[ACCEPTED]-Accessing key in factory of defaultdict-defaultdict
__missing__ of defaultdict does not pass key to factory function.
If 4
default_factoryis notNone, it is called without arguments to provide a default 3 value for the given key, this value is inserted 2 in the dictionary for the key, and returned.
Make 1 your own dictionary class with custom __missing__ method.
>>> class MyDict(dict):
... def __init__(self, factory):
... self.factory = factory
... def __missing__(self, key):
... self[key] = self.factory(key)
... return self[key]
...
>>> d = MyDict(lambda x: -x)
>>> d[1]
-1
>>> d
{1: -1}
Unfortunately not directly, as defaultdict 4 specifies that default_factory must be called 3 with no arguments:
http://docs.python.org/2/library/collections.html#collections.defaultdict
But it is possible to 2 use defaultdict as a base class that has 1 the behavior you want:
class CustomDefaultdict(defaultdict):
def __missing__(self, key):
if self.default_factory:
dict.__setitem__(self, key, self.default_factory(key))
return self[key]
else:
defaultdict.__missing__(self, key)
This works for me:
>>> a = CustomDefaultdict(factory)
>>> a
defaultdict(<function factory at 0x7f0a70da11b8>, {})
>>> print a['aaa']
{'key-md5': '47bce5c74f589f4867dbd57e9ca9f808'}
>>> print a['bbb']
{'key-md5': '08f8e0260c64418510cefb2b06eee5cd'}
In several cases where I wanted a defaultdict 5 with the key in the factory, I found an 4 lru_cache also solved my problem:
import functools
@functools.lru_cache(maxsize=None)
def use_func_as_dict(key='') # Or whatever type
with open(key, 'r') as ifile:
return ifile.readlines()
f1 = use_func_as_dict('test.txt')
f2 = use_func_as_dict('test2.txt')
# This will reuse the old value instead of re-reading the file
f3 = use_func_as_dict('test.txt')
assert f3 is f1
This actually makes 3 more sense theoretically, since you're after 2 a function of the input rather than a consistent 1 dummy fallback.
More Related questions
We use cookies to improve the performance of the site. By staying on our site, you agree to the terms of use of cookies.