Mailing List Archive

On Fri, 30 Apr 99 01:30:12 GMT, Phil Hunt <philh@vision25.demon.co.uk> wrote:
>
>Consider the % operator, eg:
>
> 'All %(a)s eat %(b)s' % {'a':'cows', 'b':'grass'}
>
>If the dictionary doesn't have all the relevant keys, an error
>occurs. Is it possible for me to change the behaviour of this so that
>if a key doesn't occur a default value of '' is assumed?

Well, my way is to make my own dictionary:

import UserDict
class DefaultDict(UserDict.UserDict):
def __init__(self, dict, default=''):
self.default = default
self.data = dict
def __getitem__(self, key):
return self.data.get(key, self.default)

'All %(a)s eat %(b)s' % DefaultDict({'a': 'cows', 'b': 'grass'})

Union dictionaries (search through several dicts) are particularly useful for
%. There's even an implementation in C from DC (MultiMapping.so)

Phil Hunt <philh@vision25.demon.co.uk> wrote:
> Consider the % operator, eg:
>
> 'All %(a)s eat %(b)s' % {'a':'cows', 'b':'grass'}
>
> If the dictionary doesn't have all the relevant keys, an error
> occurs. Is it possible for me to change the behaviour of this so that
> if a key doesn't occur a default value of '' is assumed?

class D:
def __init__(self, dict=None, **extra):
self.dict = dict or {}
self.dict.update(extra)
def __getitem__(self, key):
try:
return self.dict[key]
except KeyError:
return ""

print 'All %(a)s eat %(b)s' % {'a': 'cows', 'b': 'grass'}
print 'All %(a)s eat %(b)s' % D({'c': 'swines', 'b': 'pearls'})
print 'All %(a)s eat %(b)s' % D(a='pythoneers', b='spam')

</F>

In article <slrn7iksi3.r36.quinn@photo.ugcs.caltech.edu>
quinn@photo.ugcs.caltech.edu "Quinn Dunkan" writes:
> On Fri, 30 Apr 99 01:30:12 GMT, Phil Hunt <philh@vision25.demon.co.uk> wrote:
> >Consider the % operator, eg:
> >
> > 'All %(a)s eat %(b)s' % {'a':'cows', 'b':'grass'}
> >
> >If the dictionary doesn't have all the relevant keys, an error
> >occurs. Is it possible for me to change the behaviour of this so that
> >if a key doesn't occur a default value of '' is assumed?
>
> Well, my way is to make my own dictionary:
>
> import UserDict
> class DefaultDict(UserDict.UserDict):
> def __init__(self, dict, default=''):
> self.default = default
> self.data = dict
> def __getitem__(self, key):
> return self.data.get(key, self.default)

That's neat.

One of the nice things about Python is that you can write stuff like
this in a small amount of code -- I bet the equivalent in C++ or Java
would be somewhat longer.

This answers the other question I had: how do I run a for loop over
a dictionary so that it loops over the elements sorted by key. Presumably
I'd just override the items() method in DefaultDict.

> 'All %(a)s eat %(b)s' % DefaultDict({'a': 'cows', 'b': 'grass'})
>
> Union dictionaries (search through several dicts) are particularly useful for
> %. There's even an implementation in C from DC (MultiMapping.so)
>

--
Phil Hunt....philh@vision25.demon.co.uk