You could also use the factory methods and then the example from Francois can be written like this:
import olca_schema as o
unit_group = o.new_unit_group("Units of length", "m")
length = o.new_flow_property("Length", unit_group)
flow = o.new_product("some flow", length)
But in general, you should not create a new flow property here, as there already exists a flow property for length in openLCA which you could reference like this, if an IPC server is running on a database with reference data:
import olca_schema as o
import olca_ipc as ipc
client = ipc.Client()
length = client.find(o.FlowProperty, "Length")
flow = o.new_product("some flow", length)