PR #1711 fixes a flaky test (test_model_copy) that was failing ~50% of the time due to race conditions in how Environment.gas_limit defaults are handled. While the PR successfully resolves the immediate bug, it highlights a deeper architectural issue with global mutable state that could cause similar problems in the future.

Current Architecture

How It Works

# block_types.py
DEFAULT_BLOCK_GAS_LIMIT = 90_000_000

class EnvironmentDefaults:
    """Default environment values."""
    gas_limit: int = DEFAULT_BLOCK_GAS_LIMIT  # ← Mutable class attribute

class EnvironmentGeneric(CamelModel, Generic[NumberBoundTypeVar]):
    gas_limit: NumberBoundTypeVar = Field(
        default_factory=lambda: EnvironmentDefaults.gas_limit,  # ← Late binding
        alias="currentGasLimit",
    )

Pytest plugins modify this global state during configuration:

# filler.py:699 and execute.py:152
@pytest.hookimpl(tryfirst=True)
def pytest_configure(config: pytest.Config) -> None:
    if config.getoption("block_gas_limit"):
        EnvironmentDefaults.gas_limit = config.getoption("block_gas_limit")

Why It Exists

Design Intent:

1. Convenience: Single place to configure gas limits for all tests
2. Flexibility: Different pytest commands (fill, execute, benchmark) need different gas limits
3. CLI Control: Users can override defaults with --block-gas-limit or --transaction-gas-limit
4. Backwards Compatibility: Avoids refactoring hundreds of test files

Usage Pattern:

• 480 Environment() instantiations across 163 test files
• Only 7% explicitly set gas_limit
• 93% rely on the dynamic default from EnvironmentDefaults.gas_limit

Problems with Current Architecture

1. Non-Deterministic Plugin Ordering

Issue: Two plugins (filler.py and execute.py) both use @pytest.hookimpl(tryfirst=True), giving them identical priority. Pytest doesn't guarantee execution order when hooks have the same priority.

Impact: If both plugins try to set EnvironmentDefaults.gas_limit, the result is non-deterministic.

2. Race Conditions in Parallel Execution

Issue: Tests run with pytest -n auto --maxprocesses 6, creating multiple worker processes that can have different timing for:

• When plugins load and modify globals
• When test modules are imported
• When Environment() objects are created

Impact: Objects created at different times can have different gas_limit values, even though they use the same code (Environment()).

3. Late-Binding Default Factory

Issue: The default_factory=lambda: EnvironmentDefaults.gas_limit captures a reference to the class attribute, not its value. Every time an Environment() is created, the lambda re-evaluates.

Impact:

• Environment() at time T₁ → gas_limit = 90_000_000
• Plugin modifies EnvironmentDefaults.gas_limit = 1000
• Environment() at time T₂ → gas_limit = 1000
• Old .copy() implementation re-ran the factory → got new value!

4. Hidden Dependencies

Issue: The behavior of Environment() depends on invisible state:

• Whether pytest plugins have loaded yet
• Which CLI flags were passed
• Whether other tests have run that might affect globals

Impact: Violates the Principle of Least Surprise—test behavior is not locally reasonable.

5. Copy Semantics Violation

Issue: The old .copy() implementation broke the fundamental contract: obj.copy() == obj

PR #1711 Fix: Captures actual field values instead of re-running default_factory