factory func of collections.defaultdict should receive the "missing key" as args when called.

[lyricconch]

BPO	12473
Nosy	@amauryfa

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2011-07-02.08:31:11.474>
created_at = <Date 2011-07-02.07:21:09.476>
labels = ['invalid', 'type-bug', 'library']
title = 'factory func of collections.defaultdict should receive the "missing key" as args when called.'
updated_at = <Date 2011-07-02.08:31:11.467>
user = 'https://bugs.python.org/lyricconch'

bugs.python.org fields:

activity = <Date 2011-07-02.08:31:11.467>
actor = 'amaury.forgeotdarc'
assignee = 'none'
closed = True
closed_date = <Date 2011-07-02.08:31:11.474>
closer = 'amaury.forgeotdarc'
components = ['Library (Lib)']
creation = <Date 2011-07-02.07:21:09.476>
creator = 'lyricconch'
dependencies = []
files = []
hgrepos = []
issue_num = 12473
keywords = []
message_count = 2.0
messages = ['139624', '139625']
nosy_count = 2.0
nosy_names = ['amaury.forgeotdarc', 'lyricconch']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue12473'
versions = ['Python 2.7']

[lyricconch]

for example:

def calc(params):
    """ i am factoring numbers. """
    # an expensive CPU cost function but 
    # passin params and return result are both lightweight

cachedcalc = collections.defaultdict(calc)
result = cachedcalc[0xFFFFFFFFFFFFFFFFFFF0AC0FFF1]

[amauryfa]

-1. Besides compatibility issues, defaultdict is a dict: it contains data, and is not meant to consume CPU when accessing items. Its "default" function should return initial values, like 0 or an empty list.

I think what you want is "memoization"; a memoized function still looks like a function! there are many implementations for python, one of these is here: http://wiki.python.org/moin/PythonDecoratorLibrary#Memoize