Avoid importing IPython if notebook cells do not contain magics

CHANGES.md

@@ -30,6 +30,8 @@
 
 <!-- Changes that improve Black's performance. -->
 
+- minimize cost of introspection to detect jupyter dependencies (#3782)
+
 ### Output
 
 <!-- Changes to Black's terminal output and error messages -->

src/black/handle_ipynb_magics.py

@@ -6,6 +6,7 @@
 import secrets
 import sys
 from functools import lru_cache
+from importlib.util import find_spec
 from typing import Dict, List, Optional, Tuple
 
 if sys.version_info >= (3, 10):
@@ -57,24 +58,14 @@ class Replacement:
 
 @lru_cache
 def jupyter_dependencies_are_installed(*, verbose: bool, quiet: bool) -> bool:
-    try:
-        # isort: off
-        # tokenize_rt is less commonly installed than IPython
-        # and IPython is expensive to import
-        import tokenize_rt  # noqa:F401
-        import IPython  # noqa:F401
-
-        # isort: on
-    except ModuleNotFoundError:
-        if verbose or not quiet:
-            msg = (
-                "Skipping .ipynb files as Jupyter dependencies are not installed.\n"
-                'You can fix this by running ``pip install "black[jupyter]"``'
-            )
-            out(msg)
-        return False
-    else:
-        return True
+    retv = find_spec("tokenize_rt") is not None and find_spec("IPython") is not None
+    if not retv and (verbose or not quiet):
+        msg = (
+            "Skipping .ipynb files as Jupyter dependencies are not installed.\n"
+            'You can fix this by running ``pip install "black[jupyter]"``'
+        )
+        out(msg)
+    return retv
 
 
 def remove_trailing_semicolon(src: str) -> Tuple[str, bool]: