[llc] add quickstart

doc/llc.md

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+# Python Low-Level Client
+
+Our low-level clients provide simple, reliable connections to APIs for previews and advanced usage.
+
+Here's how to get started:
+
+```pycon
+>>> from azure.identity import DefaultAzureCredential
+>>> from azure.example import ExampleClient
+>>> from azure.example.rest import build_example_request
+>>> client = ExampleClient(endpoint='https://www.example.org/', credential=DefaultAzureCredential())
+>>> request = build_example_request()
+>>> request
+<HttpRequest [GET], url: 'https://www.example.org'>
+>>> response = client.send_request(request)
+>>> response
+<HttpResponse: 200 OK, Content-Type: text/plain>
+>>> response.raise_for_status()
+>>> response.text
+'Happy to see you!'
+```
+
+## Code Snippets
+
+**Code snippets for how to use our Low-Level clients**:
+
+1. [Sync client](#sync-client)
+2. [Async client](#async-client)
+
+## What is a Low-Level Client
+
+Our low-level clients provide simple, reliable connection to raw HTTP for previews and advanced usage. We provide caller `send_request` on the client to send requests to the service.
+Calls through this method fully harness the power of `azure-core` and [`azure-identity`][azure_identity_docs]. We provide both synchronous and asynchronous low-level clients.
+
+The basic structure of calls with low-level clients is:
+
+1. [Initialize your client](#1-initialize-your-client "Initialize Your Client")
+2. [Create a request](#2-create-a-request "Create a Request")
+3. [Send the request](#3-send-the-request "Send the Request")
+4. [Handle the response](#4-handle-the-response "Handle the Response")
+
+We will go into each step in the following sections
+
+## 1. Initialize Your Client
+
+First you import your client from the namespace of your package. For example, let's say your namespace is `azure.example` and your client's name
+is `ExampleClient`. Your import would look like
+
+```python
+from azure.example import ExampleClient
+```
+
+Most clients require authenticating through their `credential` parameter. Depending on what authentication support your library is using, you can either [authenticate with aad](#authenticating-with-aad) or [authenticate with an `AzureKeyCredential`](#authenticating-with-azurekeycredential).
+
+Additionally, most of our clients accept an `endpoint` parameter at initialization, usually a link to your own resource.
+
+### Authenticating with AAD
+
+If your client supports authenticating with an [Azure Active Directory (AAD) token credential][aad_authentication], we provide a convenient library for AAD authentication called [`azure-identity`][azure_identity_docs] that can be installed additionally with:
+
+```bash
+pip install azure-identity
+```
+
+Once [`azure-identity`][azure_identity_pip] is installed, the simplest way to authenticate is to use the [`DefaultAzureCredential`][default_azure_credential] class.
+
+The following code snippet shows you how to authenticate with a [`DefaultAzureCredential`][default_azure_credential].
+
+```python
+from azure.identity import DefaultAzureCredential
+from azure.example import ExampleClient
+
+client = ExampleClient(
+    endpoint="https://www.example.org/",
+    credential=DefaultAzureCredential()
+)
+```
+
+### Authenticating with [`AzureKeyCredential`][azure_key_credential]
+
+Some libraries support authenticating with an [`AzureKeyCredential`][azure_key_credential]. The following code snippet shows you how to authenticate with an
+[`AzureKeyCredential`][azure_key_credential]
+
+```python
+from azure.core.credentials import AzureKeyCredential
+from azure.example import ExampleClient
+
+credential = "myCredential"
+client = ExampleClient(
+    endpoint="https://www.example.org/",
+    credential=AzureKeyCredential(credential)
+)
+```
+
+## 2. Create a Request
+
+Next, you need to create the request you want to be sent to the service.
+
+We offer [request builders](#use-our-request-builders) to make creating your `HttpRequest`s easier.
+
+For more advanced users, you can also [create your `HttpRequest` fully by yourself](#create-your-own-httprequest)
+
+### Use our Request Builders
+
+Our request builders:
+
+- Keep track of the URL and method of the call, so you don't have to
+- Let you know what parameters the service needs
+- Take care of formatting your parameters
+- Will be grouped into submodules if there's a natural grouping to them.
+
+These request builders are located in the `rest` module of our libraries. If there's
+a natural grouping to request builders, these submodule groups will live inside the `rest` module.
+
+Now, let's make a request with a `json` body.
+
+```python
+from azure.example.rest import build_analyze_text_request
+
+request = build_analyze_text_request(
+    json={"document": "Hello world!"},
+    language="en",
+)
+```
+
+If the `rest` module has grouped submodules, we recommend importing the whole submodule like this to
+avoid name conflicts:
+
+```python
+from azure.example.rest import languages
+
+request = languages.build_detect_request(
+    json={"document": "世界你好！"}
+)
+```
+
+### Create Your Own [`HttpRequest`][azure_core_http_request]
+
+For more advanced scenarios, you can also create your own [`HttpRequest`][azure_core_http_request].
+
+Let's make the same request as we do in our [previous example](#use-our-request-builders)
+
+```python
+from azure.example.core.rest import HttpRequest
+
+# this URL is relative to the endpoint we passed our client
+request = HttpRequest("POST", "/helloWorld",
+    json={"document": "Hello world!"},
+    params={"language": "en"}
+)
+```
+
+## 3. Send the Request
+
+Now, we pass this request to your client's `send_request` method. This actually makes the network call.
+
+```python
+from azure.example import ExampleClient
+
+response = client.send_request(request) # makes the network call
+```
+
+## 4. Handle the Response
+
+Our `send_request` call returns an [`HttpResponse`][azure_core_http_response].
