A Python Gotcha: References as Default Parameters
Saturday, February 05, 2011.
Suppose you’re writing a Python function like this one that unpacks data into a dictionary; optionally, an existing dictionary instead of an empty one.
Surprise!
$ python
Python 2.6.4
[GCC 4.4.1] on linux2
>>> def hashcopy(src, dst={}):
... for k, v in src.items():
... dst[k] = v
... return dst
...
>>> hashcopy({1:2,3:4})
{1: 2, 3: 4}
>>> hashcopy({5:6,7:8})
{1: 2, 3: 4, 5: 6, 7: 8}I haven’t looked deeply into this, but it seems like default parameters must be bound to object instances at compile time.
In Perl 5 you typically only set default parameters at runtime, so the empty hashref you get is always the freshest in the land:
sub hashcopy
{
my $src = shift;
my $dst = shift || {};
%$dst = (%$dst, %$src);
return $dst;
}All other things equal, this is undoubtedly slower, but considerably less wtf-subtle.
blog comments powered by Disqus
