# Handling Errors { #handling-errors }

There are many situations in which you need to notify an error to a client that is using your API.

This client could be a browser with a frontend, a code from someone else, an IoT device, etc.

You could need to tell the client that:

* The client doesn't have enough privileges for that operation.
* The client doesn't have access to that resource.
* The item the client was trying to access doesn't exist.
* etc.

pkg io, type Seeker interface { Seek }
pkg io, type Seeker interface, Seek(int64, int) (int64, error)
pkg io, type WriteCloser interface { Close, Write }
pkg io, type WriteCloser interface, Close() error
pkg io, type WriteCloser interface, Write([]uint8) (int, error)
pkg io, type WriteSeeker interface { Seek, Write }
pkg io, type WriteSeeker interface, Seek(int64, int) (int64, error)

```

////

So, when you need to declare a value as required while using `Query`, you can simply not declare a default value:

{* ../../docs_src/query_params_str_validations/tutorial006_an_py39.py hl[9] *}

### Required, can be `None` { #required-can-be-none }

You can declare that a parameter can accept `None`, but that it's still required. This would force clients to send a value, even if the value is `None`.

make -j$(nproc)
make install DESTDIR=${TARGET}
cd ..

# Symlinks in the binary distribution are set up for installation in /usr, we
# need to fix up all the links to stay within /${TARGET}.
/fixlinks.sh "/${TARGET}"

# Patch to allow non-glibc 2.12 compatible builds to work.
sed -i '54i#define TCP_USER_TIMEOUT 18' "/${TARGET}/usr/include/netinet/tcp.h"

# Download specific version of libstdc++ shared library based on the value of

				// no longer be multipart, and thus the checksum becomes full-object instead. Since
				// the CopyObject API does not require that the caller send us this final checksum, we need
				// to compute it server-side, with the same type as the source object.
				if dstOpts.WantChecksum != nil && dstOpts.WantChecksum.Type.IsMultipartComposite() {
					dstOpts.WantServerSideChecksumType = dstOpts.WantChecksum.Type.Base()

## What is Erasure Code?

```

...without adding the required parameter `needy`, you will see an error like:

```JSON
{
  "detail": [
    {
      "type": "missing",
      "loc": [
        "query",
        "needy"
      ],
      "msg": "Field required",
      "input": null
    }
  ]
}
```

As `needy` is a required parameter, you would need to set it in the URL:

```

If you want to play with JWT tokens and see how they work, check <a href="https://jwt.io/" class="external-link" target="_blank">https://jwt.io</a>.

## Install `PyJWT` { #install-pyjwt }

We need to install `PyJWT` to generate and verify the JWT tokens in Python.

### Import typing's `List` { #import-typings-list }

In Python 3.9 and above you can use the standard `list` to declare these type annotations as we'll see below. 💡

But in Python versions before 3.9 (3.6 and above), you first need to import `List` from standard Python's `typing` module:

{* ../../docs_src/body_nested_models/tutorial002.py hl[1] *}

#   destroys some temporary bazel data and causes longer builds.
#
#   You will need the NVIDIA Container Toolkit for GPU testing:
#   https://github.com/NVIDIA/nvidia-container-toolkit
#
#   Note: if you interrupt a bazel command on docker (ctrl-c), you
#   will need to run `docker exec tf pkill bazel` to quit bazel.
#
#   Note: new files created from the container are owned by "root".