MermaidTrace: Visualize Your Python Code Logic

Stop drowning in cryptic logs. One line of code to transform complex execution logic into clear Mermaid sequence diagrams.

🌐 Language: English | 中文

🏗️ Architecture

flowchart TB
    %% Style Definitions
    classDef userLayer fill:#e3f2fd,stroke:#1565c0,stroke-width:2px,color:#0d47a1,rx:5,ry:5
    classDef coreLayer fill:#fff3e0,stroke:#e65100,stroke-width:2px,color:#bf360c,rx:5,ry:5
    classDef ioLayer fill:#e8f5e9,stroke:#2e7d32,stroke-width:2px,color:#1b5e20,rx:5,ry:5
    classDef vizLayer fill:#f3e5f5,stroke:#7b1fa2,stroke-width:2px,color:#4a148c,rx:5,ry:5
    classDef storage fill:#eceff1,stroke:#455a64,stroke-width:2px,color:#263238,shape:cyl

    subgraph User["👤 User Integration"]
        direction TB
        Decorator["@trace / @trace_class"]:::userLayer
        Middleware["FastAPI Middleware"]:::userLayer
        Callback["LangChain Callback"]:::userLayer
    end

    subgraph Core["⚙️ Core Engine"]
        direction TB
        Context["ContextVars<br/>(Trace Context)"]:::coreLayer
        EventBus["Event Bus<br/>(LogContext)"]:::coreLayer
        Masker["Data Masking<br/>(Sanitizer)"]:::coreLayer
    end

    subgraph IO["⚡ Async I/O Layer"]
        direction TB
        Queue["Async Queue<br/>(Non-blocking)"]:::ioLayer
        Writer["File Handler<br/>(Rotating/Timed)"]:::ioLayer
    end

    Storage[(".mmd File")]:::storage

    subgraph Viz["🌐 Visualization"]
        direction TB
        Watcher["Watchdog<br/>(File Monitor)"]:::vizLayer
        Server["FastAPI Server<br/>(SSE Support)"]:::vizLayer
        Browser["Web Browser<br/>(Mermaid.js + PanZoom)"]:::vizLayer
    end

    %% Data Flow
    Decorator --> Context
    Middleware --> Context
    Callback --> Context
    Context --> EventBus
    EventBus --> Masker
    Masker --> Queue
    Queue --> Writer
    Writer --> Storage
    Storage -.-> Watcher
    Watcher --> Server
    Server == "SSE Push" ==> Browser

    %% Link Styles
    linkStyle default stroke:#607d8b,stroke-width:2px

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⚡️ Understand MermaidTrace in 5 Seconds

1. Original Code (15+ lines)

@trace(source="User", target="OrderSys")
def create_order(user_id, items):
    # Complex business logic
    if not check_inventory(items):
        return "Out of Stock"

    # Nested logic calls
    price = calculate_price(items)
    discount = get_discount(user_id)
    final = price - discount

    # External service interactions
    res = pay_service.process(final)
    if res.success:
        update_stock(items)
        send_notif(user_id)
        return "Success"
    return "Failed"

2. Auto-Generated Sequence Diagram

sequenceDiagram
    autonumber
    User->>OrderSys: create_order(user_id, items)
    activate OrderSys
    OrderSys->>Inventory: check_inventory(items)
    Inventory-->>OrderSys: True
    OrderSys->>Pricing: calculate_price(items)
    Pricing-->>OrderSys: 100.0
    OrderSys->>UserDB: get_discount(user_id)
    UserDB-->>OrderSys: 5.0
    OrderSys->>PayService: process(95.0)
    activate PayService
    PayService-->>OrderSys: success
    deactivate PayService
    OrderSys->>Inventory: update_stock(items)
    OrderSys->>Notification: send_notif(user_id)
    OrderSys-->>User: "Success"
    deactivate OrderSys

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🚀 Dynamic Demo & Online Tryout

