Hi Albert-Jan,
As far as I know, the recommended object to subclass when subclassing a `dict`
is `UserDict`. In Python 3, it's in `collections.UserDict` and in Python 2 is
in `UserDict.UserDict`.
Here's an basic example of how it would work:
try:
from collections import UserDict
except ImportError:
from UserDict import UserDict
class FrozenDict(UserDict):
def __setitem__(self, key, item):
raise TypeError("'FrozenDict' object does not support item assignment")
According to the Fluent Python book (by Luciano Ramalho, which I recommend
wholeheartedly), subclassing built-in types is tricky because: "the code of the
built-ins (written in C) does not call special methods overridden by
user-defined classes." Therefore, other methods of `dict`, like `update` or
`__init__` will *not* call your special `__setitem__` method.
However, be aware that although the FrozenDict above is read-only, it's not
*really* frozen, i.e., it cannot be used as a key in another dict. In order to
do that, you would need to define the `__hash__` method.
This StackOverflow answer, which you might have seen, provide an implementation
of a FrozenDict that could be used as a dict.
http://stackoverflow.com/questions/2703599/what-would-a-frozen-dict-be
Cheers,
Alex
On Wed, Oct 7, 2015, at 18:10, Albert-Jan Roskam wrote:
> Hi,
> I wanted to create a read-only dict to hold some constants. I looked around
> on the internet and created two implementations:-FrozenDict (derives from
> collections.mapping)-ChillyDict (derives from dict, which seems more obvious
> to me)
> The code can be found here: http://pastebin.com/QJ3V2mSK
> Some questions:1. one doctest from FrozenDict fails: fd.keys() returns an
> empty list. Why?2. Is FrozenDict the way to use collections.mapping (aside
> from the error!). I just discovered this and i seems quite cool (pun
> intended)3. Which implementation is better, and why? I like ChillyDict better
> because it is more straightforward and shorter.
> The read-only dict does not need to be very fast, it just needs to give some
> reasonable protection against mutating values.It also needs to work under
> Python 2.7 and 3.3+.
> Thank you!
> Albert-Jan
>
>
>
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