+
+### Error handling
+
+The response you get back from `send_request` will not automatically raise if your response is an error.
+If you wish to raise an error if your response is bad, call [`.raise_for_status()`][azure_core_raise_for_status] on your returned
+response.
+
+```python
+try:
+    response.raise_for_status()  # raises an error if your response is not good
+except HttpResponseError as e:
+    print(str(e))
+```
+
+### JSON response
+
+If the response you get back should be a `json` object, you can call `.json()` on your response
+to get it `json`-deserialized.
+
+Putting this all together, see our code snippets for how you can deal with your response object
+
+```python
+
+response = client.send_request(request)
+try:
+    response.raise_for_status()  # raises an error if your response is not good
+    json_response = response.json()  # get your response as a json object
+    # Now play with your JSON response!
+
+except HttpResponseError as e:
+    print(str(e))
+```
+
+## Examples
+
+### Sync Client
+
+```python
+from azure.identity import DefaultAzureCredential
+from azure.example import ExampleClient
+from azure.example.rest import build_analyze_text_request
+from azure.core.exceptions import HttpResponseError
+
+client = ExampleClient(
+    endpoint="https://example.org",
+    credential=DefaultAzureCredential()
+)
+
+request = build_analyze_text_request(
+    json={"document": "Hello world!"},
+    language="en",
+)
+
+response = client.send_request(request)
+
+try:
+    response.raise_for_status()
+    json_response = response.json()
+    # Play with your response!
+except HttpResponseError:
+    print(str(e))
+```
+
+### Async Client
+
+```python
+from azure.identity.aio import DefaultAzureCredential
+from azure.example.aio import ExampleClient
+from azure.example.rest import build_analyze_text_request
+from azure.core.exceptions import HttpResponseError
+
+request = build_analyze_text_request(
+    json={"document": "Hello world!"},
+    language="en",
+)
+
+with DefaultAzureCredential() as credential:
+    with ExampleClient(endpoint="https://example.org", credential=credential) as client:
+        response = await client.send_request(request)
+
+        try:
+            response.raise_for_status()
+            await response.load_body()
+            json_response = response.json()
+            # Play with your response!
+        except HttpResponseError:
+            print(str(e))
+```
+
+## Troubleshooting
+
+### Errors
+
+All errors thrown by `.raise_for_error()` are [exceptions defined in `azure-core`][azure_core_exceptions].
+
+### Logging
+
+Our low-level clients also have logging support. They use the standard
+[logging][python_logging] library for logging.
+Basic information about HTTP sessions (URLs, headers, etc.) is logged at INFO
+level.
+
+Detailed DEBUG level logging, including request/response bodies and un-redacted
+headers, can be enabled on a client with the `logging_enable` keyword argument.
+
+```python
+from azure.identity import DefaultAzureCredential
+from azure.example import ExampleClient
+
+client = ExampleClient(
+    endpoint="https://example.org",
+    credential=DefaultAzureCredential(),
+    logging_enable=True
+)
+```
+
+### File an Issue
+
+You can file issues for the Low Level Client [here][issues] in our repo. You can label your issue with the `"Low Level Client"` label.
+
+<!-- LINKS -->
+
+[azure_core_docs]: https://docs.microsoft.com/python/api/overview/azure/core-readme?view=azure-python
+[azure_identity_docs]: https://docs.microsoft.com/python/api/overview/azure/identity-readme?view=azure-python
+[http_response]: https://docs.microsoft.com/python/api/azure-core/azure.core.pipeline.transport.httpresponse?view=azure-python
+[azure_identity_pip]: https://pypi.org/project/azure-identity/
+[aad_authentication]: https://docs.microsoft.com/azure/cognitive-services/authentication?tabs=powershell#authenticate-with-an-authentication-token
+[identity_credentials]: https://github.com/Azure/azure-sdk-for-python/tree/master/sdk/identity/azure-identity#credentials
+[default_azure_credential]: https://docs.microsoft.com/python/api/azure-identity/azure.identity.defaultazurecredential?view=azure-python
+[azure_key_credential]: https://docs.microsoft.com/python/api/azure-core/azure.core.credentials.azurekeycredential?view=azure-python
+[bearer_token_credential_policy]: https://docs.microsoft.com/python/api/azure-core/azure.core.pipeline.policies.bearertokencredentialpolicy?view=azure-python
+[azure_key_credential_policy]: https://docs.microsoft.com/python/api/azure-core/azure.core.pipeline.policies.azurekeycredentialpolicy?view=azure-python
+[azure_core_exceptions]: https://docs.microsoft.com/python/api/azure-core/azure.core.exceptions?view=azure-python
+[azure_core_http_request]: https://docsupport.blob.core.windows.net/$web/azure-core/azure.core.html#azure.core.protocol.HttpRequest
+[azure_core_http_response]: https://docsupport.blob.core.windows.net/$web/azure-core/azure.core.html#azure.core.protocol.HttpResponse
+[azure_core_async_http_response]: https://docsupport.blob.core.windows.net/$web/azure-core/azure.core.html#azure.core.protocol.AsyncHttpResponse
+[azure_core_raise_for_status]: https://docsupport.blob.core.windows.net/$web/azure-core/azure.core.html#azure.core.protocol.HttpResponse.raise_for_status
+[python_logging]: https://docs.python.org/3.5/library/logging.html
+[code_of_conduct]: https://opensource.microsoft.com/codeofconduct/
+[coc_faq]: https://opensource.microsoft.com/codeofconduct/faq/
+[coc_contact]: mailto:opencode@microsoft.com
+[issues]: https://github.com/Azure/azure-sdk-for-python/issues