feat: make dask optional so osw imports without it (#219)

- move dask into a new `parallel` extra and import it lazily. util.py is
  imported unconditionally by core, wtsite and wiki_tools, so a missing dask
  broke `import osw` outright - which ruled out Pyodide, where there is no
  dask build.

Author: simontaurus

pyproject.toml

@@ -47,7 +47,6 @@ dependencies = [
     # used in custom_types.py on py3.10; was missing before (latent
     # ImportError on 3.10), surfaced by ty/deptry during the migration
     'backports.strenum; python_version < "3.11"',
-    "dask",
     "tqdm",
     "pybars3-wheel",
     # the osw CLI (src/osw/cli); a base dependency, not an extra, so `pip
@@ -71,6 +70,10 @@ Download = "https://pypi.org/project/osw/#files"
 
 [project.optional-dependencies]
 wikitext = ["mwparserfromhell"]
+# Only osw.utils.util.async_parallelize needs it, and nothing in the package calls
+# that. Keeping it in the base dependencies made `import osw` impossible anywhere
+# dask is unavailable - notably Pyodide, which has no dask build.
+parallel = ["dask"]
 DB = [
     "psycopg2",
     "sqlalchemy",
@@ -106,7 +109,7 @@ workflow = [
     "anyio>=4.9,<4.14",
 ]
 tutorial = ["osw[dataimport]"]
-all = ["osw[dataimport,DB,UI,S3,wikitext,mcp]"]
+all = ["osw[dataimport,DB,UI,S3,wikitext,mcp,parallel]"]
 
 [project.scripts]
 # command-line access to a live OSL instance, built from the same

src/osw/utils/util.py

@@ -12,8 +12,6 @@
 from pathlib import Path
 from typing import IO, Callable, Dict, Iterable, List, Optional, Union
 
-import dask
-from dask.diagnostics import ProgressBar
 from tqdm.asyncio import tqdm
 
 _logger = logging.getLogger(__name__)
@@ -585,7 +583,21 @@ def file_in_paths(
 
 
 def async_parallelize(func: Callable, iterable: Iterable, **kwargs):
-    """Work in progress"""
+    """Work in progress. Requires the optional ``parallel`` extra.
+
+    dask is imported here rather than at module scope because this is the only
+    thing in osw that uses it. At module scope it made ``import osw`` fail
+    outright wherever dask is unavailable - util.py is imported unconditionally
+    by core, wtsite and wiki_tools - which rules out Pyodide, where there is no
+    dask build at all.
+    """
+    # ty cannot resolve these: dask is the `parallel` extra and is deliberately
+    # absent from the dev environment, which is the condition this import exists to
+    # tolerate. Adding the extra to the dev group would make it resolve - that is what
+    # pyproject does for mcp - but it would install dask everywhere to type check the
+    # one place that is allowed to run without it.
+    import dask  # ty: ignore[unresolved-import]
+    from dask.diagnostics import ProgressBar  # ty: ignore[unresolved-import]
 
     async def inner(func_, iterable_, **kwargs_):
         async with AsyncMessageBuffer() as msg_buf:
@@ -646,6 +658,11 @@ def wrapped_function(*args, **kwargs):
     from datetime import datetime
     from time import sleep
 
+    # Only the demo helpers below use dask; see async_parallelize for why it is
+    # not imported at module scope. Needs the optional `parallel` extra.
+    import dask  # ty: ignore[unresolved-import]
+    from dask.diagnostics import ProgressBar  # ty: ignore[unresolved-import]
+
     print("Redirecting print messages to a buffer.")
     message_buffer = MessageBuffer()
     io_buffer = StringIO()