Authenticate Python apps to Azure services during local development using service principals

When creating cloud applications, developers need to debug and test applications on their local workstation. When an application is run on a developer's workstation during local development, it still must authenticate to any Azure services used by the app. This article covers how to set up dedicated application service principal objects to be used during local development.

Dedicated application service principals for local development allow you to follow the principle of least privilege during app development. Since permissions are scoped to exactly what is needed for the app during development, app code is prevented from accidentally accessing an Azure resource intended for use by a different app. This also prevents bugs from occurring when the app is moved to production because the app was overprivileged in the dev environment.

An application service principal is set up for the app when the app is registered in Azure. When registering apps for local development, it's recommended to:

• Create separate app registrations for each developer working on the app. This will create separate application service principals for each developer to use during local development and avoid the need for developers to share credentials for a single application service principal.
• Create separate app registrations per app. This scopes the app's permissions to only what is needed by the app.

During local development, environment variables are set with the application service principal's identity. The Azure SDK for Python reads these environment variables and uses this information to authenticate the app to the Azure resources it needs.

1 - Register the application in Azure

Application service principal objects are created with an app registration in Azure. This can be done using either the Azure portal or Azure CLI.

Azure portal

Sign in to the Azure portal and follow these steps.

Instructions	Screenshot
In the Azure portal: Enter app registrations in the search bar at the top of the Azure portal. Select the item labeled App registrations under the Services heading on the menu that appears below the search bar.
On the App registrations page, select + New registration.
On the Register an application page, fill out the form as follows. Name → Enter a name for the app registration in Azure. It is recommended this name include the app name, the user the app registration is for, and an identifier like 'dev' to indicate this app registration is for use in local development. Supported account types → Accounts in this organizational directory only. Select Register to register your app and create the application service principal.
On the App registration page for your app: Application (client) ID → This is the app id the app will use to access Azure during local development. Copy this value to a temporary location in a text editor as you will need it in a future step. Directory (tenant) id → This value will also be needed by your app when it authenticates to Azure. Copy this value to a temporary location in a text editor it will also be needed it in a future step. Client credentials → You must set the client credentials for the app before your app can authenticate to Azure and use Azure services. Select Add a certificate or secret to add credentials for your app.
On the Certificates & secrets page, select + New client secret.
The Add a client secret dialog will pop out from the right-hand side of the page. In this dialog: Description → Enter a value of Current. Expires → Select a value of 24 months. Select Add to add the secret.
On the Certificates & secrets page, you will be shown the value of the client secret. Copy this value to a temporary location in a text editor as you will need it in a future step. IMPORTANT: This is the only time you will see this value. Once you leave or refresh this page, you will not be able to see this value again. You may add an additional client secret without invalidating this client secret, but you will not see this value again.

Azure CLI

Azure CLI commands can be run in the Azure Cloud Shell or on a workstation with the Azure CLI installed.

First, use the az ad sp create-for-rbac command to create a new service principal for the app. This will also create the app registration for the app at the same time.

az ad sp create-for-rbac --name {service-principal-name}

The output of this command will look like the following. It's recommended to copy this output into a temporary file in a text editor as you'll need these values in a future step as this is the only place you ever see the client secret (password) for the service principal. You can, however, add a new password later without invalidating the service principal or existing passwords if need be.

{
  "appId": "00000000-0000-0000-0000-000000000000",
  "displayName": "{service-principal-name}",
  "password": "abcdefghijklmnopqrstuvwxyz",
  "tenant": "11111111-1111-1111-1111-111111111111"
}

2 - Create an Azure AD security group for local development

Since there typically multiple developers who work on an application, it's recommended to create an Azure AD group to encapsulate the roles (permissions) the app needs in local development rather than assigning the roles to individual service principal objects. This offers the following advantages.

• Every developer is assured to have the same roles assigned since roles are assigned at the group level.
• If a new role is needed for the app, it only needs to be added to the Azure AD group for the app.
• If a new developer joins the team, a new application service principal is created for the developer and added to the group, assuring the developer has the right permissions to work on the app.

Azure portal

Instructions	Screenshot
Navigate to the Azure Active Directory page in the Azure portal by typing Azure Active Directory into the search box at the top of the page and then selecting Azure Active Directory from under services.
On the Azure Active Directory page, select Groups from the left-hand menu.
On the All groups page, select New group.
On the New Group page: Group type → Security Group name → A name for the security group, typically created from the application name. It is also helpful to include a string like local-dev in the name of the group to indicate the purpose of the group. Group description → A description of the purpose of the group. Select the No members selected link under Members to add members to the group.
On the Add members dialog box: Use the search box to filter the list of principal names in the list. Select the application service principals for local development for this app. As objects are selected, they will be greyed out and move to the Selected items list at the bottom of the dialog. When finished, select the Select button.
Back on the New group page, select Create to create the group. The group will be created and you will be taken back to the All groups page. It may take up to 30 seconds for the group to appear and you may need to refresh the page due to caching in the Azure portal.

