Choosing Immutable Variables in Vyper: When to Use, Trade‑offs, and How to Validate

Why Consider Immutables?

In Vyper, an immutable variable is set once during contract deployment and never changes. This guarantees that the value is fixed, reduces the number of storage slots, and allows the compiler to inline the value, cutting runtime gas.

Decision Matrix

Option	Supported Types	Gas at Deployment	Gas at Runtime	Use‑case Fit
Regular state variable	All Vyper types	High – storage write	High – storage read/write	Values that change after deployment
Immutable variable	int128, address, bytes32, bool	Medium – storage write in constructor	Low – inlined at compile time	Constants that must be known at deployment but never change
Constant (compile‑time)	All literal types	None – baked into bytecode	None – directly inlined	Hard‑coded values that do not need constructor input

When Immutables Are the Right Choice

• Fixed configuration – e.g., the owner address, a pre‑determined fee, or a token symbol.
• Gas‑critical paths – any function that reads the value frequently benefits from inlining.
• Security guarantees – once set, the value cannot be altered by a malicious actor.

Limitations to Keep in Mind

• Only int128, address, bytes32, and bool are allowed.
• Dynamic arrays, mappings, and structs cannot be immutable.
• Providing a large value via the constructor can inflate deployment gas; if the value is huge, consider a constant or external storage.
• Misusing immutables for data that needs to change leads to irreversible contract behavior.

Concrete Implementation

Below is a minimal contract that stores an immutable owner address and exposes it via a view function.

# @version ^0.3.9

owner: immutable(address)

@external
def __init__(_owner: address):
    # The constructor assigns the immutable once.
    owner = _owner

@view
@external
def get_owner() -> address:
    return owner

Compile and Inspect

1. Run vyper --version and confirm you are on 0.3.9 or newer.
2. Compile with vyper contract.vy. The output ABI will contain get_owner only; the immutable is not exposed as a storage slot.
3. Use vyper --abi contract.vy to view the ABI and verify that no owner variable is listed.
4. Check the bytecode with vyper --bytecode contract.vy and look for the address value in the data section – it should be inlined.

Deploy and Verify Runtime Value

1. Deploy the contract via a script or Remix, passing the desired owner address as the constructor argument.
2. Call get_owner() and confirm it matches the address you supplied.
3. Deploy a second instance with a different address and repeat the call; each instance will have its own immutable value.

Gas Comparison

Using a tool like eth-gas-reporter or ganache-cli, you can compare the gas used by a function that reads a regular state variable versus one that reads an immutable. Typically, the immutable path saves 30–50% of the gas spent on storage reads.

Practical Checklist Before Adding an Immutable

• Is the value known at deployment time?
• Does it need to change later?
• Is the type one of the supported ones?
• Will the deployment gas cost be acceptable?
• Can the value be inlined as a constant instead?

Rollback Considerations

Because immutable variables cannot be altered after deployment, there is no rollback path. If you anticipate the need to change the value, use a regular state variable or a proxy pattern instead.

Final Takeaway

Use immutable in Vyper when you need a value that is set once, is cheap to read, and must never change. It’s a powerful tool for reducing gas and hardening contracts, but its type restrictions and lack of mutability mean it’s not a silver bullet for all configuration data.