Extract the validation of a sha within the repo to a common function

cherry_picker/cherry_picker/cherry_picker.py

@@ -74,7 +74,7 @@ def upstream(self):
 
     @property
     def sorted_branches(self):
-        """Return the branches to cherry-pick to, sorted by version"""
+        """Return the branches to cherry-pick to, sorted by version."""
         return sorted(
             self.branches,
             reverse=True,
@@ -349,12 +349,15 @@ def continue_cherry_pick(self):
             click.echo(f"Current branch ({cherry_pick_branch}) is not a backport branch.  Will not continue. \U0001F61B")
 
     def check_repo(self):
-        # CPython repo has a commit with
-        # SHA=7f777ed95a19224294949e1b4ce56bbffcb1fe9f
-        cmd = ['git', 'log', '-r', self.config['check_sha']]
+        """
+        Check that the repository is for the project we're configured to operate on.
+
+        This function performs the check by making sure that the sha specified in the config
+        is present in the repository that we're operating on.
+        """
         try:
-            subprocess.check_output(cmd, stderr=subprocess.STDOUT)
-        except subprocess.SubprocessError:
+            validate_sha(self.config['check_sha'])
+        except ValueError:
             raise InvalidRepoException()
 
 
@@ -416,6 +419,9 @@ def cherry_pick_cli(dry_run, pr_remote, abort, status, push, config_path,
 def get_base_branch(cherry_pick_branch):
     """
     return '2.7' from 'backport-sha-2.7'
+
+    raises ValueError if the specified branch name is not of a form that
+        cherry_picker would have created
     """
     prefix, sha, base_branch = cherry_pick_branch.split('-', 2)
 
@@ -425,19 +431,30 @@ def get_base_branch(cherry_pick_branch):
     if not re.match('[0-9a-f]{7,40}', sha):
         raise ValueError(f'branch name has an invalid sha: {sha}')
 
-    cmd = ['git', 'log', '-r', sha]
-    try:
-        subprocess.check_output(cmd, stderr=subprocess.STDOUT)
-    except subprocess.SubprocessError:
-        raise ValueError(f'The sha listed in the branch name, {sha}, is not present in the repository')
+    # Validate that the sha refers to a valid commit within the repo
+    # Throws a ValueError if the sha is not present in the repo
+    validate_sha(sha)
 
     # Subject the parsed base_branch to the same tests as when we generated it
-    # This throws a ValueError if the base_branch doesn't need our requirements
+    # This throws a ValueError if the base_branch doesn't meet our requirements
     version_from_branch(base_branch)
 
     return base_branch
 
 
+def validate_sha(sha):
+    """
+    Validate that a hexdigest sha is a valid commit in the repo
+
+    raises ValueError if the sha does not reference a commit within the repo
+    """
+    cmd = ['git', 'log', '-r', sha]
+    try:
+        subprocess.check_output(cmd, stderr=subprocess.STDOUT)
+    except subprocess.SubprocessError:
+        raise ValueError(f'The sha listed in the branch name, {sha}, is not present in the repository')
+
+
 def version_from_branch(branch):
     """
     return version information from a git branch name