On 5/09/19 15:30, Peter Otten wrote:
>> 2) Is it possible to make MyClass automatically a subclass of an other
>> class
>> through the metaclass?
>>
> While you can modify `bases` before passing it on to `type` this starts to
> get a bit messy. Maybe you need a real metaclass which unlike the registered
> function above is shared by the subclasses...
>
> import random
>
> def register(f):
> f.registered = True
> return f
>
> class Foo: pass
> class Bar: pass
>
> class RegisterMeta(type):
> def __new__(cls, name, bases, namespace):
> namespace["my_cool_functions"] = [
> n for n, v in namespace.items()
> if getattr(v, "registered", False)
> ]
> return type.__new__(
> cls, name,
> bases + (random.choice([Foo, Bar]),),
> namespace
> )
>
> class RegisterBase(metaclass=RegisterMeta):
> def __getitem__(self, i):
> return self.my_cool_functions[i]
>
> class MyClass(RegisterBase):
> @register
> def foo(self):
> pass
> @register
> def bar(self):
> pass
> def other(self):
> pass
>
> print(MyClass.my_cool_functions)
> print(MyClass()[0])
> print(MyClass.my_cool_functions is RegisterBase.my_cool_functions) # False
> print(MyClass.__bases__, RegisterBase.__bases__)
>
>
> ...or something else entirely. Can you provide some context?
Sure I am researching the possibility of writing an easy to use
lexing/parsing tool. The idea is to write your lexer/parser as
follows:
class Calculator(metaclass = ...):
def __init__(self):
self.names = set()
self.table = {}
@token(r'\d+')
def NUMBER(self, st):
return int(st)
@token(r'\w+')
def VAR(self, st):
self.names.add(st)
return st
@production(r"VAR '=' NUMBER")
def assign(self, prd):
name = prd[0]
val = prd[1]
if name in self.names:
self.table[name] = value
else:
raise CalcError("variable (%s) not available" % name)
calc = Calculator()
calc("a = 7")
So the token en production decorators register a regex/prodcution with
a specific method to be called in specific circumstances when parsing
a string.
So I need the lexing and parsing algorithms available to this class,
either by adding methods to the class or by making a subclass of the
class where they are implemented.
--
Antoon Pardon.
--
https://mail.python.org/mailman/listinfo/python-list
