dict.setdefault: Bug: default argument is ALWAYS evaluated, i.e. no short-circuit eval

[albertneu]

BPO	10930
Nosy	@amauryfa, @ericvsmith

^{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 2011-01-18.12:14:12.338>
created_at = <Date 2011-01-18.10:54:36.126>
labels = ['type-bug', 'invalid']
title = 'dict.setdefault: Bug: default argument is ALWAYS evaluated, i.e. no short-circuit eval'
updated_at = <Date 2011-01-18.15:28:04.916>
user = 'https://bugs.python.org/albertneu'

bugs.python.org fields:

activity = <Date 2011-01-18.15:28:04.916>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2011-01-18.12:14:12.338>
closer = 'amaury.forgeotdarc'
components = []
creation = <Date 2011-01-18.10:54:36.126>
creator = 'albert.neu'
dependencies = []
files = []
hgrepos = []
issue_num = 10930
keywords = []
message_count = 3.0
messages = ['126456', '126458', '126467']
nosy_count = 3.0
nosy_names = ['amaury.forgeotdarc', 'eric.smith', 'albert.neu']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue10930'
versions = ['Python 2.6']

[albertneu]

Hello!

Is it intentional, that the default argument is ALWAYS evaluated, even if it is not needed???

Is it not a bug, that this method has no short-circuit eval (http://en.wikipedia.org/wiki/Short-circuit_evaluation)
??

Example1:
=========

infinite = 1e100

one_div_by = {0.0 : infinite}

def func(n):
    return one_div_by.setdefault(float(n), 1/float(n))

for i in [1, 2, 3, 4]:
    print i, func(i)
print one_div_by
# works!!

for i in [0, 1, 2, 3, 4]:     # added 0 -> FAIL!
    print i, func(i)
print one_div_by
# fail!!

Example2:
=========

fib_d = {0 : 0, 1 : 1}

def fibonacci(n):
    return fib_d.setdefault(n, fibonacci(n-1) + fibonacci(n-2))

for i in range(10):
    print i, fibonacci(i)
print fib_d

[amauryfa]

setdefault() is a method, its arguments are evaluated then the function is called. This is not a bug, and this behavior cannot change.

If you are trying to "cache" the computation of a function, you should try "memoizing" techniques, like the one mentioned here: http://code.activestate.com/recipes/52201-memoizing-cacheing-function-return-values/
Then you can write::

    @Memoize
    def fib(n):
        return fib(n-1) + fib(n-2)
    fib.memo = {(0,): 1, (1,): 1}

    @Memoize
    def func(n):
        return 1/float(n)
    func.memo = {(0.0,): infinite}

[ericvsmith]

Or use a collections.defaultdict, which has a factory function as a constructor argument. It sort of depends on what you're trying to do.