Connecting and Reordering

Where we left off

Lesson 4's flow ran Chat Input, then Prompt, then Gemini, then Chat Output, in that order, left to right on the canvas. It's tempting to assume that's because they're arranged left to right. This lesson shows that's a coincidence: what actually decides run order is the edges, full stop.

Do this yourself

Reopen Lesson 4's flow (still in your browser).

1. Drag the Chat Output node so it sits on the far left, and drag Chat Input to the far right, physically reversing their layout.
2. Open the Playground and run it again. Confirm it behaves exactly the same, Chat Input still runs first, Chat Output still runs last, despite Chat Output now sitting to the left of Chat Input on screen.
3. Drag the nodes back into a sensible layout (or don't, it doesn't affect behavior at all).

Why this is true

A flow's real shape lives in its edges: "Prompt's user_input reads from Chat Input's output" is a fact about a connection, not about pixel coordinates. Langflow (and, underneath, lfx) computes run order with a topological sort: it looks at every edge, and produces an order where each component runs only after everything it depends on has already run. Position on the canvas is purely for you, the human reading the flow, to make its shape easier to see.

The code, piece by piece

chat_output = ChatOutput()
chat_input = ChatInput()
prompt = PromptComponent()
prompt.set(template="{user_input}", user_input=chat_input.message_response)
chat_output.set(input_value=prompt.build_prompt)

These four lines instantiate ChatOutput before ChatInput, on purpose, the reverse of every earlier lesson. The .set() calls still correctly say "Prompt reads from Chat Input" and "Chat Output reads from Prompt", those connections are what matters, not which line came first in the file.

graph = Graph(start=chat_input, end=chat_output)
graph.prepare()
order = [vertex.display_name for vertex in graph.topological_sort()]

topological_sort() is the same computation the canvas uses to decide run order, walking the edges to find a valid sequence. It doesn't know or care that ChatOutput() was instantiated first in this file, it only reads the connections.