Add environmental variables interpolation support

Author: dbagan13

src/tentaclio/credentials/reader.py

@@ -2,6 +2,7 @@
 import io
 import logging
 import os
+import re
 
 import yaml
 
@@ -13,6 +14,8 @@
 
 SECRETS = "secrets"
 TENTACLIO_SECRETS_FILE = "TENTACLIO__SECRETS_FILE"
+ENV_VARIABLE_PATTERN = r"\${[a-zA-Z_]+[a-zA-Z0-9_]*}"
+NOT_VALID_ENV_VARIABLE_VALUES = "[${}]"
 
 
 class TentaclioFileError(Exception):
@@ -33,7 +36,9 @@ class TentaclioFileError(Exception):
 
     def __init__(self, message: str):
         """Intialise a new TentaclioFileError."""
-        message = self.ERROR_TEMPLATE.format(message=message, tentaclio_file=self.TENTACLIO_FILE)
+        message = self.ERROR_TEMPLATE.format(
+            message=message, tentaclio_file=self.TENTACLIO_FILE
+        )
         super().__init__(message)
 
 
@@ -85,6 +90,25 @@ def _load_from_file(
         raise TentaclioFileError("File not found") from e
 
 
+def replace_env_variables(url: str) -> str:
+    """Replace the environment variables inside the url by its value in the environment.
+
+    Environment variables match the following format: ${ENV_VARIABLE_NAME}
+    """
+    pattern = re.compile(ENV_VARIABLE_PATTERN)
+    env_variables = re.findall(pattern, url)
+    for env_variable in env_variables:
+        env_variable_name = re.sub(NOT_VALID_ENV_VARIABLE_VALUES, "", env_variable)
+        env_value = os.getenv(env_variable_name)
+        if not env_value:
+            raise EnvironmentError(
+                f"Error while reading variable '{env_variable_name}' from the environment. "
+                f"Check that the variable exists in your environment."
+            )
+        url = url.replace(env_variable, env_value)
+    return url
+
+
 def add_credentials_from_reader(
     injector: injection.CredentialsInjector, yaml_reader: protocols.Reader
 ) -> injection.CredentialsInjector:
@@ -99,6 +123,7 @@ def add_credentials_from_reader(
     creds = _load_creds_from_yaml(yaml_reader)
     for name, url in creds.items():
         logger.info(f"Adding secret: {name}")
+        url = replace_env_variables(url)
         try:
             injector.register_credentials(urls.URL(url))
         except Exception as e: