Basic document hook (snippets)

hooks.py

doc_events = {
	"Sales Order": {
		"on_submit": "my_custom_app.workflows.make_stock_entry_on_so_submit"
	},
  "on_cancel": "my_custom_app.workflows.cancel_stock_entry_on_so_cancel"
	}
}

workflows.py

# located in the same directory of my_custom_app as hooks.py

@frappe.whitelist()
def make_stock_entry_on_so_submit(so, method): # required arguments
  doc = frappe.new_doc("Stock Entry")
  # move data to your stock entry record from sales order
  doc.save()
  doc.submit()
  so.stock_entry = doc.name # so.stock_entry is a custom field
  return so


@frappe.whitelist()
def cancel_stock_entry_on_so_cancel(so, method): # required arguments
  doc = frappe.get_doc("Stock Entry", so.stock_entry) # decide how you want to store a related stock entry, probably a custom field
  doc.cancel()
  so.stock_entry = ""
  return so