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:
You should be iterating over the tuple and checking if the key is in the dict not the other way around, if you don't check if the key exists and it is not in the dict you are going to get a key error:
print({k:d[k] for k in l if k in d})
Some timings:
{k:d[k] for k in set(d).intersection(l)}
In [22]: %%timeit
l = xrange(100000)
{k:d[k] for k in l}
....:
100 loops, best of 3: 11.5 ms per loop
In [23]: %%timeit
l = xrange(100000)
{k:d[k] for k in set(d).intersection(l)}
....:
10 loops, best of 3: 20.4 ms per loop
In [24]: %%timeit
l = xrange(100000)
l = set(l)
{key: d[key] for key in d.viewkeys() & l}
....:
10 loops, best of 3: 24.7 ms per
In [25]: %%timeit
l = xrange(100000)
{k:d[k] for k in l if k in d}
....:
100 loops, best of 3: 17.9 ms per loop
I don't see how {k:d[k] for k in l}
is not readable or elegant and if all elements are in d then it is pretty efficient.
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.
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}
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
d = {1:2, 3:4, 5:6, 7:8}
l = (1,5)
{key: d[key] for key in l}
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}