Revendor editor and make dateutil optional

Re-implemented the ``python-editor`` dependency as a small internal
function to avoid the need for external dependencies.
The implementation is based on the original
version in 7b91b325ff43a0e9235e0f15b57391fa92352626.

Make the ``python-dateutil`` library an optional dependency.
This library is only required if the ``timezone`` option
is used in the Alembic configuration.
An extra require named ``tz`` is available with
``pip install alembic[tz]`` to install it.

Fixes: #674
Fixes: #856
Change-Id: I07f17b2fea01e3a3d677ce95333fe3e8d8d438fd

1 files changed, 10 insertions, 2 deletions

diff --git a/alembic/script/base.py b/alembic/script/base.py
index 3f61864..723c011 100644
--- a/alembic/script/base.py
+++ b/alembic/script/base.py
@@ -5,14 +5,17 @@ import re
 import shutil
 import sys
 
-from dateutil import tz
-
 from . import revision
 from . import write_hooks
 from .. import util
 from ..runtime import migration
 from ..util import compat
 
+try:
+    from dateutil import tz
+except ImportError:
+    tz = None  # noqa
+
 _sourceless_rev_file = re.compile(r"(?!\.\#|__init__)(.*\.py)(c|o)?$")
 _only_source_rev_file = re.compile(r"(?!\.\#|__init__)(.*\.py)$")
 _legacy_rev = re.compile(r"([a-f0-9]+)\.py$")
@@ -515,6 +518,11 @@ class ScriptDirectory(object):
 
     def _generate_create_date(self):
         if self.timezone is not None:
+            if tz is None:
+                raise util.CommandError(
+                    "The library 'python-dateutil' is required "
+                    "for timezone support"
+                )
             # First, assume correct capitalization
             tzinfo = tz.gettz(self.timezone)
             if tzinfo is None: