I am using this code in python to upsert multiple records using the composite funcitonality.

records = [
"/External_ID__c/334",
{"attributes" : {"type" : "Account"},
"NumberOfEmployees" : 200},
"External_ID__c/335",
{"attributes" : {"type" : "Account"},
"NumberOfEmployees" : 200}
]
res = sf.restful(method='PATCH', path='composite/sobjects', json=dict(records=records))
print(res)

Not sure how to reference external Id.

Thanks