What's the best way to initialize a dict of dicts in Python? [duplicate]

问题

A lot of times in Perl, I\'ll do something like this:

$myhash{foo}{bar}{baz} = 1

How would I translate this to Python? So far I have:

if not \'foo\' in myhash:
    myhash[\'foo\'] = {}
if not \'bar\' in myhash[\'foo\']:
    myhash[\'foo\'][\'bar\'] = {}
myhash[\'foo\'][\'bar\'][\'baz\'] = 1

Is there a better way?

回答1:

class AutoVivification(dict):
    """Implementation of perl's autovivification feature."""
    def __getitem__(self, item):
        try:
            return dict.__getitem__(self, item)
        except KeyError:
            value = self[item] = type(self)()
            return value

Testing:

a = AutoVivification()

a[1][2][3] = 4
a[1][3][3] = 5
a[1][2]['test'] = 6

print a

Output:

{1: {2: {'test': 6, 3: 4}, 3: {3: 5}}}

回答2:

If the amount of nesting you need is fixed, collections.defaultdict is wonderful.

e.g. nesting two deep:

myhash = collections.defaultdict(dict)
myhash[1][2] = 3
myhash[1][3] = 13
myhash[2][4] = 9

If you want to go another level of nesting, you'll need to do something like:

myhash = collections.defaultdict(lambda : collections.defaultdict(dict))
myhash[1][2][3] = 4
myhash[1][3][3] = 5
myhash[1][2]['test'] = 6

edit: MizardX points out that we can get full genericity with a simple function:

import collections
def makehash():
    return collections.defaultdict(makehash)

Now we can do:

myhash = makehash()
myhash[1][2] = 4
myhash[1][3] = 8
myhash[2][5][8] = 17
# etc

回答3:

Is there a reason it needs to be a dict of dicts? If there's no compelling reason for that particular structure, you could simply index the dict with a tuple:

mydict = {('foo', 'bar', 'baz'):1} # Initializes dict with a key/value pair
mydict[('foo', 'bar', 'baz')]      # Returns 1

mydict[('foo', 'unbar')] = 2       # Sets a value for a new key

The parentheses are required if you initialize the dict with a tuple key, but you can omit them when setting/getting values using []:

mydict = {}                        # Initialized the dict
mydict['foo', 'bar', 'baz'] = 1    # Sets a value
mydict['foo', 'bar', 'baz']        # Returns 1

回答4:

I guess the literal translation would be:

 mydict = {'foo' : { 'bar' : { 'baz':1}}}

Calling:

 >>> mydict['foo']['bar']['baz']

gives you 1.

That looks a little gross to me, though.

(I'm no perl guy, though, so I'm guessing at what your perl does)

来源：https://stackoverflow.com/questions/651794/whats-the-best-way-to-initialize-a-dict-of-dicts-in-python

标签

python

autovivification