dictionary wart

[Jesper Olsen]

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

Intuitively I would imagine that

a={}
a.set_default(my_default)

would do this -ie. a[my_new_key] should now return the the default
value my_default instead of creating an exception. This would be
similar to how it is done in other languages, eg. Ruby:

a={}
a.default=value

But surprisingly, in python set_default is a dictionary method used
for looking up a key...

Jesper

[Paul Rubin]

jo****@mail2world.com (Jesper Olsen) writes:

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

Intuitively I would imagine that

a={}
a.set_default(my_default)

would do this -ie. a[my_new_key] should now return the the default
value my_default instead of creating an exception.

No, instead of a[my_new_key] use a.get(my_new_key, my_default).
That does what you want.

[Ben Finney]

On 18 Mar 2004 04:41:38 -0800, Jesper Olsen wrote:

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

But surprisingly, in python set_default is a dictionary method used
for looking up a key...

(Probably you mean setdefault(), since there's no set_default attribute
in any default Python object.)

A dict, by design, has a defined set of keys, like all mapping types.
Your "dictionary default" would change a dictionary so that it
effectively had an infinite set of keys.

<http://www.python.org/doc/current/lib/typesmapping.html>

It sounds like you want an iterator, rather than a dict.

<http://www.python.org/doc/current/lib/typeiter.html>

But without knowing the problem you're trying to solve, we can only
guess at the solution.

--
\ "I got a postcard from my best friend, it was a satellite |
`\ picture of the entire Earth. On the back he wrote, 'Wish you |
_o__) were here'." -- Steven Wright |
Ben Finney <http://bignose.squidly.org/>

[Jesper]

Paul Rubin wrote:

jo****@mail2world.com (Jesper Olsen) writes:

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

Intuitively I would imagine that

a={}
a.set_default(my_default)

would do this -ie. a[my_new_key] should now return the the default
value my_default instead of creating an exception.

No, instead of a[my_new_key] use a.get(my_new_key, my_default).
That does what you want.

Unfortunately not - get() is different, because you can not assign to it.
For instance in a dictionary which is mapping strings to integers (or
lists), I would like to do

a[my_key]+=5

expressing that with get() would be awkward.

Jesper

[Fuzzyman]

On Thu, 18 Mar 2004 13:46:19 GMT, Jesper <jo****@mail2world.com>
wrote:

Paul Rubin wrote:

jo****@mail2world.com (Jesper Olsen) writes:

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

Intuitively I would imagine that

a={}
a.set_default(my_default)

would do this -ie. a[my_new_key] should now return the the default
value my_default instead of creating an exception.

No, instead of a[my_new_key] use a.get(my_new_key, my_default).
That does what you want.

Unfortunately not - get() is different, because you can not assign to it.
For instance in a dictionary which is mapping strings to integers (or
lists), I would like to do

a[my_key]+=5

expressing that with get() would be awkward.

Jesper

You can however subclass dict to create something that *behaves* like
a dictionary, but for unknown keys returns a default :

Regards,

Fuzzy

http://www.voidspace.org.uk/atlantib...thonutils.html

e.g. (untested - but should work)
class defaultDict(dict):
"""A dictionary that returns a default for non-existent keys.
Won't work for methods like pop unless you also define them as
well."""
def __init__(self, default):
dict.__init__(self)
self.default = default

def __getitem__(self, item):
if self.has_key(item):
return dict.__getitem__(self, key)
else:
return self.default

if __name__ == '__main__':
a = defaultDict(3)
a['test'] = 4
print a['test']
print a['fish']


---
Everyone has talent. What is rare is the courage to follow talent to the dark place where it leads. -Erica Jong
Ambition is a poor excuse for not having sense enough to be lazy. -Milan Kundera

http://www.voidspace.org.uk
Where Headspace Meets Cyberspace
Cyberpunk and Science Resource Site
Exploring the worlds of Psychology, Spirituality, Science and Computing
--
http://www.fuchsiashockz.co.uk
http://groups.yahoo.com/group/void-shockz
http://www.learnlandrover.com

[Fuzzyman]

On Thu, 18 Mar 2004 15:06:48 +0000, Fuzzyman <mi*****@foord.net>
wrote:

On Thu, 18 Mar 2004 13:46:19 GMT, Jesper <jo****@mail2world.com>
wrote:

Paul Rubin wrote:

jo****@mail2world.com (Jesper Olsen) writes:

Does python have a way of defining a dictionary default?
I think not, but are there any plans to incorporate it?

Intuitively I would imagine that

a={}
a.set_default(my_default)

would do this -ie. a[my_new_key] should now return the the default
value my_default instead of creating an exception.
No, instead of a[my_new_key] use a.get(my_new_key, my_default).
That does what you want.
Unfortunately not - get() is different, because you can not assign to it.
For instance in a dictionary which is mapping strings to integers (or
lists), I would like to do

a[my_key]+=5

expressing that with get() would be awkward.

Jesper

You can however subclass dict to create something that *behaves* like
a dictionary, but for unknown keys returns a default :

Regards,

Fuzzy

http://www.voidspace.org.uk/atlantib...thonutils.html

e.g. (untested - but should work)
class defaultDict(dict):
"""A dictionary that returns a default for non-existent keys.
Won't work for methods like pop unless you also define them as
well."""
def __init__(self, default):
dict.__init__(self)
self.default = default

def __getitem__(self, item):
if self.has_key(item):
return dict.__getitem__(self, key)
else:
return self.default

Oops - that would be (and I see that someone else already suggested
this anyway )...

return dict.__getitem__(self, item)

if __name__ == '__main__':
a = defaultDict(3)
a['test'] = 4
print a['test']
print a['fish']


---
Everyone has talent. What is rare is the courage to follow talent to the dark place where it leads. -Erica Jong
Ambition is a poor excuse for not having sense enough to be lazy. -Milan Kundera

http://www.voidspace.org.uk
Where Headspace Meets Cyberspace
Cyberpunk and Science Resource Site
Exploring the worlds of Psychology, Spirituality, Science and Computing

---
Everyone has talent. What is rare is the courage to follow talent to the dark place where it leads. -Erica Jong
Ambition is a poor excuse for not having sense enough to be lazy. -Milan Kundera

http://www.voidspace.org.uk
Where Headspace Meets Cyberspace
Cyberpunk and Science Resource Site
Exploring the worlds of Psychology, Spirituality, Science and Computing
--
http://www.fuchsiashockz.co.uk
http://groups.yahoo.com/group/void-shockz
http://www.learnlandrover.com

[Skip Montanaro]

Jesper> For instance in a dictionary which is mapping strings to
Jesper> integers (or lists), I would like to do

Jesper> a[my_key]+=5

Jesper> expressing that with get() would be awkward.

Though not *terribly* awkward:

a[my_key] = a.get(my_key, 0) + 5

I agree it's a bit clumsy, but I think it's the best you can do if you don't
want to subclass dict (I don't really like {}.setdefault()):

class ddict(dict):
def __init__(self, default=None, init=True):
self.default = default
self.init = init

def __getitem__(self, key):
if key in self:
return self.get(key)
else:
if self.init:
self[key] = self.default
return self.default

d = ddict(default="Jesper", init=False)
print "abc" in d
print d["abc"]
d = ddict(default=0)
d["abc"] += 5
print d["abc"]

I suspect there's more to it than I've implemented, but that should get you
pointed in the right direction.

Skip