How to Expand a Contract and Migrate Data in Python

Expand an old data contract by renaming fields and adding defaults, then migrate to a final version with deepcopy isolation.

Medium Python 3.9+ Aug 9, 2026 Production deployment patterns 14 views 0 copies

contract migration deepcopy data-transformation

Python code

50 lines

Python 3.9+

import json
from copy import deepcopy

# Mock data representing a user record (old contract)
old_contract = {
    "id": 1,
    "name": "Alice",
    "email": "alice@example.com",
    "age": 30,
    "status": "active"
}

# Expanded contract: adds fields with defaults and renames some fields
expand_rules = {
    "id": "user_id",
    "name": "full_name",
    "email": "email",
    "age": "age",
    "status": "account_status"
}

# Migration: from old to expanded contract (add fields, rename keys)
def expand_contract(data, rules):
    expanded = {}
    for old_key, new_key in rules.items():
        if old_key in data:
            expanded[new_key] = data[old_key]
    # Add new fields with default values
    expanded.setdefault("created_at", "2024-01-01")
    expanded.setdefault("last_login", None)
    expanded.setdefault("premium", False)
    return expanded

# Mock: simulate data after expansion
expanded_data = expand_contract(old_contract, expand_rules)

# Contract migration: from expanded to final contract (contract version 2)
def migrate_contract(data):
    mig = deepcopy(data)
    # Rename again or restructure
    mig["name"] = mig.pop("full_name")
    mig["id"] = mig.pop("user_id")
    return mig

# Usage demonstration
if __name__ == "__main__":
    print("Old contract:", json.dumps(old_contract))
    print("Expanded:", json.dumps(expanded_data))
    final = migrate_contract(expanded_data)
    print("Migrated:", json.dumps(final))

Output

stdout

Old contract: {"id": 1, "name": "Alice", "email": "alice@example.com", "age": 30, "status": "active"}
Expanded: {"user_id": 1, "full_name": "Alice", "email": "alice@example.com", "age": 30, "account_status": "active", "created_at": "2024-01-01", "last_login": null, "premium": false}
Migrated: {"email": "alice@example.com", "age": 30, "account_status": "active", "created_at": "2024-01-01", "last_login": null, "premium": false, "name": "Alice", "id": 1}

How it works

The expand_contract function iterates over rename rules, mapping old keys to new names, and uses setdefault to add new fields with sensible defaults. The migrate_contract function uses deepcopy to avoid mutating the input, then renames keys to produce the final contract version. This two-step pattern keeps field additions separate from schema migrations, making each step testable. The reliance on setdefault guarantees backward compatibility for records that already contain some new fields.

Common mistakes

Forgetting `deepcopy` and mutating the original data during migration
Assuming every old key exists instead of using `.get()` or `if key in data`
Hard-coding defaults that don't match production values
Not versioning the contract, making rollback difficult

Variations

Use a dataclass with `asdict()` for typed contract validation
Apply a chain of migration functions with a version map for multi-step upgrades

Real-world use cases

Migrating legacy user records to a new schema when rolling out a platform-wide API version.
Expanding event payloads in a Kafka pipeline to add optional fields without breaking old consumers.
Upgrading database records during a live deployment by applying rename and default-value rules.

How to Expand a Contract and Migrate Data in Python

Python code

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How it works

Common mistakes

Variations

Real-world use cases

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How it works

Common mistakes

Variations

Real-world use cases

More from Production deployment patterns

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