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pythondefault-arguments

Good uses for mutable function argument default values?

It is a common mistake in Python to set a mutable object as the default value of an argument in a function. Here's an example taken from this excellent write-up by David Goodger:

>>> def bad_append(new_item, a_list=[]):
        a_list.append(new_item)
        return a_list
>>> print bad_append('one')
['one']
>>> print bad_append('two')
['one', 'two']

The explanation why this happens is here.

And now for my question: Is there a good use-case for this syntax?

I mean, if everybody who encounters it makes the same mistake, debugs it, understands the issue and from thereon tries to avoid it, what use is there for such syntax?

Solution

  • Canonical answer is this page: http://effbot.org/zone/default-values.htm

    It also mentions 3 "good" use cases for mutable default argument:

    • binding local variable to current value of outer variable in a callback
    • cache/memoization
    • local rebinding of global names (for highly optimized code)