Only load the IPython and Jupyter integration modules when Dash is im…

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - [#3646](https://github.com/plotly/dash/pull/3646) Remove React 16 support (`16.14.0` is no longer an accepted value for `REACT_VERSION` / `_set_react_version`).
 
 ### Changed
+- [#4037](https://github.com/plotly/dash/pull/4037) Only load the IPython and Jupyter integration modules when Dash is imported inside IPython. Roughly halves `import dash` time in environments that have IPython installed.
 - [#3987](https://github.com/plotly/dash/pull/3987) Forward FastAPI reload scope options (`reload_dirs`, `reload_excludes`, and `reload_includes`) to Uvicorn when reloading.
 - [#3986](https://github.com/plotly/dash/pull/3986) Adjust `_run_before_hooks` in the `fastapi` backend to honor a response returned by a `before_request` function, matching the `flask` backend's behavior.
 

dash/_jupyter.py

@@ -15,6 +15,9 @@
 from werkzeug.serving import make_server
 
 try:
+    if "IPython" not in sys.modules:
+        # Not inside IPython: skip the heavy imports, they double the cost of `import dash`.
+        raise ImportError
     from IPython import get_ipython  # type: ignore[attr-defined]
     from IPython.display import IFrame, display, Javascript  # type: ignore[import-not-found]
     from IPython.core.display import HTML  # type: ignore[import-not-found]

tests/unit/test_jupyter_import.py

@@ -0,0 +1,27 @@
+import subprocess
+import sys
+
+import pytest
+
+pytest.importorskip("IPython")
+
+
+def _run(*args):
+    result = subprocess.run(
+        [sys.executable, *args], capture_output=True, text=True, check=False
+    )
+    assert result.returncode == 0, result.stderr
+
+
+def test_import_dash_does_not_load_ipython():
+    _run("-c", "import dash, sys; assert 'IPython' not in sys.modules")
+
+
+def test_jupyter_support_loads_inside_ipython():
+    _run(
+        "-m",
+        "IPython",
+        "-c",
+        "from dash import _jupyter; "
+        "assert _jupyter._dep_installed and _jupyter.jupyter_dash.in_ipython",
+    )