HEADSUP default parameters

[Robert Collins]

I just got rid of the 'deleting paths' test stipple from bzrtools.

It was caused by a default parameter that is not None:

def foo(to_file=sys.stdout):
    # stuff here.


The reason this caused stipple is that the default parameter is
evaluated at compile time, causing any redirection to not take effect.

The correct way to do this is:

def foo(to_file=None):
    if to_file is None:
        to_file = sys.stdout


Similar bugs can be trigged by *any* default parameter other than None.

So this is a HEADSUP to code reviewers: please double-check that any
default parameter with a value of None is really what was intended.

Rob
-- 
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