Allow init into existing but empty directory

The "alembic init" command will now proceed if the target directory exists
as long as it's still empty.  Previously, it would not proceed if the
directory existed. The new behavior is modeled from what git does, to
accommodate for container or other deployments where an Alembic target
directory may need to be already mounted instead of being created with
alembic init.  Pull request courtesy Aviskar KC.

Fixes: #571
Closes: #574
Pull-request: https://github.com/sqlalchemy/alembic/pull/574
Pull-request-sha: ab7b8f342a89c752460d171cd50d1f958c7d59c9

Change-Id: I6b0cff9a7e940d9d878aa339891bda33744a2a4c

3 files changed, 76 insertions, 7 deletions

diff --git a/alembic/command.py b/alembic/command.py
index 3faafdd..c02db62 100644
--- a/alembic/command.py
+++ b/alembic/command.py
@@ -37,18 +37,21 @@ def init(config, directory, template="generic"):
 
     """
 
-    if os.access(directory, os.F_OK):
-        raise util.CommandError("Directory %s already exists" % directory)
+    if os.access(directory, os.F_OK) and os.listdir(directory):
+        raise util.CommandError(
+            "Directory %s already exists and is not empty" % directory
+        )
 
     template_dir = os.path.join(config.get_template_directory(), template)
     if not os.access(template_dir, os.F_OK):
         raise util.CommandError("No such template %r" % template)
 
-    util.status(
-        "Creating directory %s" % os.path.abspath(directory),
-        os.makedirs,
-        directory,
-    )
+    if not os.access(directory, os.F_OK):
+        util.status(
+            "Creating directory %s" % os.path.abspath(directory),
+            os.makedirs,
+            directory,
+        )
 
     versions = os.path.join(directory, "versions")
     util.status(
diff --git a/docs/build/unreleased/571.rst b/docs/build/unreleased/571.rst
new file mode 100644
index 0000000..82c7647
--- /dev/null
+++ b/docs/build/unreleased/571.rst
@@ -0,0 +1,12 @@
+.. change::
+    :tags: usecase, commands
+    :tickets: 571
+
+    The "alembic init" command will now proceed if the target directory exists
+    as long as it's still empty.  Previously, it would not proceed if the
+    directory existed. The new behavior is modeled from what git does, to
+    accommodate for container or other deployments where an Alembic target
+    directory may need to be already mounted instead of being created with
+    alembic init.  Pull request courtesy Aviskar KC.
+
+
diff --git a/tests/test_command.py b/tests/test_command.py
index 653028f..746471b 100644
--- a/tests/test_command.py
+++ b/tests/test_command.py
@@ -800,3 +800,57 @@ class CommandLineTest(TestBase):
                             # not too long
                             assert len(help_text) < 80
         assert not commands, "Commands without help text: %s" % commands
+
+    def test_init_file_exists_and_is_not_empty(self):
+        with mock.patch(
+            "alembic.command.os.listdir", return_value=["file1", "file2"]
+        ), mock.patch("alembic.command.os.access", return_value=True):
+            directory = "alembic"
+            assert_raises_message(
+                util.CommandError,
+                "Directory %s already exists and is not empty" % directory,
+                command.init,
+                self.cfg,
+                directory=directory,
+            )
+
+    def test_init_file_exists_and_is_empty(self):
+        def access_(path, mode):
+            if "generic" in path or path == "foobar":
+                return True
+            else:
+                return False
+
+        def listdir_(path):
+            if path == "foobar":
+                return []
+            else:
+                return ["file1", "file2", "alembic.ini.mako"]
+
+        with mock.patch(
+            "alembic.command.os.access", side_effect=access_
+        ), mock.patch("alembic.command.os.makedirs") as makedirs, mock.patch(
+            "alembic.command.os.listdir", side_effect=listdir_
+        ), mock.patch(
+            "alembic.command.ScriptDirectory"
+        ):
+            command.init(self.cfg, directory="foobar")
+            eq_(makedirs.mock_calls, [mock.call("foobar/versions")])
+
+    def test_init_file_doesnt_exist(self):
+        def access_(path, mode):
+            if "generic" in path:
+                return True
+            else:
+                return False
+
+        with mock.patch(
+            "alembic.command.os.access", side_effect=access_
+        ), mock.patch("alembic.command.os.makedirs") as makedirs, mock.patch(
+            "alembic.command.ScriptDirectory"
+        ):
+            command.init(self.cfg, directory="foobar")
+            eq_(
+                makedirs.mock_calls,
+                [mock.call("foobar"), mock.call("foobar/versions")],
+            )