Possible constant assignment operators ":=" and "::=" for Python

[Michele Simionato]

Edward Elliott wrote:
> Michele Simionato wrote:
> >> >>> A = []  # let's declare a "constant" here
> >> >>> b = A   # and let's assign the constant here
> >> >>> b.append('1') # OOPS!
> >
> > But it makes no sense to use a mutable object for a constant!
> > The user should use a tuple,
>
> Sure.  Now show me the builtin immutable equivalent of a dict.

There is none. However it is pretty easy to get one:

import UserDict

class ReadOnlyDict(UserDict.DictMixin):
    def __init__(self, dic):
        self._dic = dic
    def __getitem__(self, name):
        return self._dic[name]
    def keys(self):
        return self._dic.keys()
    def __getitem__(self, name):
        raise TypeError('this dictionary is read-only')
    def __delitem__(self, name):
        raise TypeError('this dictionary is read-only')

I am sure you already know that, this snipped is for the benefit of the
other
readers of this thread. Of course, the ReadOnlyDict can be modified by
modifying ._dic, but the point was all about avoiding accidental
modifications.

> > or a custom list-like type where
> > all methods with side effects are removed, so it effectively acts
> > as a tuple.
>
> Ugh, not worth the trouble imo.

Agreed, use a tuple if you need a tuple.

   Michele Simionato