What's the best way to initialize a dict of dicts in Python?
Question:
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?
Answers:
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)
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}}}
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
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
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?
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)
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}}}
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
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