# Extending OpenAPI { #extending-openapi }

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process { #the-normal-process }

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

   * Returns the current sum. The returned value is NOT an atomic snapshot; invocation in
   * the absence of concurrent updates returns an accurate result, but concurrent updates that occur
   * while the sum is being calculated might not be incorporated.
   *
   * @return the sum
   */
  @Override
  public long sum() {
    long sum = base;
    Cell[] as = cells;
    if (as != null) {
      int n = as.length;

    this.canonicalForm = canonicalForm;
  }

  /**
   * Returns a {@code HostSpecifier} built from the provided {@code specifier}, which is already
   * known to be valid. If the {@code specifier} might be invalid, use {@link #from(String)}
   * instead.
   *
   * <p>The specifier must be in one of these formats:
   *
   * <ul>
   *   <li>A domain name, like {@code google.com}

        assertNotNull(notFound);

        // Test potentially retriable errors
        SmbException sharingViolation = new SmbException(NtStatus.NT_STATUS_SHARING_VIOLATION, false);
        // Sharing violations might be retriable in some contexts
        // The implementation determines this
        assertNotNull(sharingViolation);
    }

    @Test
    @DisplayName("Should preserve stack trace information")

          canUseHeaderValue = false
        } else {
          headerValue = value
        }
      }
      name.equals("Pragma", ignoreCase = true) -> {
        // Might specify additional cache-control params. We invalidate just in case.
        canUseHeaderValue = false
      }
      else -> {
        continue@loop
      }
    }

    var pos = 0

For example, if you want to read or manipulate the request body before it is processed by your application.

/// danger

This is an "advanced" feature.

If you are just starting with **FastAPI** you might want to skip this section.

///

## Use cases { #use-cases }

Some use cases include:

	In Namespaces that previously did not have the "DefaultDeny" annotation, you should delete any existing NetworkPolicy objects. These had no effect in the v1beta1 API, but with v1 semantics they might cause some traffic to be unintentionally blocked.


### Storage

* `description`: A long description that can contain Markdown text.
* `value`: This is the actual example shown, e.g. a `dict`.
* `externalValue`: alternative to `value`, a URL pointing to the example. Although this might not be supported by as many tools as `value`.

You can use it like this:

{* ../../docs_src/schema_extra_example/tutorial005_an_py310.py hl[23:49] *}

        // that. If any of the BROWSER_COMPATIBLE_DATE_FORMAT_STRINGS ended in "'GMT'" we'd have
        // to also check that position.getIndex() == value.length() otherwise parsing might have
        // terminated early, ignoring things like "+01:00". Leaving this as != 0 means that any
        // trailing junk is ignored.
        return result
      }
    }
  }
  return null
}

import junit.framework.TestCase;
import org.jspecify.annotations.NullUnmarked;

/**
 * Tests that all package-private {@code writeReplace} methods are overridden in any existing
 * subclasses. Without such overrides, optimizers might put a {@code writeReplace}-containing class
 * and its subclass in different packages, causing the serialization system to fail to invoke {@code