Fixing Mcp-lambda-handler Docstring Parameter Parsing With Type Hints

July 7, 2025 by StackCamp Team 70 views

mcp-lambda-handler Docstring Parameter Parsing Issue with Type Hints

This article addresses a critical bug in the mcp-lambda-handler library that affects the parsing of docstring parameters when type hints are included. The issue arises in the mcp_lambda_handler.py file, where the logic for extracting parameter names from Google-style docstrings fails to correctly identify parameters when type hints are present. This leads to parameter descriptions not being properly matched, impacting the overall functionality and usability of the library. This article delves into the specifics of the bug, its impact, and a proposed solution.

Understanding the Bug

The Problem

The core issue lies in how the mcp-lambda-handler parses Google-style docstrings. When docstrings include type hints within the parameter descriptions (e.g., param1 (int): Description), the parsing logic incorrectly extracts the parameter name, including the type hint. Specifically, the code splits the parameter description at the first colon (:) without accounting for the possibility of type hints enclosed in parentheses. This results in the parameter name being captured as param1 (int) instead of just param1.

Code Snippet

The problematic code snippet is found within the mcp_lambda_handler.py file. At line 198, the parameter name is extracted using the following logic:

param_name = line.split(":")[0].strip()

This line of code splits the string at the first colon and takes the first part as the parameter name. While this works for simple docstrings without type hints, it fails when type hints are included. For instance, if the docstring line is param1 (int): The first parameter., the extracted param_name becomes param1 (int).

The Consequence

Later in the code, at line 246, the extracted parameter names are used to match against the actual function parameter names. The code attempts to find the parameter description using the following check:

if param_name in arg_descriptions:

Since the actual parameter names in the function definition do not include type hints (e.g., param1), the lookup fails. This results in the parameter descriptions not being correctly associated with the function parameters, leading to incorrect or missing documentation and potentially affecting other functionalities that rely on accurate parameter information.

Impact of the Bug

The incorrect parsing of docstring parameters has several significant implications:

Documentation Issues

The primary impact is on the accuracy and completeness of the generated documentation. When parameter descriptions are not correctly matched, the documentation may lack crucial information about the function's parameters. This makes it harder for developers to understand how to use the function, leading to potential errors and increased development time. Clear and accurate documentation is essential for any library or framework, and this bug directly undermines that goal.

Functionality Limitations

In some cases, the inability to correctly parse parameter descriptions can limit the functionality of tools and processes that rely on this information. For example, automated testing tools or code analysis tools might not be able to correctly interpret the function's parameters, leading to incomplete or inaccurate results. This can hinder the development and maintenance process, making it harder to ensure the quality and reliability of the code.

Developer Experience

The bug also affects the overall developer experience. When developers encounter issues with documentation or tools, it can lead to frustration and decreased productivity. Spending time debugging issues related to incorrect parameter parsing can be a significant drain on resources, especially in large projects with many functions and parameters. A smooth and efficient development process is crucial for maintaining developer morale and ensuring project success, and this bug detracts from that ideal.

Compatibility Problems

This issue specifically affects projects that follow the Google-style docstring convention, which is widely used in the Python community. The inability to correctly parse docstrings with type hints creates compatibility problems for projects that adhere to this standard. This can lead to inconsistencies and make it harder to integrate mcp-lambda-handler into existing workflows and projects.

Reproducing the Bug

To demonstrate the bug, consider the following steps:

Step 1: Create a Lambda Function

First, create a Lambda function with a Google-style docstring that includes type hints. Here’s an example:

def my_function(param1, param2):
    """
    Function description.
    
    Args:
        param1 (int): The first parameter.
        param2 (str): The second parameter.
    
    Returns:
        str: Result description.
    """
    pass

This function my_function takes two parameters, param1 and param2, with their types specified in the docstring using type hints (int and str respectively).

