[Python-ideas] kwargs for return
Stephen J. Turnbull
turnbull.stephen.fw at u.tsukuba.ac.jp
Sun Jan 27 06:28:00 EST 2019
Thomas Güttler Lists writes:
> status = backend.transmit_data()
> But later you want to add something to the [API's return value].
Common Lisp has this feature. There, it's called "multiple values".
The called function uses a special form that creates a multiple value.
The primary value is always available, whether the return is a normal
value or a multiple value. Another special form is used to access the
secondary values (and if it's not used, any secondary values are
immediately discarded). In Python-like pseudo-code:
def divide(dividend, divisor):
# A special form, represented as syntax.
return_multiple_values (dividend // divisor, dividend % divisor)
def greatest_smaller_multiple(dividend, divisor):
result = divisor * divide(dividend, divisor)
# Secondary values are not accessible from result.
def recreate_dividend(dividend, divisor):
# The ``multiple_values`` builtin is magic: there is no other way
# to "see" multiple values.
# It returns a tuple.
# For keyword returns, have a convention that the second value
# is a dict or use an appropriate "UnboxedMultipleValues" class.
vs = multiple_values(divide(dividend, divisor))
return vs * divisor + vs
But Common Lisp experience suggests
(1) if you want to use this feature to change the API, you generally
want to do a full refactoring anyway, because
(2) the called function can (and often enough to be problematic,
*does*) creep in the direction of *requiring* attention to the
secondary values, can (and sometimes *does*) lead to subtle bugs
at call sites that only use the primary value.
It's unclear to me that this feature can be safely used in the way
your example suggests.
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