I have a dictionary, and would like to pass a part of it to a function, that part being given by a list (or tuple) of keys. Like so:
# the dictionary
d = {1:2, 3:4, 5:6, 7:8}
# the subset of keys I'm interested in
l = (1,5)
Now, ideally I'd like to be able to do this:
>>> d[l]
{1:2, 5:6}
... but that's not working, since it will look for a key named (1,5)
.
And d[1,5]
isn't even valid Python (though it seems it would be handy).
I know I can do this:
>>> dict([(key, value) for key,value in d.iteritems() if key in l])
{1: 2, 5: 6}
or this:
>>> dict([(key, d[key]) for key in l])
which is more compact ... but I feel there must be a "better" way of doing this. Am I missing a more elegant solution?
(I'm using Python 2.7)
This question is related to
python
dictionary
set intersection
and dict comprehension
can be used here
# the dictionary
d = {1:2, 3:4, 5:6, 7:8}
# the subset of keys I'm interested in
l = (1,5)
>>>{key:d[key] for key in set(l) & set(d)}
{1: 2, 5: 6}
Write a dict
subclass that accepts a list of keys as an "item" and returns a "slice" of the dictionary:
class SliceableDict(dict):
default = None
def __getitem__(self, key):
if isinstance(key, list): # use one return statement below
# uses default value if a key does not exist
return {k: self.get(k, self.default) for k in key}
# raises KeyError if a key does not exist
return {k: self[k] for k in key}
# omits key if it does not exist
return {k: self[k] for k in key if k in self}
return dict.get(self, key)
Usage:
d = SliceableDict({1:2, 3:4, 5:6, 7:8})
d[[1, 5]] # {1: 2, 5: 6}
Or if you want to use a separate method for this type of access, you can use *
to accept any number of arguments:
class SliceableDict(dict):
def slice(self, *keys):
return {k: self[k] for k in keys}
# or one of the others from the first example
d = SliceableDict({1:2, 3:4, 5:6, 7:8})
d.slice(1, 5) # {1: 2, 5: 6}
keys = 1, 5
d.slice(*keys) # same
On Python 3 you can use the itertools islice
to slice the dict.items()
iterator
import itertools
d = {1: 2, 3: 4, 5: 6}
dict(itertools.islice(d.items(), 2))
{1: 2, 3: 4}
Note: this solution does not take into account specific keys. It slices by internal ordering of d
, which in Python 3.7+ is guaranteed to be insertion-ordered.
d = {1:2, 3:4, 5:6, 7:8}
l = (1,5)
{key: d[key] for key in l}
Use a set to intersect on the dict.viewkeys()
dictionary view:
l = {1, 5}
{key: d[key] for key in d.viewkeys() & l}
This is Python 2 syntax, in Python 3 use d.keys()
.
This still uses a loop, but at least the dictionary comprehension is a lot more readable. Using set intersections is very efficient, even if d
or l
is large.
Demo:
>>> d = {1:2, 3:4, 5:6, 7:8}
>>> l = {1, 5}
>>> {key: d[key] for key in d.viewkeys() & l}
{1: 2, 5: 6}
Source: Stackoverflow.com