🎬 Quick Demo

(Master Preview: Multi-file browsing, live-reload, and interactive pan/zoom)

sequenceDiagram
    participant CLI as mermaid-trace CLI
    participant App as Python App
    participant Web as Live Preview

    Note over CLI, Web: Enable Live Preview Mode
    CLI->>Web: Start HTTP Server (localhost:8000)
    App->>App: Run Logic (with @trace decorator)
    App->>App: Auto-update flow.mmd
    Web->>Web: File Change Detected (Hot Reload)
    Web-->>CLI: Render Latest Diagram

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(From adding decorators to browser live preview in 10 seconds)

🛠️ Try Online (Google Colab)

No local setup required. Experience core features in your browser:

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🎯 Why MermaidTrace? (Use Cases)

1. Master "Legacy" Codebases

Pain: Taking over a complex, undocumented legacy project with tangled function calls. Solution: Add @trace_class or @trace to entry points and run the code once. Value: Instantly generate a complete execution path map to understand the architecture.

2. Automated Technical Docs

Pain: Manual sequence diagrams are time-consuming and quickly become outdated. Solution: Integrate MermaidTrace during development. Value: Diagrams stay 100% in sync with your code logic automatically.

3. Debug Complex Recursion & Concurrency

Pain: Nested calls or async tasks produce interleaved logs that are impossible to read. Solution: Use built-in async support and intelligent collapsing. Value: Visualize recursion depth and concurrency flow to pinpoint logic bottlenecks.

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🚀 Quick Start in 3 Steps

1. Install

pip install mermaid-trace

2. Add Decorators

from mermaid_trace import trace, configure_flow

# Configure output file
configure_flow("my_flow.mmd")

@trace(source="User", target="AuthService")
def login(username):
    return verify_db(username)

@trace(source="AuthService", target="DB")
def verify_db(username):
    return True

login("admin")

3. View Diagram

Run the built-in CLI tool to preview in real-time (with hot-reload):

# Preview a single file (Auto-reload, Pan/Zoom supported)
mermaid-trace serve my_flow.mmd

# Preview a directory (File browser, Multi-file switching)
mermaid-trace serve .

🔗 LangChain Integration

Visualize LLM chains, agents, and RAG retrieval with a single handler:

from mermaid_trace.integrations.langchain import MermaidTraceCallbackHandler

handler = MermaidTraceCallbackHandler(host_name="MyAIApp")
# Pass to any LangChain object
chain.invoke({"input": "..."}, config={"callbacks": [handler]})

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✨ Key Features

• Decorator-Driven: Simply add @trace or @trace_interaction to functions.
• Auto-Instrumentation: Use @trace_class to trace a whole class at once.
• Data Privacy: Automatic Data Masking for sensitive fields (passwords, tokens).
• Performance Control: Configurable Sampling Rate for high-throughput systems.
• Distributed Tracing: Support for W3C Trace Context, B3, and custom headers to link microservices.
• Third-Party Patching: Use patch_object to trace calls inside external libraries.
• Async Support: Seamlessly works with asyncio coroutines and concurrency.
• Enhanced Web UI: Interactive preview server with file browsing, auto-reload, and pan/zoom support.
• Intelligent Collapsing: Automatically collapses repetitive calls and identifies loops.
• FastAPI Integration: Middleware for zero-config HTTP request tracing with header propagation.
• LangChain Integration: Callback Handler for LLM chains and agent visualization.
• Detailed Exceptions: Captures full stack traces for errors, displayed in the diagram.

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📚 Documentation

Core Documentation

User Guide · API Reference · Contributing Guidelines · Changelog · License

Code Comment Documents

Category	Links
Core Modules	Context · Decorators · Events · Formatter
Handlers	Async Handler · Mermaid Handler
Integrations	FastAPI
Others	init · CLI

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🤝 Contributing

We welcome contributions! Please see CONTRIBUTING.md for details.

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📄 License

MIT