Code Snippet — July 27, 2026 ### Deep Merge Nested Dicts with Conflict Re

💻 Code Snippet — July 27, 2026

Deep Merge Nested Dicts with Conflict Resolution (Python)

dict.update() silently replaces nested dicts. This recursive merge supports conflict strategies, list handling, and None-skipping:

from copy import deepcopy

def deep_merge(base, override, strategy="override",
               list_strategy="replace", skip_none=True):
    """Recursively merge nested dicts."""
    result = deepcopy(base)
    for key, new_val in override.items():
        if skip_none and new_val is None:
            continue
        if key not in result:
            result[key] = deepcopy(new_val)
            continue
        old_val = result[key]
        if isinstance(old_val, dict) and isinstance(new_val, dict):
            result[key] = deep_merge(old_val, new_val, strategy, list_strategy, skip_none)
            continue
        if isinstance(old_val, list) and isinstance(new_val, list):
            if list_strategy == "replace":
                result[key] = deepcopy(new_val)
            elif list_strategy == "append":
                result[key] = old_val + deepcopy(new_val)
            elif list_strategy == "union":
                seen, merged = set(), []
                for i in old_val + new_val:
                    h = str(i) if isinstance(i, (dict, list)) else i
                    if h not in seen: seen.add(h); merged.append(i)
                result[key] = merged
            continue
        if strategy == "override": result[key] = deepcopy(new_val)
        elif strategy == "error": raise ValueError(f"Conflict at {key}")
    return result

Usage: reduce(deep_merge, [defaults, env, user])

💡 3 conflict strategies (override/preserve/error), 3 list modes (replace/append/union). skip_none=True → None means "don't touch". Zero deps, Python 3.8+.