我可以获取setdefault设置的默认值

Question

If I set a default value on a dict, is it possible to get the default value back after its been overwritten?

Like this:

dd = {}
dd.setdefault('beverage':'A nice cup of tea')
...
dd['beverage'] = 'Yellow urine'
...
if dd['beverage'] not in list_of_valid_beverages:
    # If the beverage has been contaminated revert to default.
    # Something like any of the following?
    dd['beverage'] = dd.getdefault('beverage')
    dd['beverage'].clear()
    del dd['beverage']
...
print dd['beverage']

My guess is that this is not possible with an ordinary dict. Ones the statement

dd['beverage'] = 'Yellow urine'

is executed the tea is forever lost. But it would be a nice feature :-)

Anyone with deeper knowledge about this?

Answer 1

No, it's not possible. All setdefault does is the following logic:

if key not in the_dict:
    the_dict[key] = value
return the_dict[key]

It does not treat the "default" value specially, and doesn't store it anywhere other than the location that you're overwriting.

If you want a set of persistent defaults, you're best off just keeping a separate dict of the default values.