coluche, about politicians)

[Brad Allen]

On Fri, Feb 5, 2010 at 9:49 AM, Jean-Michel Pichavant
<jeanm...@sequans.com> wrote:

> Anyway why would you want to use the tuple form ? it's beaten in every
> aspect by the dictionary form.

I'm subclassing a namedtuple, and adding some additional functionality
such as __getitem__, __setitem__, so that the namedtuple also behaves
like a dict.

[Raymond Hettinger]

On Feb 5, 6:57 am, bradallen <bradallen...@gmail.com> wrote:
> Hello,
>
> For container class derived from namedtuple, but which also behaves
> like a dictionary by implementing __getitem__ for non-integer index
> values, is there a special reserved method which allows intercepting %
> string formatting operations? I would like for my container type to
> behave appropriately depending on whether the string formatting
> operation has a string like this :
>
> "whatever %s yadayada" % mycontainer  # needs to act like a tuple
>
> "whatever %(key)s yadayada" % mycontainer  # needs to act like a
> dictionary

The implementation for str.__mod__ refuses to
treat tuples and tuple subclasses as a dictionary.
Since namedtuples are a subclass of tuple, you're
not going to have any luck with this one.

To see actual C code, look at PyString_Format() in
http://svn.python.org/view/python/trunk/Objects/stringobject.c?view=markup

PyObject *dict = NULL;
. . .
if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
!PyObject_TypeCheck(args, &PyBaseString_Type))
dict = args;

Since automatic conversion is out, you can instead use
the namedtuple._asdict() method for an explicit conversion:

>>> from collections import namedtuple
>>> Point = namedtuple('Point', 'x y')
>>> p = Point(5, 12)
>>> 'x: %(x)s' % p._asdict()
'x: 5'

Raymond

[Steven D'Aprano]

On Fri, 05 Feb 2010 16:49:00 +0100, Jean-Michel Pichavant wrote:

> Anyway why would you want to use the tuple form ? it's beaten in every
> aspect by the dictionary form.

Except convenience, efficiency and readability.

"%s %s" % (1, 2)

versus

"%(a)s %(b)s" % {'a': 1, 'b': 2}

I'm all in favour of the second form when you need it. But you certainly
don't need it all time.

--
Steven