magical expanding hash

[braver]

I need a magical expanding hash with the following properties:

* it creates all intermediate keys

meh['foo']['bar] = 1

-- works even if meh['foo'] didn't exist before

* allows pushing new elements to leaves which are arrays

meh['foo']['list] << elem1
meh['foo']['list] << elem2

* allows incrementing numeric leaves

meh['foo']['count'] += 7

* serializable

I have such a class in ruby. Can python do that?

[James Stroud]

braver wrote:

I need a magical expanding hash with the following properties:

* it creates all intermediate keys

meh['foo']['bar] = 1

-- works even if meh['foo'] didn't exist before

* allows pushing new elements to leaves which are arrays

meh['foo']['list] << elem1
meh['foo']['list] << elem2

* allows incrementing numeric leaves

meh['foo']['count'] += 7

* serializable

I have such a class in ruby. Can python do that?

Is this too magical?
class meh(dict):
def __getitem__(sel f, item):
if self.has_key(it em):
return dict.__getitem_ _(self, item)
else:
anitem = meh()
dict.__setitem_ _(self, item, anitem)
return anitem
m = meh()

m['bob']['carol']['ted'] = 2

print m['bob']['carol']['ted']

[Paul Rubin]

"braver" <de*********@gm ail.com> writes:

I need a magical expanding hash with the following properties: ...
I have such a class in ruby. Can python do that?

Python's built-in dict objects don't do that but you could write such
a class pretty straightforward ly.

[Giovanni Bajo]

James Stroud wrote:

I need a magical expanding hash with the following properties:

* it creates all intermediate keys

meh['foo']['bar] = 1

-- works even if meh['foo'] didn't exist before

* allows pushing new elements to leaves which are arrays

meh['foo']['list] << elem1
meh['foo']['list] << elem2

* allows incrementing numeric leaves

meh['foo']['count'] += 7

* serializable

I have such a class in ruby. Can python do that?

Is this too magical?
class meh(dict):
def __getitem__(sel f, item):
if self.has_key(it em):
return dict.__getitem_ _(self, item)
else:
anitem = meh()
dict.__setitem_ _(self, item, anitem)
return anitem

Actually what the OP wants is already a method of dict, it's called
setdefault(). It's not overloaded by "[]" because it's believed to be better to
be able to say "I want auto-generation" explicitally rather than implicitly: it
gives the user more power to control, and to enforce stricter rules.

class meh(dict): .... def __getitem__(sel f, item):
.... return dict.setdefault (self, item, meh())
.... a = meh()
a["foo"]["bar"] = 2
a["foo"]["dup"] = 3
print a["foo"]["bar"] 2 print a

{'foo': {'dup': 3, 'bar': 2}}
So I advise using this class, and suggest the OP to try using setdefault()
explicitally to better understand Python's philosophy.

BTW: remember that setdefault() is written "setdefault ()" but it's read
"getorset() ".
--
Giovanni Bajo

[Diez B. Roggisch]

>

BTW: remember that setdefault() is written "setdefault ()" but it's read
"getorset() ".

I can only second that. The misleading name has - well, mislead me :)

Regards,

Diez

[Paul Rubin]

"Diez B. Roggisch" <de***@nospam.w eb.de> writes:

BTW: remember that setdefault() is written "setdefault ()" but it's read
"getorset() ".

I can only second that. The misleading name has - well, mislead me :)

Hmm,

x[a][b][c][d] = e # x is a "magic" dict

becomes

x.setdefault(a, {}).setdefault( b,{}).setdefaul t(c,{})[d] = e

if I understand correctly. Ugh.

[Steven Bethard]

Paul Rubin wrote:

Hmm,

x[a][b][c][d] = e # x is a "magic" dict

becomes

x.setdefault(a, {}).setdefault( b,{}).setdefaul t(c,{})[d] = e

if I understand correctly. Ugh.

Agreed. I really hope that Python 3.0 applies Raymond Hettinger's
suggestion "Improved default value logic for Dictionaries" from
http://wiki.python.org/moin/Python3%2e0Suggestions

This would allow you to make the setdefault() call only once, instead of
on every lookup:

class meh(dict):
def __init__(self, *args, **kwargs):
super(meh, self).__init__( *args, **kwargs)
self.setdefault (function=meh)

STeVe

[braver]

Nice. What about pushing to leaves which are arrays, or incrementing
leaves which are numbers? If the array leaf didn't exist, or a number
wasn't set yet, << must create an empty array and push the element from
the RHS into it, and += must init the leaf to 0 and add the RHS to it.
Here's the corresponding ruby:

# ruby!

class MagicalExpandin gHash < Hash
def initialize(*par ams)
if params.first.is _a? MagicalExpandin gHash
@parentObj, @parentKey = params[0..1]
params = params[2..-1]
end
super(*params) { |h,k|
h[k] = MagicalExpandin gHash.new(self, k)
}
end
def <<(elem)
if @parentObj[@parentKey].empty?
@parentObj[@parentKey] = [ elem ]
else
raise ArgumentError, "Can't push onto populated index", caller
end
end
def +(elem)
unless elem.is_a? Numeric
raise ArgumentError, "Can't add a non-Numeric value", caller
end
if @parentObj[@parentKey].empty?
@parentObj[@parentKey] = elem
else
raise ArgumentError, "Can't add to populated index", caller
end
end

def to_hash
h = Hash.new
self.each_pair {|k,v| h[k]=(v.class==self .class)? v.to_hash : v }
return h
end

def from_hash(h)
h.each_pair {|k,v| self[k]=(v.is_a? Hash) ?
self.class.new. from_hash(v) : v}
end

def marshal_dump
self.to_hash
end
def marshal_load(h)
from_hash(h)
end
end

# examples
if $0 == __FILE__
meh = MagicalExpandin gHash.new

meh['usa']['france'] << 'tocqueville'
meh['usa']['france'] << 'freedom fries'
meh['life']['meaning'] += 42

puts meh.inspect
# => {"usa"=>{"franc e"=>["tocquevill e", "freedom fries"]},
"life"=>{"meani ng"=>42}}
end

[Paul Rubin]

"braver" <de*********@gm ail.com> writes:

Nice. What about pushing to leaves which are arrays, or incrementing
leaves which are numbers? If the array leaf didn't exist, or a number
wasn't set yet, << must create an empty array and push the element from
the RHS into it, and += must init the leaf to 0 and add the RHS to it.

Are you trying to simulate Ruby syntax or just implement those functions?
Implementing the functions is easy enough. If you want Ruby syntax,
use Ruby.

[braver]

Actually, the behavior is important to translate perl into ruby. Can
it be implemented in python looking similarly?