Proposal: Light-weight call-by-name syntax in Python
The following syntax
a : b
is to be interpreted as:
a(lambda: b)
Effectively, this gives a "light-weight macro system" to Python,
since it allows with little syntax to indicate that the argument to
a function is not to be immediately invoked.
It is a generalization of special-case syntax proposals like
delayed: <expr>
In this proposal, `delayed' can be a normal callable.
Motivating examples follow:
# Logging
# The following assumes the logging library has been extended to support
# a callable argument in addition to a string.
import logging
logging.debug: "I have a %s" % expensive_function()
# Spawn parallel tasks
# This would work with the existing concurrent.futures unmodified.
from concurrent.futures import ThreadPoolExecutor
executor = ThreadPoolExecutor(max_workers=4)
future = executor.submit: some_long_running_function(another_long_one())
# Asyncio
loop.call_soon_threadsafe: do_something(42)
# Custom asserts
from some_module_implementing_contracts import precondition
def foo(x):
precondition: x >= 0
return math.sqrt(x)
# Lazy evaluation
class delayed:
def __init__(self, callback):
self.callback = callback
self.value = None
def __call__(self):
if self.callback is not None:
self.value = self.callback()
self.callback = None
return self.value
promise = delayed: a() + b()
print(promise())
# default dictionary
dd = collections.defaultdict: MyDefaultObject(42)
print(dd["foo"])
# => prints MyDefaultObject(42)
Notes on syntax:
a : b is my preferred syntax.
It would conflict with existing use of : in a few places, e.g.
in if and while. My preferred approach is that we would
need to write (a : b) there.
An alternative is a as-yet-unused token such as :: .
Stephan
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