Minimal Python Language Server

Language servers provide features like autocompletion, go-to-definition, find-all-references, and diagnostics (linting/error checking) to IDEs and editors. This challenge asks you to build a minimal language server for Python, focusing on providing basic diagnostics (linting) using the pylint library. This is a valuable exercise in understanding how language servers work and how to integrate external tools into a standardized protocol.

Problem Description

You are tasked with creating a simple Python language server that analyzes Python files for potential errors and style issues using pylint. The server should listen for incoming requests from a Language Server Protocol (LSP) client and respond with diagnostic information. The core functionality is to take a file path as input, run pylint on that file, and return a list of diagnostic messages (errors, warnings, etc.) in the LSP format. The server should handle cases where pylint is not installed and provide appropriate error responses.

Key Requirements:

• LSP Compliance: The server must adhere to the basic LSP protocol for diagnostics.
• Pylint Integration: Utilize the pylint library to perform static analysis.
• Error Handling: Gracefully handle cases where pylint is not installed.
• Diagnostic Reporting: Return diagnostic messages with appropriate severity levels (Error, Warning, Info, etc.) and line/column information.
• Asynchronous Operation: While not strictly required for a minimal implementation, consider using asynchronous operations (e.g., asyncio) for improved responsiveness, especially when running pylint.

Expected Behavior:

1. The server should listen for incoming LSP requests (specifically, the textDocument/didOpen and textDocument/didChange notifications, though you can initially focus on didOpen for simplicity).
2. Upon receiving a request with a file path, the server should:
   • Check if pylint is installed. If not, return an error response to the client.
   • Run pylint on the specified file.
   • Parse the pylint output and convert it into a list of LSP diagnostic messages.
   • Send the list of diagnostic messages back to the client.
3. The server should be able to handle multiple file requests concurrently.

Edge Cases to Consider:

• pylint not installed.
• Invalid file paths.
• Large files (consider performance implications).
• Files with non-Python content.
• Empty files.

Examples

Example 1:

Input: File path: "example.py" containing: "print(123)"
Output: [
    {
        "range": {
            "start": {
                "line": 1,
                "character": 0
            },
            "end": {
                "line": 1,
                "character": 15
            }
        },
        "message": "Indentation error",
        "severity": 1  // 1 represents Error in LSP
    }
]
Explanation: `pylint` might flag an indentation error (depending on the project's configuration). The output is a list of diagnostics, each with a range (line and character positions) and a message. Severity 1 indicates an error.

Example 2:

Input: File path: "example.py" containing: "x = 1 + 1"
Output: []
Explanation: If the file contains no linting errors, the output is an empty list.

Example 3: (Edge Case)

Input: File path: "nonexistent_file.py"
Output: { "code": -32000, "message": "Pylint not installed." }
Explanation: If pylint is not installed, the server should return an error response with a code and a descriptive message.

Constraints

• Python Version: Python 3.7 or higher.
• Dependencies: pylint (installable via pip). You may use other libraries for LSP communication (e.g., python-lsp-server, pygls).
• Performance: The server should respond to requests within a reasonable timeframe (e.g., under 2 seconds for moderately sized files). While optimization is not the primary focus, avoid excessively slow operations.
• LSP Version: LSP 3.17 or later.
• File Size: Assume files are less than 1MB.

Notes

• You don't need to implement the entire LSP specification. Focus on the core diagnostic functionality.
• Consider using a library like python-lsp-server or pygls to simplify LSP communication. These libraries handle much of the protocol boilerplate.
• The pylint output format can be complex. You'll need to parse it carefully to extract the relevant information (file path, line number, column number, message, severity).
• Start with a simple example file and gradually increase the complexity.
• Think about how to handle different pylint severity levels and map them to the LSP severity levels (Error, Warning, Info, Hint, etc.).
• Error messages from pylint might contain extra information. Focus on extracting the core message and location.