text: # ------------------------------------
  # Copyright (c) Microsoft Corporation.
  # Licensed under the MIT License.
  # ------------------------------------
  import os
  
  from azure.core.exceptions import ClientAuthenticationError
  from .._constants import EnvironmentVariables
  from .client_credential import CertificateCredential, ClientSecretCredential
  from .user import UsernamePasswordCredential
  
  
  try:
      from typing import TYPE_CHECKING
  except ImportError:
      TYPE_CHECKING = False
  
  if TYPE_CHECKING:
      from typing import Any, Mapping, Optional, Union
      from azure.core.credentials import AccessToken
  
      EnvironmentCredentialTypes = Union["CertificateCredential", "ClientSecretCredential", "UsernamePasswordCredential"]
  
  
  class EnvironmentCredential:
      """A credential configured by environment variables.
  
      This credential is capable of authenticating as a service principal using a client secret or a certificate, or as
      a user with a username and password. Configuration is attempted in this order, using these environment variables:
  
      Service principal with secret:
        - **AZURE_CLIENT_ID**: the service principal's client ID
        - **AZURE_CLIENT_SECRET**: one of the service principal's client secrets
        - **AZURE_TENANT_ID**: ID of the service principal's tenant. Also called its 'directory' ID.
  
      Service principal with certificate:
        - **AZURE_CLIENT_ID**: the service principal's client ID
        - **AZURE_CLIENT_CERTIFICATE_PATH**: path to a PEM-encoded certificate file including the private key
        - **AZURE_TENANT_ID**: ID of the service principal's tenant. Also called its 'directory' ID.
  
      User with username and password:
        - **AZURE_CLIENT_ID**: the application's client ID
        - **AZURE_USERNAME**: a username (usually an email address)
        - **AZURE_PASSWORD**: that user's password
        - **AZURE_TENANT_ID**: (optional) ID of the service principal's tenant. Also called its 'directory' ID.
          If not provided, defaults to the 'organizations' tenant, which supports only Azure Active Directory work or
          school accounts.
      """
  
      def __init__(self, **kwargs):
          # type: (Mapping[str, Any]) -> None
          self._credential = None  # type: Optional[EnvironmentCredentialTypes]
  
          if all(os.environ.get(v) is not None for v in EnvironmentVariables.CLIENT_SECRET_VARS):
              self._credential = ClientSecretCredential(
                  client_id=os.environ[EnvironmentVariables.AZURE_CLIENT_ID],
                  client_secret=os.environ[EnvironmentVariables.AZURE_CLIENT_SECRET],
                  tenant_id=os.environ[EnvironmentVariables.AZURE_TENANT_ID],
                  **kwargs
              )
          elif all(os.environ.get(v) is not None for v in EnvironmentVariables.CERT_VARS):
              self._credential = CertificateCredential(
                  client_id=os.environ[EnvironmentVariables.AZURE_CLIENT_ID],
                  tenant_id=os.environ[EnvironmentVariables.AZURE_TENANT_ID],
                  certificate_path=os.environ[EnvironmentVariables.AZURE_CLIENT_CERTIFICATE_PATH],
                  **kwargs
              )
          elif all(os.environ.get(v) is not None for v in EnvironmentVariables.USERNAME_PASSWORD_VARS):
              self._credential = UsernamePasswordCredential(
                  client_id=os.environ[EnvironmentVariables.AZURE_CLIENT_ID],
                  username=os.environ[EnvironmentVariables.AZURE_USERNAME],
                  password=os.environ[EnvironmentVariables.AZURE_PASSWORD],
                  tenant=os.environ.get(EnvironmentVariables.AZURE_TENANT_ID),  # optional for username/password auth
                  **kwargs
              )
  
      def get_token(self, *scopes, **kwargs):  # pylint:disable=unused-argument
          # type: (*str, **Any) -> AccessToken
          """Request an access token for `scopes`.
  
          :param str scopes: desired scopes for the token
          :rtype: :class:`azure.core.credentials.AccessToken`
          :raises: :class:`azure.core.exceptions.ClientAuthenticationError`
          """
          if not self._credential:
              raise ClientAuthenticationError(message="Incomplete environment configuration.")
          return self._credential.get_token(*scopes, **kwargs)
  