Function attributes: a bug?

[Erik Max Francis]

Tj wrote:

> But there's a bug -- if I generate a function using this, use it for a
> while to build up a list, then call make_list_accum() again, it will
> want to init itself with the other list by default, sharing it!  This
> is surprising behavior, and doesn't happen if I don't use this
> optional init form.

This doesn't have anything to do with functio attributes; what you're
discovering is that default arguments are only evaluated once:

>>> def f(x, l=[]):
...  l.append(x)
...  print l
... 
>>> f(1)
[1]
>>> f(2)
[1, 2]
>>> f(3)
[1, 2, 3]

When the default argument is mutable, and you want to make sure you
don't run into this issue, use an immutable sentinel in the argument and
test for it in the body:

>>> def g(x, l=None): 
...  if l is None:
...   l = []
...  l.append(x) 
...  print l
... 
>>> g(1)
[1]
>>> g(2)
[2]
>>> g(3)
[3]

-- 
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