# Very common: Use None and replace it in the function
def bisect_right(a, x, lo=0, hi=None, *, key=None):
if hi is None:
hi = len(a)
Note that if this is changed to
def bisect_right(a, x, lo=0, hi[whatever]len(a), *, key=None):
then you'll lose the ability to pass hi=None to mean the end of the array. For this argument of bisect_right, None actually makes sense since it's used for that purpose in slices, and it would probably be a backward-compatibility problem to drop support for it also. So you will still have to write
def bisect_right(a, x, lo=0, hi[whatever]len(a), *, key=None):
if hi is None:
hi = len(a)
Self-referential expressions will result in UnboundLocalError::
def spam(eggs=>eggs): # Nope
That seems like not a rule of its own, but a special case of this rule: deferred arguments that haven't been assigned to yet are unbound (and not, say, set to some implementation-specific value analogous to _USE_GLOBAL_DEFAULT).
def bisect(a, hi=>len(a)):
That looks like an anonymous function that takes a single parameter named hi, ignores it, and returns the length of a free variable named a. It may even mean that in Python someday in expression contexts. But it's definitely not what it means here.
hi<=len(a) would make a bit more sense.
def bisect(a, hi=:len(a)):
This one annoys me the least, though I can't say why.
def bisect(a, hi?=len(a)):
? seems reasonable for omitted/defaulted arguments, but that's not what this PEP is about. I can't see why call-time evaluation would have a ? and def-time evaluation wouldn't.
def bisect(a, hi!=len(a)):
Same basic problem as => (and <=). Is hi equal to len(a), or not?
def bisect(a, hi=`len(a)`):
It looks like the backticks are part of the expression rather than the function-parameter syntax, but they aren't, and never could be - even if there was a deferred-evaluation syntax at the expression level, it couldn't be used here, because the lexical environment would be wrong.
Since default arguments behave largely the same whether they're early or late
bound,
They seem very different to me. They're evaluated in a different lexical scope, and many use cases for this extension depend on the changed scoping.