Why the result changes each time?

Question

def sum_specified_dims(mdd, dims, keys = [], result = {}):
    for key,value in mdd.items():
        keys.append(key)
        if type(value) == dict:
            sum_specified_dims(value, dims, keys, result)
        else:
            pointer = result
            for dim in dims:
                index = dim - 1
                key_value = keys[index]
                if not key_value in pointer:
                    pointer[key_value] = {}
                pointer = pointer[key_value]
            if key_of_sum in pointer:
                pointer[key_of_sum] += value
            else:
                pointer[key_of_sum] = value
        keys.pop()
    return result

key_of_sum='total'
keys=[1]
import datetime
mdd={datetime.date(2013, 7, 16): {3.0: {'2.0': 8}}}
>>> sum_specified_dims(mdd,keys)
{datetime.date(2013, 7, 16): {'total': 8}}
>>> sum_specified_dims(mdd,keys)
{datetime.date(2013, 7, 16): {'total': 16}}
>>> sum_specified_dims(mdd,keys,[],{})
{datetime.date(2013, 7, 16): {'total': 8}}

If I omit the 3rd,4th parameters, the result will accumulate each time.

But it's OK if I don't omit any parameters.

Anyone can explain why this happens, is it a python bug?

UPDATE

it seems not always happens:

>>> def f(a=2):
...     print a
... 
>>> f(3)
3
>>> f()
2

Integers are immutable, while lists and dictionaries are mutable. It's all explained in the linked question. — Blender, Jul 23 '13 at 07:29
@user2559257 : you didn't, you're not mutating the default argument in that case. — mikołak, Jul 23 '13 at 07:30
Try to avoid this "keys = [], result = {}" as mutable parameters. See this link -> http://docs.python-guide.org/en/latest/writing/gotchas.html#mutable-default-arguments — Alberto Megía, Jul 23 '13 at 07:37

score 3 · Accepted Answer · answered Jul 23 '13 at 07:26

3

The reason is that mutable (e.g. lists) default arguments in python 'remember' their earlier values.

See http://effbot.org/zone/default-values.htm for an interesting explanation.

answered Jul 23 '13 at 07:26

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Why the result changes each time?

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