Dictionaries and default values

Question

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

Answer 1

For multiple different defaults try this:

connectionDetails = { "host": "www.example.com" }
defaults = { "host": "127.0.0.1", "port": 8080 }

completeDetails = {}
completeDetails.update(defaults)
completeDetails.update(connectionDetails)
completeDetails["host"]  # ==> "www.example.com"
completeDetails["port"]  # ==> 8080

Answer 2

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"

Answer 3

There is a method in python dictionaries to do this: dict.setdefault

connectionDetails.setdefault('host',someDefaultValue)
host = connectionDetails['host']

However this method sets the value of connectionDetails['host'] to someDefaultValue if key host is not already defined, unlike what the question asked.

Answer 4

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

Answer 5

Like this:

host = connectionDetails.get('host','someDefault')

Answer 6

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"