Steven Bethard wrote:
[EMAIL PROTECTED] wrote:
However, is there a good reason why default parameters aren't evaluated as the function is called? (apart from efficiency and backwards compatibility)?
So, one of my really common use cases that takes advantage of the fact that default parameters are evaluated at function definition time:
def foo(bar, baz, matcher=re.compile(r'...')): ... text = matcher.sub(r'...', text) ...
Surely "re.compile(r'...')" is effectively a constant ? So your above code is equivalent to :
aConst = re.compile(r'...') def foo(bar, baz, matcher=aConst): ... text = matcher.sub(r'...', text) ...
Basically, yes. Though you add an extra name to the module or class namespace by defining 'aConst'. I consider this a minor pollution of the namespace since 'matcher' is only used in the function, not the module or class.
But if we're arguing for equivalencies, the oft-asked for
def foo(lst=[]):
...where [] is evaluated for each function call, is equivalent for most purposes to:
def foo(lst=None):
if lst is None:
lst = []There's a minor pollution of the range of values 'lst' can take on -- if you need lst to be able to take on the value None, you'll want to rewrite this something like:
no_value = object()
def foo(lst=no_value):
if lst is no_value:
lst = []My point here is that there's always going to be some equivalent expression (turing completeness etc.) I was just pointing out a quite reasonable use of the current default parameter evaluation system.
Steve -- http://mail.python.org/mailman/listinfo/python-list
