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

3 files changed, 72 insertions, 13 deletions

diff --git a/alembic/util/__init__.py b/alembic/util/__init__.py
index cfdab49..2a3d8a8 100644
--- a/alembic/util/__init__.py
+++ b/alembic/util/__init__.py
@@ -1,4 +1,5 @@
 from .compat import raise_  # noqa
+from .editor import open_in_editor  # noqa
 from .exc import CommandError
 from .langhelpers import _with_legacy_names  # noqa
 from .langhelpers import asbool  # noqa
@@ -19,7 +20,6 @@ from .messaging import status  # noqa
 from .messaging import warn  # noqa
 from .messaging import write_outstream  # noqa
 from .pyfiles import coerce_resource_to_filename  # noqa
-from .pyfiles import edit  # noqa
 from .pyfiles import load_python_file  # noqa
 from .pyfiles import pyc_file_from_path  # noqa
 from .pyfiles import template_to_file  # noqa
diff --git a/alembic/util/editor.py b/alembic/util/editor.py
new file mode 100644
index 0000000..c27f0f3
--- /dev/null
+++ b/alembic/util/editor.py
@@ -0,0 +1,71 @@
+import os
+from os.path import exists
+from os.path import join
+from os.path import splitext
+from subprocess import check_call
+
+from .compat import is_posix
+from .exc import CommandError
+
+
+def open_in_editor(filename, environ=None):
+    """
+    Opens the given file in a text editor. If the environment variable
+    ``EDITOR`` is set, this is taken as preference.
+
+    Otherwise, a list of commonly installed editors is tried.
+
+    If no editor matches, an :py:exc:`OSError` is raised.
+
+    :param filename: The filename to open. Will be passed  verbatim to the
+        editor command.
+    :param environ: An optional drop-in replacement for ``os.environ``. Used
+        mainly for testing.
+    """
+
+    try:
+        editor = _find_editor(environ)
+        check_call([editor, filename])
+    except Exception as exc:
+        raise CommandError("Error executing editor (%s)" % (exc,)) from exc
+
+
+def _find_editor(environ=None):
+    candidates = _default_editors()
+    for i, var in enumerate(("EDITOR", "VISUAL")):
+        if var in environ:
+            user_choice = environ[var]
+            if exists(user_choice):
+                return user_choice
+            if os.sep not in user_choice:
+                candidates.insert(i, user_choice)
+
+    for candidate in candidates:
+        path = _find_executable(candidate, environ)
+        if path is not None:
+            return path
+    raise OSError(
+        "No suitable editor found. Please set the "
+        '"EDITOR" or "VISUAL" environment variables'
+    )
+
+
+def _find_executable(candidate, environ):
+    # Assuming this is on the PATH, we need to determine it's absolute
+    # location. Otherwise, ``check_call`` will fail
+    if not is_posix and splitext(candidate)[1] != ".exe":
+        candidate += ".exe"
+    for path in environ.get("PATH", "").split(os.pathsep):
+        value = join(path, candidate)
+        if exists(value):
+            return value
+    return None
+
+
+def _default_editors():
+    # Look for an editor. Prefer the user's choice by env-var, fall back to
+    # most commonly installed editor (nano/vim)
+    if is_posix:
+        return ["sensible-editor", "editor", "nano", "vim", "code"]
+    else:
+        return ["code.exe", "notepad++.exe", "notepad.exe"]
diff --git a/alembic/util/pyfiles.py b/alembic/util/pyfiles.py
index 8dfb9db..1bc5be3 100644
--- a/alembic/util/pyfiles.py
+++ b/alembic/util/pyfiles.py
@@ -10,7 +10,6 @@ from .compat import has_pep3147
 from .compat import load_module_py
 from .compat import load_module_pyc
 from .compat import py3k
-from .compat import raise_
 from .exc import CommandError
 
 
@@ -75,17 +74,6 @@ def pyc_file_from_path(path):
         return None
 
 
-def edit(path):
-    """Given a source path, run the EDITOR for it"""
-
-    import editor
-
-    try:
-        editor.edit(path)
-    except Exception as exc:
-        raise_(CommandError("Error executing editor (%s)" % (exc,)), from_=exc)
-
-
 def load_python_file(dir_, filename):
     """Load a file from the given path as a Python module."""