Suggest: make gas limit configurable in Python SDK signer

[HeadyZhang]

Summary

The reference Web3EvmSigner in the Python SDK hardcodes gas: 300000 for both write_contract() and send_transaction(). Making this configurable would improve flexibility for different contract call complexities and network conditions.

Affected file

python/x402/mechanisms/evm/signers.py

• Line 479: write_contract() — "gas": 300000
• Line 505: send_transaction() — "gas": 300000

Current behavior

# write_contract (L475-482)
tx = func(*args).build_transaction(
    {
        "from": self._account.address,
        "nonce": self._w3.eth.get_transaction_count(self._account.address),
        "gas": 300000,
        "gasPrice": self._w3.eth.gas_price,
    }
)

# send_transaction (L500-507)
tx = {
    "from": self._account.address,
    "to": Web3.to_checksum_address(to),
    "data": data,
    "nonce": self._w3.eth.get_transaction_count(self._account.address),
    "gas": 300000,
    "gasPrice": self._w3.eth.gas_price,
}

Suggested change

Add a configurable default with optional per-call override:

class Web3EvmSigner:
    DEFAULT_GAS_LIMIT = 300_000

    def __init__(self, w3, account, *, gas_limit: int = DEFAULT_GAS_LIMIT):
        self._w3 = w3
        self._account = account
        self._gas_limit = gas_limit

    def send_transaction(self, to: str, data: bytes, *, gas: int | None = None) -> str:
        tx = {
            "from": self._account.address,
            "to": Web3.to_checksum_address(to),
            "data": data,
            "nonce": self._w3.eth.get_transaction_count(self._account.address),
            "gas": gas or self._gas_limit,
            "gasPrice": self._w3.eth.gas_price,
        }
        # ...

Alternatively, using eth.estimate_gas() as a default would adapt to actual call complexity:

"gas": gas or self._w3.eth.estimate_gas({"to": to, "data": data, "from": self._account.address}),

This is a minor ergonomics improvement — the current hardcoded value works fine for most cases. Happy to open a PR if helpful.