A **virtual environment** is a directory with some files in it.

///

/// info

This page will teach you how to use **virtual environments** and how they work.

If you are ready to adopt a **tool that manages everything** for you (including installing Python), try [uv](https://github.com/astral-sh/uv).

///

## Create a Project { #create-a-project }

First, create a directory for your project.

## Decision

We do not use Java serialization.
Instead, we use custom serialization where we explicitly describe how data objects should be serialized and deserialized.

For internal purposes, we use binary formats for their brevity.
We use the `Serializer` abstraction to separate the actual implementation of serialization from its uses.

   * However, we can't usually write {@code throw t;} when {@code t} has a static type of {@link
   * Throwable}. But we <i>can</i> write {@code sneakyThrow(t);}.
   *
   * <p>We sometimes also use {@code sneakyThrow} for testing how our code responds to
   * sneaky checked exception.
   *
   * @return never; this method declares a return type of {@link Error} only so that callers can

  /** Returns the sample elements that this generate populates its container with. */
  SampleElements<E> samples();

  /**
   * Creates a new container containing the given elements. TODO: would be nice to figure out how to
   * use E... or E[] as a parameter type, but this doesn't seem to work because Java creates an
   * array of the erased type.
   */
  T create(Object... elements);

  /**

   * However, we can't usually write {@code throw t;} when {@code t} has a static type of {@link
   * Throwable}. But we <i>can</i> write {@code sneakyThrow(t);}.
   *
   * <p>We sometimes also use {@code sneakyThrow} for testing how our code responds to
   * sneaky checked exception.
   *
   * @return never; this method declares a return type of {@link Error} only so that callers can

contribution quickly and easily:

- [ ] Your pull request should address just one issue, without pulling in other changes.
- [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
- [ ] Each commit in the pull request should have a meaningful subject line and body.
  Note that commits might be squashed by a maintainer on merge.

        }
    ]
}
```

/// info

Notice how the `images` key now has a list of image objects.

///

## Deeply nested models { #deeply-nested-models }

You can define arbitrarily deeply nested models:

{* ../../docs_src/body_nested_models/tutorial007_py310.py hl[7,12,18,21,25] *}

/// info

Notice how `Offer` has a list of `Item`s, which in turn have an optional list of `Image`s

  /** Returns the sample elements that this generate populates its container with. */
  SampleElements<E> samples();

  /**
   * Creates a new container containing the given elements. TODO: would be nice to figure out how to
   * use E... or E[] as a parameter type, but this doesn't seem to work because Java creates an
   * array of the erased type.
   */
  T create(Object... elements);

  /**

  private var currentTask: SerialTask = TestThreadSerialTask

  /** The coordinator task if it's waiting, and how it will resume. Guarded by `this`. */
  private var waitingCoordinatorTask: SerialTask? = null
  private var waitingCoordinatorInterrupted = false
  private var waitingCoordinatorNotified = false

  /** How many times a new task has been started. Guarded by `this`. */
  private var contextSwitchCount = 0

   * However, we can't usually write {@code throw t;} when {@code t} has a static type of {@link
   * Throwable}. But we <i>can</i> write {@code sneakyThrow(t);}.
   *
   * <p>We sometimes also use {@code sneakyThrow} for testing how our code responds to
   * sneaky checked exception.
   *
   * @return never; this method declares a return type of {@link Error} only so that callers can