There’s a PEP suggested for 3.11 handling this
def fn(s, n=>len(s)):
Of the alternatives they offer, ":=" kind of looks nicest to me:
def fn(s, n:=len(s)):
Semantically, the "?=" alternative might be best, since it kind of makes it clear it's a default, and doesn't conflict with anything else in the language:
def fn(s, n?=len(s)):
edit: At this point I'd shift from "might be best" to "almost certainly is best". I'm now a zealous "?=" advocate.
`x ?= default` is perfect IMHO. There's already prior art (Makefiles) that this means exactly "present? else = ..." with lazy evaluation of RHS expression.
It also would allow eliminating the "if x is SENTINEL" pattern, where None is a valid non-default argument.
The only potential downside is that standard, function definition-time default arguments also imply "present? else = ...". But I think of all the options available, this one makes the most sense.
def foo(..., cache={}):
...
I'm not sure it handles the use case op is talking about.
IE def foo(bar=list()): # this creates a mutable list shard by all calls of the function. Very unlikely something you want.
Maybe the pep would allow for def foo(bar=>list()):
But the pep does not address this use case at all
>Function parameters can have default values which are calculated during function definition and saved. This proposal introduces a new form of argument default, defined by an expression to be evaluated at function call time.