Step 2: Use `mcp-lambda-handler`

Next, use the mcp-lambda-handler to process this function. The exact method for doing this will depend on your specific setup and how you are using the library. However, the core issue will manifest itself when the library attempts to parse the docstring and extract the parameter descriptions.

Step 3: Observe the Issue

When mcp-lambda-handler processes the function, it will incorrectly parse the parameter names. Instead of extracting param1 and param2, it will extract param1 (int) and param2 (str). This can be observed by examining the internal data structures used by the library or by observing the generated documentation.

Expected vs. Actual Behavior

The expected behavior is that the library should correctly extract the parameter names without the type hints. This would allow the parameter descriptions to be properly matched and used in documentation or other processes. The actual behavior is that the library includes the type hints in the extracted parameter names, leading to a mismatch and the parameter descriptions being ignored.

Proposed Solution

To address this bug, the parameter name extraction logic needs to be modified to remove type hints before storing the names in the arg_descriptions dictionary. A simple and effective solution is to add a step that splits the parameter name at the opening parenthesis ( and takes the first part. This will remove the type hint while preserving the actual parameter name.

Modified Code

Here’s the proposed modification to the code at line 198 in mcp_lambda_handler.py:

# Extract parameter name, removing type hints if present
param_line = line.split(":")[0].strip()
# Remove type hints in parentheses (e.g., "param1 (int)" -> "param1")
param_name = param_line.split("(")[0].strip()

Explanation

The modified code first extracts the parameter line by splitting at the colon, as before. Then, it splits the resulting string at the opening parenthesis (. This effectively separates the parameter name from the type hint. The first part of the split (index 0) is the parameter name without the type hint. The strip() method is used to remove any leading or trailing whitespace.

Benefits

This solution offers several benefits:

Correct Parameter Extraction: It correctly extracts parameter names even when type hints are present in the docstrings.
Backward Compatibility: It maintains backward compatibility with docstrings that do not include type hints, as the split operation will still work correctly in those cases.
Simplicity: It is a simple and straightforward fix that is easy to understand and implement.
Efficiency: The added step does not introduce significant overhead, so it does not negatively impact performance.

Additional Context

Google-Style Docstrings

This bug specifically affects Google-style docstrings, which are a popular convention in the Python community. These docstrings are documented in detail in the sphinxcontrib-napoleon documentation. The Google style includes the use of type hints within the parameter descriptions, making this bug particularly relevant for projects that adhere to this style.

File and Lines

The issue is located in the src/mcp-lambda-handler/awslabs/mcp_lambda_handler/mcp_lambda_handler.py file. The problematic line is 198, where the parameter name extraction occurs. The related line is 246, where the parameter lookup is performed.

Impact and Urgency

The impact of this bug is significant, as it affects the accuracy of documentation and the functionality of tools that rely on correct parameter information. The fix is relatively straightforward, making it a high-priority issue to address.

Conclusion

The bug in mcp-lambda-handler that affects docstring parameter parsing with type hints is a critical issue that needs to be addressed. The proposed solution, which involves modifying the parameter name extraction logic to remove type hints, is simple, effective, and maintains backward compatibility. By implementing this fix, the mcp-lambda-handler library can better support Google-style docstrings and provide accurate parameter information, improving the overall developer experience and the reliability of the library. This fix ensures that parameter descriptions are correctly associated with function parameters, leading to more accurate documentation and enhanced functionality for tools and processes that rely on this information. Addressing this bug is essential for maintaining the quality and usability of the mcp-lambda-handler library and ensuring its compatibility with standard Python docstring conventions.

By resolving this issue, developers can confidently use mcp-lambda-handler with Google-style docstrings, knowing that their parameter descriptions will be correctly parsed and utilized. This enhances the overall developer experience and ensures that the library functions as expected in a wide range of use cases. The fix is a crucial step in improving the robustness and reliability of mcp-lambda-handler, making it a more valuable tool for the Python community.