Azure CLI

The az ad group create command is used to create groups in Azure Active Directory. The --display-name and --main-nickname parameters are required. The name given to the group should be based on the name of the application. It's also useful to include a phrase like 'local-dev' in the name of the group to indicate the purpose of the group.

az ad group create \
    --display-name MyDisplay \
    --mail-nickname MyDisplay  \
    --description \<group-description>

To add members to the group, you'll need the object ID of the application service principal which is different that the application id. Use the az ad sp list to list the available service principals. The --filter parameter command accepts OData style filters and can be used to filter the list as shown. The --query parameter limits to columns to only those of interest.

az ad sp list \
    --filter "startswith(displayName, 'msdocs')" \
    --query "[].{objectId:objectId, displayName:displayName}" \
    --output table

The az ad group member add command can then be used to add members to groups.

az ad group member add \
    --group \<group-name> \
    --member-id \<object-id>  \

3 - Assign roles to the application

Next, you need to determine what roles (permissions) your app needs on what resources and assign those roles to your app. In this example, the roles will be assigned to the Azure Active Directory group created in step 2. Roles can be assigned a role at a resource, resource group, or subscription scope. This example will show how to assign roles at the resource group scope since most applications group all their Azure resources into a single resource group.

Azure portal

Instructions	Screenshot
Locate the resource group for your application by searching for the resource group name using the search box at the top of the Azure portal. Navigate to your resource group by selecting the resource group name under the Resource Groups heading in the dialog box.
On the page for the resource group, select Access control (IAM) from the left-hand menu.
On the Access control (IAM) page: Select the Role assignments tab. Select + Add from the top menu and then Add role assignment from the resulting drop-down menu.
The Add role assignment page lists all of the roles that can be assigned for the resource group. Use the search box to filter the list to a more manageable size. This example shows how to filter for Storage Blob roles. Select the role that you want to assign. Select Next to go to the next screen.
The next Add role assignment page allows you to specify what user to assign the role to. Select User, group, or service principal under Assign access to. Select + Select members under Members A dialog box will open on the right-hand side of the Azure portal.
In the Select members dialog: The Select text box can be used to filter the list of users and groups in your subscription. If needed, type the first few characters of the local development Azure AD group you created for the app. Select the local development Azure AD group associated with your application. Select Select at the bottom of the dialog to continue.
The Azure AD group will now show as selected on the Add role assignment screen. Select Review + assign to go to the final page and then Review + assign again to complete the process.

Azure CLI

An application service principal is assigned a role in Azure using the az role assignment create command.

az role assignment create --assignee "{appId}" \
    --scope /subscriptions/"{subscriptionName}" \
    --role "{roleName}" \
    --resource-group "{resourceGroupName}"

To get the role names that a service principal can be assigned to, use the az role definition list command.

az role definition list \
    --query "sort_by([].{roleName:roleName, description:description}, &roleName)" \
    --output table

For example, to allow the application service principal with the appId of 00000000-0000-0000-0000-000000000000 read, write, and delete access to Azure Storage blob containers and data to all storage accounts in the msdocs-python-sdk-auth-example resource group, you would assign the application service principal to the Storage Blob Data Contributor role using the following command.

az role assignment create --assignee "00000000-0000-0000-0000-000000000000" \
    --scope /subscriptions/"Storage Blob Data Subscriber" \
    --role "Storage Blob Data Contributor" \
    --resource-group "msdocs-python-sdk-auth-example"

For information on assigning permissions at the resource or subscription level using the Azure CLI, see the article Assign Azure roles using the Azure CLI.

4 - Set local development environment variables

The DefaultAzureCredential object will look for the service principal information in a set of environment variables at runtime. Since most developers work on multiple applications, it's recommended to use a package like python-dotenv to access environment from a .env file stored in the application's directory during development. This scopes the environment variables used to authenticate the application to Azure such that they can only be used by this application.

The .env file is never checked into source control since it contains the application secret key for Azure. The standard .gitignore file for Python automatically excludes the .env file from check-in.

To use the python-dotenv package, first install the package in your application.

pip install python-dotenv

Then, create a .env file in your application root directory. Set the environment variable values with values obtained from the app registration process as follows:

• AZURE_CLIENT_ID → The app ID value.
• AZURE_TENANT_ID → The tenant ID value.
• AZURE_CLIENT_SECRET → The password/credential generated for the app.

AZURE_CLIENT_ID=00000000-0000-0000-0000-000000000000
AZURE_TENANT_ID=11111111-1111-1111-1111-111111111111
AZURE_CLIENT_SECRET=abcdefghijklmnopqrstuvwxyz

Finally, in the startup code for your application, use the python-dotenv library to read the environment variables from the .env file on startup.

from dotenv import load_dotenv

if ( os.environ['ENVIRONMENT'] == 'development'):
    print("Loading environment variables from .env file")
    load_dotenv(".env")

5 - Implement DefaultAzureCredential in your application

To authenticate Azure SDK client objects to Azure, your application should use the DefaultAzureCredential class from the azure.identity package. In this scenario, DefaultAzureCredential will detect the environment variables AZURE_CLIENT_ID, AZURE_TENANT_ID, and AZURE_CLIENT_SECRET are set and read those variables to get the application service principal information to connect to Azure with.

Start by adding the azure.identity package to your application.

pip install azure-identity

Next, for any Python code that creates an Azure SDK client object in your app, you'll want to:

1. Import the DefaultAzureCredential class from the azure.identity module.
2. Create a DefaultAzureCredential object.
3. Pass the DefaultAzureCredential object to the Azure SDK client object constructor.

An example of this is shown in the following code segment.

from azure.identity import DefaultAzureCredential
from azure.storage.blob import BlobServiceClient

# Acquire a credential object
token_credential = DefaultAzureCredential()

blob_service_client = BlobServiceClient(
        account_url="https://<my_account_name>.blob.core.windows.net",
        credential=token_credential)