Question

I use a dict as a short-term cache. I want to get a value from the dictionary, and if the dictionary didn’t already have that key, set it, e.g.:

val = cache.get('the-key', calculate_value('the-key'))
cache['the-key'] = val

In the case where 'the-key' was already in cache, the second line is not necessary. Is there a better, shorter, more expressive idiom for this?

Answer 1

yes, use:

val = cache.setdefault('the-key', calculate_value('the-key'))

An example in the shell:

>>> cache = {'a': 1, 'b': 2}
>>> cache.setdefault('a', 0)
1
>>> cache.setdefault('b', 0)
2
>>> cache.setdefault('c', 0)
0
>>> cache
{'a': 1, 'c': 0, 'b': 2}

See: http://docs.python.org/release/2.5.2/lib/typesmapping.html