Crate env_wrapper

A wrapper around the standard std::env functions that allows for a test double to be injected during testing.

Motivation

Testing code that relies on the state of environment variables can be fragile, since the state may change between tests or be polluted by other tests. The ideal solution is to have a private set of environment variables per test, so these problems cannot happen.

Approach

This crate introduces the RealEnvironment (a wrapper around the functions in std::env) and FakeEnvironment structs, which implement the Environment trait. Instead of using std::env directly, use RealEnvironment with dependency injection so each of your tests can have a private set of environment variables.

Example

Scenario: An app looks for the presence of the CONFIG_LOCATION environment variable. If it isn’t set, it uses a default location.

use env_wrapper::{Environment, RealEnvironment};

const CONFIG_LOCATION_ENV_VAR_NAME: &str = "CONFIG_LOCATION";
const DEFAULT_CONFIG_LOCATION: &str = "/etc/my_app/service.conf";

fn main() {
    // In the production code, inject RealEnvironment.
    let real_env = RealEnvironment;
    let config_location = get_config_location(real_env);
}

fn get_config_location(env: impl Environment) -> String {
    match env.var(CONFIG_LOCATION_ENV_VAR_NAME) {
        Ok(location) => location,
        _ => DEFAULT_CONFIG_LOCATION.to_string(),
    }
}

#[test]
fn when_the_user_has_set_the_config_location_env_var_then_use_that_location() {
    use env_wrapper::FakeEnvironment;

    // Arrange
    // Each test should have a separate instance of FakeEnvironment.
    let mut fake_env = FakeEnvironment::new();
    let user_specified_location = "/a/user/specified/location";
    fake_env.set_var(CONFIG_LOCATION_ENV_VAR_NAME, user_specified_location);
     
    // Act
    // In the test code, inject FakeEnvironment.
    let location = get_config_location(fake_env);

    // Assert
    assert_eq!(location, user_specified_location);
}

Structs

FakeEnvironment

A fake process environment, suitable for testing.

RealEnvironment

The process’s environment. Wraps the standard std::env functions.

Traits

Environment

Represents a process’s environment.