Good uses for mutable function argument default values?

Question

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?

Answer 1

You can use it to cache values between function calls:

def get_from_cache(name, cache={}):     if name in cache: return cache[name]     cache[name] = result = expensive_calculation()     return result 

but usually that sort of thing is done better with a class as you can then have additional attributes to clear the cache etc.

Answer 2

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)