Assuming connectionDetails is a Python dictionary, what's the best, most elegant, most "pythonic" way of refactoring code like this?
if "host" in connectionDetails: host = connectionDetails["host"] else: host = someDefaultValue 9 Answers
Like this:
host = connectionDetails.get('host', someDefaultValue) You can also use the defaultdict like so:
from collections import defaultdict a = defaultdict(lambda: "default", key="some_value") a["blabla"] => "default" a["key"] => "some_value" You can pass any ordinary function instead of lambda:
from collections import defaultdict def a(): return 4 b = defaultdict(a, key="some_value") b['absent'] => 4 b['key'] => "some_value" While .get() is a nice idiom, it's slower than if/else (and slower than try/except if presence of the key in the dictionary can be expected most of the time):
>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="try:\n a=d[1]\nexcept KeyError:\n a=10") 0.07691968797894333 >>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="try:\n a=d[2]\nexcept KeyError:\n a=10") 0.4583777282275605 >>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="a=d.get(1, 10)") 0.17784020746671558 >>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="a=d.get(2, 10)") 0.17952161730158878 >>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="if 1 in d:\n a=d[1]\nelse:\n a=10") 0.10071221458065338 >>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", ... stmt="if 2 in d:\n a=d[2]\nelse:\n a=10") 0.06966537335119938 For multiple different defaults try this:
connectionDetails = { "host": "" } defaults = { "host": "127.0.0.1", "port": 8080 } completeDetails = {} completeDetails.update(defaults) completeDetails.update(connectionDetails) completeDetails["host"] # ==> "" completeDetails["port"] # ==> 8080 This is not exactly the question asked for but there is a method in python dictionaries: dict.setdefault
host = connectionDetails.setdefault('host',someDefaultValue) However this method sets the value of connectionDetails['host'] to someDefaultValue if key host is not already defined, unlike what the question asked.
(this is a late answer)
An alternative is to subclass the dict class and implement the __missing__() method, like this:
class ConnectionDetails(dict): def __missing__(self, key): if key == 'host': return "localhost" raise KeyError(key) Examples:
>>> connection_details = ConnectionDetails(port=80) >>> connection_details['host'] 'localhost' >>> connection_details['port'] 80 >>> connection_details['password'] Traceback (most recent call last): File "python", line 1, in <module> File "python", line 6, in __missing__ KeyError: 'password' Testing @Tim Pietzcker's suspicion about the situation in PyPy (5.2.0-alpha0) for Python 3.3.5, I find that indeed both .get() and the if/else way perform similar. Actually it seems that in the if/else case there is even only a single lookup if the condition and the assignment involve the same key (compare with the last case where there is two lookups).
>>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="try:\n a=d[1]\nexcept KeyError:\n a=10") 0.011889292989508249 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="try:\n a=d[2]\nexcept KeyError:\n a=10") 0.07310474599944428 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="a=d.get(1, 10)") 0.010391917996457778 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="a=d.get(2, 10)") 0.009348208011942916 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="if 1 in d:\n a=d[1]\nelse:\n a=10") 0.011475925013655797 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="if 2 in d:\n a=d[2]\nelse:\n a=10") 0.009605801998986863 >>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", .... stmt="if 2 in d:\n a=d[2]\nelse:\n a=d[1]") 0.017342638995614834 You can use a lamba function for this as a one-liner. Make a new object connectionDetails2 which is accessed like a function...
connectionDetails2 = lambda k: connectionDetails[k] if k in connectionDetails.keys() else "DEFAULT" Now use
connectionDetails2(k) instead of
connectionDetails[k] which returns the dictionary value if k is in the keys, otherwise it returns "DEFAULT"
I am sure that all these answers are ok but it shows that there is no 'nice' way of doing this. I use dictionaries instead of case statements all the time and to add a default clause I just call the following function:
def choose(choise, choises, default): """Choose a choice from the choises given """ return choises[choise] if choise in choises else default That way I can use normal dictionaries without special default clauses etc.
