Dynamic Inventory script output vs JSON file - ansible

Question

I'm writing a dynamic inventory script which queries Docker containers. It outputs JSON, which I can save to a file and use, but I get parse errors from Ansible when I try to use the script directly.
[root#297b1ca0cfa4 /]# docker-dynamic-inventory > inv.json
[root#297b1ca0cfa4 /]# cat inv.json
{"all": {"hosts": {"inv_clyde_1": null, "inv_blinky_1": null, "inv_inky_1": null, "inv_pinky_1": null, "admiring_chandrasekhar": null}, "_meta": {"hostvars": {}}, "vars": {"ansible_connection": "docker"}}}
[root#297b1ca0cfa4 /]# ansible all -i inv.json -m ping
inv_clyde_1 | FAILED! => {
"failed": true,
"msg": "docker command not found in PATH"
}
Note that I don't care if the ping fails, getting that far means that my inventory works. Ansible is successfully interpreting the JSON and the inventory it represents. Now compare this with using the script directly:
[root#297b1ca0cfa4 /]# ansible all -i /usr/bin/docker-dynamic-inventory -m ping
[WARNING]: * Failed to parse /usr/bin/docker-dynamic-inventory with script plugin:
You defined a group 'all' with bad data for the host list:
{u'hosts': {u'inv_clyde_1': None, u'inv_inky_1': None,
u'admiring_chandrasekhar': None, u'inv_pinky_1': None, u'inv_blinky_1': None},
u'_meta': {u'hostvars': {}}, u'vars': {u'ansible_connection': u'docker'}}
Ansible's docs on Inventory show it using a dictionary and null values to represent hosts, which is why I do that here.
Apart from the fact that Ansible prints the dict it read in from JSON, I'm not seeing what's different/wrong here. Why does the stored JSON output work where the script won't?

Answer 1

So it turns out all is a special group, but only when interpreted with the script parser. In a static inventory, all can be a dictionary of keys with null values, but when coming from a script, the host value for all must be a list of strings.
{"all":
{"hosts": ["admiring_chandrasekhar", "inv_inky_1", "inv_pinky_1",
"inv_blinky_1", "inv_clyde_1"],
"_meta": {"hostvars": {}},
"vars": {"ansible_connection": "docker"}}}