gistfile1.txt

[debug] server.go:936 Calling POST /images/{name:.*}/tag
2014/04/09 19:17:50 POST /v1.6/images/scratch/tag
[/var/lib/docker|8af4e1a1] +job tag(scratch, , )
Repository name can't be empty
[/var/lib/docker|8af4e1a1] -job tag(scratch, , ) = ERR (1)
[error] server.go:962 Error: Repository name can't be empty
[error] server.go:89 HTTP Error: statusCode=500 Repository name can't be empty

gistfile2.py

import docker

c = docker.Client()
c.tag('scratch', 'foobar')