Make resource identifiers immutable

[kyleknap]

Before you could switch out the identifiers on a resource, which can have unintended affects if you switch the identifier and then make subsequent calls with the resource. Now identifiers are read-only, much like how attributes are read-only:

In [1]: import boto3

In [2]: s3 = boto3.resource('s3')

In [3]: bucket = s3.Bucket('mybucketfoo')

In [4]: bucket.name = 'foo'
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-5d85c15550cd> in <module>()
----> 1 bucket.name = 'foo'

AttributeError: can't set attribute

cc @jamesls @mtdowling @rayluo

[rayluo]

LGTM

[jamesls]

Should we be returning None if the internal attr doesn't exist? Seems like that would only happen if we had a bug, in which case we'd want it to fail loudly.

[kyleknap]

Yeah I am fine with that. I do not think that it will happen unless there is a bug. Furthermore, since this is an identifier in general, its value cannot be None so it make sense not to default to None.

[kyleknap]

Scratch what I said, I remembered why I set it a default to None. It is because when the resource is instantiated there is check to make sure that all of the identifiers are non-None values: https://github.com/boto/boto3/blob/develop/boto3/resources/base.py#L117. So if I did not use a default, I would get a bad error message like: 'test.Queue' object has no attribute '_url' instead of the current message of: ValueError: Required parameter url not set because the identifier getter is called before the ValueError is thrown. I would prefer to keep it as is unless you have any other suggestions.

[jamesls]

Yeah that's fine, but let's add a comment here that explains that.

[kyleknap]

I can do that.

[jamesls]

Just had a question about a failure case, otherwise looks good.

[jamesls]