Why does setting a default parameter value make this function a closure?

I'm writing an application where Tags are linkable and there's a need to retrieve the entire chain of linked Tags. Self-reference is not allowed. Running the following code ends up with some very strange results:

class Tag(object):
  def __init__(self, name):
    self.name = name
    self.links = []

  def __repr__(self):
    return "<Tag {0}>".format(self.name)

  def link(self, tag):
    self.links.append(tag)


def tag_chain(tag, known=[]):
  chain = []
  if tag not in known:
    known.append(tag)
  print "Known: {0}".format(known)

  for link in tag.links:
    if link in known:
      continue
    else:
      known.append(link)
    chain.append(link)
    chain.extend(tag_chain(link, know开发者_StackOverflow社区n))
  return chain

a = Tag("a")
b = Tag("b")
c = Tag("c")
a.link(b)
b.link(c)
c.link(a)

o = tag_chain(a)
print "Result:", o
print "------------------"
o = tag_chain(a)
print "Result:", o

Results:

Known: [<Tag a>]
Known: [<Tag a>, <Tag b>]
Known: [<Tag a>, <Tag b>, <Tag c>]
Result: [<Tag b>, <Tag c>]
------------------
Known: [<Tag a>, <Tag b>, <Tag c>]
Result: []

So, somehow, I've accidentally created a closure. As far as I can see, known should have gone out of scope and died off once the function call completed.

If I change the definition of chain_tags() to not set a default value, the problem goes away:

...
def tag_chain(tag, known):
...
o = tag_chain(a, [])
print "Result:", o
print "------------------"
o = tag_chain(a, [])
print "Result:", o

Why is this?

This is a common mistake in Python:

def tag_chain(tag, known=[]):
  # ...

known=[] doesn't mean that if known is unsupplied, make it an empty list; in fact, it binds known to an "anonymous" list. Each time that known defaults to that list, it is the same list.

The typical pattern to do what you intended here, is:

def tag_chain(tag, known=None):
    if known is None:
        known = []
    # ...

which correctly initializes known to an empty list if it is not provided.

Maybe as an extra eplanation what happens: Default parameters are simply stored on the function object itself (ie tag_chain.func_defaults in Py2 ) and used to extend the argument when needed:

>>> def x(a=['here']):
...     a.append(a[-1]*2)
...
>>> x
<function x at 0x0053DB70>
>>> x.func_defaults
(['here'],)

In this example you can watch the default list grow there:

>>> x()
>>> x.func_defaults
(['here', 'herehere'],)
>>> x()
>>> x.func_defaults
(['here', 'herehere', 'herehereherehere'],)

Modifying default arguments is somewhat like changing class variables.