prepend_sys_path does not work with absolute paths on windows

[mwerezak]

Describe the bug

If you put an absolute path in prepend_sys_path in your alembic.ini on windows (which will happen if you use %(here)s), the path will get split on the ':' character that separates the drive letter.

Expected behavior

Absolute paths to not get broken.

To Reproduce
Use an absolute path in prepend_sys_path like C:/something or %(here)s/something

Then print out sys.path, e.g. for path in sys.path: print(path) you get:

C
\something
C
\path\to\here\something

# Insert code here

Error
ModuleNotFoundError: No module named ...
because env.py can't import my modules.

Versions.

• OS: Windows 10
• Python: 3.11.5
• Alembic: 1.12.0
• SQLAlchemy: 1.4.49
• Database: Postgres
• DBAPI: psycopg2

Additional context

alembic/alembic/script/base.py

Lines 212 to 218 in 3e48ed0

	prepend_sys_path = config.get_main_option("prepend_sys_path")
	if prepend_sys_path:
	sys.path[:0] = list(
	_split_on_space_comma_colon.split(prepend_sys_path)
	)

[zzzeek]

@CaselIT is this also #1284 ?

[zzzeek]

PRs with tests welcome here

[CaselIT]

@CaselIT is this also #1284 ?

it should not, since that depend on the value of version_path_separator

[zzzeek]

Proposed in #1331 we add a new config value with legacy fallback behavior as follows:

# sys.path path, will be prepended to sys.path if present.
# defaults to the current working directory.  for multiple paths, the path separator
# is defined by prepend_sys_path_separator
prepend_sys_path = .

# prepend sys path separator; As mentioned above, this is the character used to split
# sys_path paths.  The default within new alembic.ini files is "os", which uses os.pathsep.
# If this key is omitted entirely, it falls back to the legacy behavior of splitting on spaces and/or commas.
# Valid values for prepend_sys_path_separator are:
#
# prepend_sys_path_separator = :
# prepend_sys_path_separator = ;
# prepend_sys_path_separator = space
prepend_sys_path_separator = os  # Use os.pathsep. Default configuration used for new projects.

[CaselIT]

it's probably the most sensible option. it makes little sense to have the two paths in the config handled in a different way

[zzzeek]

I guess if we were on pyproject.toml this whole thing would be a non issue since it supports lists :)

[CaselIT]

does not support %(here)s though