Resolving ERROR:root:code for hash md5 was not found in Mercurial on macOS Due to Python Hash Module Issues

Problem Background and Error Analysis

On macOS Catalina 10.15.1, after installing Python 2.7 via Homebrew, executing Mercurial commands such as hg commit --amend triggers a series of ERROR:root:code for hash md5 was not found errors. The stack trace reveals that the issue originates from Python's hashlib.py module. During initialization, this module attempts to load hash algorithms like MD5 and SHA1 through the __get_builtin_constructor function, but due to OpenSSL version incompatibility, a ValueError: unsupported hash type exception is raised.

Further analysis of the stack trace shows that Mercurial's util.py module references hashlib.md5 upon import. Since the hashlib module fails to initialize correctly, this leads to an AttributeError: 'module' object has no attribute 'md5'. This indicates that the core problem lies in Python's hash module depending on the system OpenSSL library, with the current OpenSSL version unable to provide the required hash algorithm support.

Root Cause Investigation

This issue typically occurs after running brew upgrade openssl. Homebrew upgrades OpenSSL to a newer version (e.g., 1.1.x), but Python 2.7's hashlib module may still be linked to an older version (e.g., 1.0.2t). The version mismatch prevents hash algorithms from loading properly. Users might attempt to resolve this with brew link openssl --force, but macOS security policies refuse to link the system-provided openssl@1.1, leaving the problem unresolved.

Solution and Implementation Steps

Based on the best answer, the solution is to switch back to a compatible OpenSSL version. First, check the currently installed OpenSSL versions:

$ ls /usr/local/Cellar/openssl

Assuming the output shows version 1.0.2t, execute the switch command:

$ brew switch openssl 1.0.2t

This command cleans the old version and creates symbolic links, ensuring the system uses the specified OpenSSL version. After switching, verify the fix in a Python virtual environment. For example, run:

(my-venv) $ python -c "import hashlib;m=hashlib.md5();print(m.hexdigest())"

If the output is d41d8cd98f00b204e9800998ecf8427e (the MD5 hash of an empty string), the hash module is functioning correctly, and Mercurial commands should no longer produce errors.

Code Examples and In-Depth Analysis

To better understand the issue, let's analyze a simplified version of Python's hashlib module. The following example demonstrates how hash algorithms depend on OpenSSL:

import sys
import hashlib

def check_hash_support():
    try:
        # Attempt to create an MD5 hash object
        m = hashlib.md5()
        m.update(b"test")
        print("MD5 supported:", m.hexdigest())
    except ValueError as e:
        print("MD5 error:", e)
    
    # Check all available algorithms
    print("Available algorithms:", hashlib.algorithms_available)

if __name__ == "__main__":
    check_hash_support()

With an incompatible OpenSSL version, this code will throw ValueError: unsupported hash type md5. After fixing, the algorithm list will include md5, sha1, etc., verifying the hash module's dependency on system libraries.

Additional Notes and Best Practices

Beyond switching OpenSSL versions, users should consider upgrading to Python 3.x, as Python 2.7 is no longer maintained and newer versions offer better OpenSSL dependency management. On macOS, when using Homebrew to manage Python and OpenSSL, pay attention to version compatibility. Check current links with:

$ brew info openssl
$ python -c "import ssl; print(ssl.OPENSSL_VERSION)"

Ensure the OpenSSL version used by Python matches the system. For Mercurial users, if issues persist, try reinstalling Mercurial or using conda environment management tools to avoid system library conflicts.

Conclusion

The ERROR:root:code for hash md5 was not found error fundamentally stems from incompatibility between Python's hash module and the OpenSSL version. Switching to a compatible version via brew switch openssl effectively resolves the problem. This paper provides comprehensive technical guidance through error analysis, cause investigation, and solutions. Developers should prioritize system library version management, especially when using package managers like Homebrew, to ensure compatibility between Python and its dependencies and prevent similar runtime errors.