Get 'None' or a default value if key does not exist

[yannlambret]

Hello,

Thanks for working on this. I've just started using etcd v3 so maybe I'm missing something, but I find that the default behavior of the client's 'get' method when requesting a non-existing key is inappropriate. I think we should take advantage of a behavior similar to the Python dictionary, i.e. be able to get at least 'None' instead of catching an exception.

This is especially true when we want to test if a key already exist. Moreover, it would be more consistent with the 'etcdctl' CLI tool that does not display anything when a non-existing key is requested, an return a 0 exit code.

[kragniz]

You'd prefer something like this?

>>> import etcd3
>>> etcd = etcd3.client()
>>> etcd.get('not-a-key')
(None, <etcd3.client.KVMetadata object at 0x7f5e32621790>)
>>> etcd.get('not-a-key', default=b'toot')
(b'toot', <etcd3.client.KVMetadata object at 0x7f5e32621650>)

[yannlambret]

I would indeed, I think it would allow to write more concise code in most situations. What do you think ?

[zimnx]

Official etcd2 client throws exception when key is not found, I would prefer current behavior. Current implementations can simply switch type of Exception handled without modifying logic around client.

[yannlambret]

There is no official Python client for etcd, just third party libraries referenced in the documentation - as this one is. If you take a look at the behavior of the go etcd client for v3 API (which is the only one maintained by CoreOS as part of the project), it returns a data structure as part of the response (resp.Kvs) which is empty if a key is not found, and the error returned by the .Get method is 'nil' in this case.

[minus7]

I'd mimic the behaviour of dict: raise KeyError (or a subclass thereof containing the KVMetadata object) if a non-existent key is accessed without providing a default. To check if a key exists in/__contains__ should be used. (A successful get returning a tuple isn't particularly nice either, maybe a proxy object would work there better?)

Disclaimer: I've never used etcd nor this library.

[kragniz]

Addressed by #73