available, instead of erroring out, the dependency result will be
                `None`.

                This is useful when you want to have optional authentication.

                It is also useful when you want to have authentication that can be
                provided in one of multiple optional ways (for example, in a query
                parameter or in an HTTP Bearer token).

                is not available, instead of erroring out, the dependency result will
                be `None`.

                This is useful when you want to have optional authentication.

                It is also useful when you want to have authentication that can be
                provided in one of multiple optional ways (for example, with OpenID
                Connect or in a cookie).

But still, it is very powerful, and allows you to declare arbitrarily deeply nested dependency "graphs" (trees).

/// tip

All this might not seem as useful with these simple examples.

But you will see how useful it is in the chapters about **security**.

And you will also see the amounts of code it will save you.

  private final Object event;
  private final Object subscriber;
  private final Method subscriberMethod;

  /**
   * @param eventBus The {@link EventBus} that handled the event and the subscriber. Useful for
   *     broadcasting a new event based on the error.
   * @param event The event object that caused the subscriber to throw.
   * @param subscriber The source subscriber context.

		debug.SetMemoryLimit(int64(ctx.MemLimit))
	}

	// Do not use RLIMIT_AS as that is not useful and at times on systems < 4Gi
	// this can crash the Go runtime if the value is smaller refer
	// - https://github.com/golang/go/issues/38010
	// - https://github.com/golang/go/issues/43699
	// So do not add `sys.SetMaxMemoryLimit()` this is not useful for any practical purposes.
	return nil

        But Guava aims to provide features that are useful across boundaries of projects, companies,
        or even industries — utilities useful for a sizable proportion of all Java programmers
        everywhere. If you can give enough detail such that any of us can imagine coming across
        a similar need in our own work, that's extremely helpful in studying how broadly useful the
        feature will be.

  - type: textarea

package org.apache.maven.exception;

import java.util.Collections;
import java.util.List;

/**
 * Provide a summary of the exception, containing:<ul>
 * <li>the exception itself,</li>
 * <li>useful end-user message,</li>
 * <li>useful reference to a solution, or set of solutions: this is usually a wiki page url in
 * <a href="http://cwiki.apache.org/confluence/display/MAVEN/">http://cwiki.apache.org/confluence/display/MAVEN/</a>,
 * </li>

     */
    boolean required() default false;

    /**
     * Specifies that this parameter cannot be configured directly by the user (as in the case of POM-specified
     * configuration). This is useful when you want to force the user to use common POM elements rather than plugin
     * configurations, as in the case where you want to use the artifact's final name as a parameter. In this case, you

// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//

Environment variables could be useful for handling application **settings**, as part of the **installation** of Python, etc.

## Create and Use Env Vars

You can **create** and use environment variables in the **shell (terminal)**, without needing Python: