On Thu, Sep 09, 2021 at 09:16:23AM -0700, Guido van Rossum wrote:
> assert cond, raise=ExcType(args)
How about this?
if not cond: raise ExcType(args)
This is 100% backwards compatible, doesn't not abuse `assert`, will not
be removed when running under -O, and takes exactly the same number of
characters to type.
If we are seriously considering DBC for Python, here's is my initial
proposal for function and method contracts. I don't have a proposal for
class invariants as yet.
def function(args):
"""Docstring"""
require:
# Preconditions checked before calling the function body.
block
ensure(result):
# Postconditions checked after calling the function body.
block
# body of function
`require` and `ensure` would be soft-keywords, and legal only inside a
`def` block, after the docstring and before the body of the function.
If present, the `require` block is executed before the body of the
function. The precondition has access to the function arguments, but
otherwise runs in its own namespace. Since it runs before the body, none
of the body's local variables will be defined (apart from the function
arguments).
If present, the `ensure` block is executed after the body of the
function. It also runs in its own namespace, but has access to the
function's body namespace. It also receives the function body's return
result as a named argument. The parameter name "result" is not
mandatory.
Both contract blocks are expected to either raise AssertionError, or
just return. Raising other exceptions is permitted but discouraged.
Catching these exceptions is permitted but discouraged except for the
purpose of logging.
When running under -O, both condition blocks are ignored. (At some time
in the future we may add a fine-grained interface to disable them.)
I have chosen the names `require` and `ensure` to match the names used
by Eiffel.
https://www.eiffel.org/doc/solutions/Design_by_Contract_and_Assertions
If the `require` and `ensure` blocks are all assertions, we can treat
the blocks as executable *specifications documention*:
def function(spam, eggs, cheese, aardvark):
require:
assert isinstance(spam, Food)
assert -1 <= eggs < MAXEGGS
assert cheese.colour = 'blue'
assert aardvark is not None
# function body
That should allow third-party checkers and tools to either do static
pre- and post-condition checking, or to generate documentation from the
conditions.
--
Steve
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