In Python 3.10 we will no longer be burdened by the old parser (though 3rd party tooling needs to catch up).One thing that the PEG parser makes possible in about 20 lines of code is something not entirely different from the old print statement. I have a prototype:Python 3.10.0a0 (heads/print-statement-dirty:5ed19fcc1a, Jun 9 2020, 16:31:17)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin
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using dumb terminal settings.
>>> print 2+2
4
>>> print "hello world"
hello world
>>> print "hello", input("Name:")
Name:Guido
hello Guido
>>> print 1, 2, 3, sep=", "
1, 2, 3
>>>But wait, there's more! The same syntax will make it possible to call *any* function:>>> len "abc"
3>>>Or any method:>>> import sys
>>> sys.getrefcount "abc"
24
>>>Really, *any* method:>>> class C:
... def foo(self, arg): print arg
...
>>> C().foo 2+2
4
>>>There are downsides too, though. For example, you can't call a method without arguments:
<built-in function print>
>>>Worse, the first argument cannot start with a parenthesis or bracket:>>> print (1, 2, 3)
1 2 3
>>> C().foo (1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: C.foo() takes 2 positional arguments but 4 were given
>>> print (2+2), 42
4
(None, 42)
>>> C().foo [0]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'method' object is not subscriptable
>>>No, it's not April 1st. I am seriously proposing this (but I'll withdraw it if the response is a resounding "boo, hiss"). After all, we currently have a bunch of complexity in the parser just to give a helpful error message to people used to Python 2's print statement:
>>> print 1, 2, 3
File "<stdin>", line 1
print 1, 2, 3
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(1, 2, 3)?
>>>And IIRC there have been a number of aborted attempts at syntactic hacks to allow people to call functions (like print) without parentheses, although (I think) none of them made it into a PEP. The PEG parser makes this much simpler, because it can simply backtrack -- by placing the grammar rule for this syntax (tentatively called "call statement") last in the list of alternatives for "small statement" we ensure that everything that's a valid expression statement (including print() calls) is still an expression statement with exactly the same meaning, while still allowing parameter-less function calls, without lexical hacks. (There is no code in my prototype that checks for a space after 'print' -- it just checks that there's a name, number or string following a name, which is never legal syntax.)One possible extension I didn't pursue (yet -- dare me!) is to allow parameter-less calls inside other expressions. For example, my prototype does not support things like this:>>> a = (len "abc")
File "<stdin>", line 1
a = (len "abc")
^
SyntaxError: invalid syntax
>>>I think that strikes a reasonable balance between usability and reduced detection of common errors.I could also dial it back a bit, e.g. maybe it's too much to allow 'C().foo x' and we should only allow dotted names (sufficient to access functions in imported modules and method calls on variables). Or maybe we should only allow simple names (allowing 'len x' but disallowing 'sys.getrefcount x'. Or maybe we should really only bring back print statements.I believe there are some other languages that support a similar grammar (Ruby? R? Raku?) but I haven't investigated.Thoughts?
_______________________________________________----Guido van Rossum (python.org/~guido)Pronouns: he/him (why is my pronoun here?)
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