is it a bug ??

[Terry Reedy]

> def g(a=[]):
> print a
> if len(a)<5:
> a = a + [1]

This creates a new list which is passed on to g, which creates another
new list....

> g(a)
> print a

This prints a *different* list than the first print statement.
To better see what is goint on print id(a), a in both places
Note: since you pass an explicit argument at every call after the
first, the difference you see is not about default args, and using
that for the first call only confuses the issue.

>>> def g(a):
...   print id(a),a
...   if len(a)<5:
...     a = a + [1]
...     g(a)
...   print id(a),a
...
>>> g([])
7938416 []
7962144 [1]
7962096 [1, 1]
7962048 [1, 1, 1]
7962000 [1, 1, 1, 1]
7961952 [1, 1, 1, 1, 1]
7961952 [1, 1, 1, 1, 1]
7961952 [1, 1, 1, 1, 1]
7962000 [1, 1, 1, 1]
7962048 [1, 1, 1]
7962096 [1, 1]
7962144 [1]

Note that there is a different list for each length.

> def f(a=[]):
> print a
> if len(a)<5:
> a.append(1)

This mutates the list in place (and passes it on)

> f(a)
> print a

So all prints print the same list, which never shrinks.  Again, print
the id also.

Terry J. Reedy