feat: add Local client

This is just a safe fallback for local testing.
If environment variables which activate the
creation of the GitHub client aren't set, then
the Local client is created. It acts on the most
recent commit of the repo in the working directory.

Minor edit to GitHubClient so that it raises an
EnvironmentError exception if INPUT_GITHUB_URL
environment variable is not defined. This allows
main to detect the error and fall back to trying
to use the Local client

GitHubClient.py

@@ -9,6 +9,8 @@ class GitHubClient(object):
 
     def __init__(self):
         self.github_url = os.getenv('INPUT_GITHUB_URL')
+        if not self.github_url:
+            raise EnvironmentError
         self.base_url = f'{self.github_url}/'
         self.repos_url = f'{self.base_url}repos/'
         self.repo = os.getenv('INPUT_REPO')

LocalClient.py

@@ -0,0 +1,16 @@
+import subprocess
+
+class LocalClient(object):
+    def __init__(self):
+        self.diff_url = None
+        self.commits = ['placeholder'] # content doesn't matter, just length
+        self.insert_issue_urls = False
+
+    def get_last_diff(self):
+        return subprocess.run(['git', 'diff', 'HEAD^..HEAD'], stdout=subprocess.PIPE).stdout.decode('utf-8')
+
+    def create_issue(self, issue):
+        return [201, None]
+
+    def close_issue(self, issue):
+        return 200

main.py

@@ -11,12 +11,21 @@ import operator
 from collections import defaultdict
 from TodoParser import *
 from LineStatus import *
+from LocalClient import *
 from GitHubClient import *
 
 
 if __name__ == "__main__":
-    # Create a basic client for communicating with GitHub, automatically initialised with environment variables.
-    client = GitHubClient()
+    # Try to create a basic client for communicating with the remote version control server, automatically initialised with environment variables.
+    try:
+        # try to build a GitHub client
+        client = GitHubClient()
+    except EnvironmentError:
+        # don't immediately give up
+        client = None
+    # if needed, fall back to using a local client for testing
+    client = client or LocalClient()
+
     # Check to see if the workflow has been run manually.
     # If so, adjust the client SHA and diff URL to use the manually supplied inputs.
     manual_commit_ref = os.getenv('MANUAL_COMMIT_REF')