Summary

This PR addresses the implementation approach in #161 with a cleaner solution using lazy imports instead of dummy functions.

Problem with PR #161's approach

PR #161 defines a dummy function at import time:

try:
    from jill.install import install_julia
except ModuleNotFoundError:
    def install_julia():
        raise RuntimeError("Could not install Julia as the Jill module was not found.")

This approach:

• Creates a confusing placeholder function that only exists to raise an error
• Delays error detection until the function is actually called
• Makes the code harder to understand

Better Solution: Lazy Import

Move the jill import to where it's actually used:

def _ensure_julia_installed():
    if not _find_julia():
        print("No Julia version found. Installing Julia.")
        try:
            from jill.install import install_julia
        except ImportError:
            raise RuntimeError(
                "Julia is not installed and the 'jill' package is not available. "
                "Please install Julia manually or install jill: pip install jill"
            )
        install_julia()
        # ...

Benefits

1. Cleaner code: No dummy functions that just raise errors
2. Better error messages: Clear, immediate error messages when there's a problem
3. Lazy loading: Only imports jill when Julia installation is actually needed
4. Same functionality: Jill remains a default dependency, no change in user experience

Changes

• diffeqpy/__init__.py:
  • Remove top-level from jill.install import install_julia
  • Add lazy import with proper error handling in _ensure_julia_installed() and install() functions
• setup.py: No changes (jill remains a required dependency)

Testing

• Syntax validation passes
• Same functionality as before, just cleaner implementation

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