Make lazy_load.load thread-safe

Closes #88

Author: stefanv

lazy_loader/__init__.py

@@ -10,12 +10,16 @@
 import inspect
 import os
 import sys
+import threading
 import types
 import warnings
 
 __all__ = ["attach", "load", "attach_stub"]
 
 
+threadlock = threading.Lock()
+
+
 def attach(package_name, submodules=None, submod_attrs=None):
     """Attach lazily loaded submodules, functions, or other attributes.
 
@@ -171,42 +175,45 @@ def myfunc():
         Actual loading of the module occurs upon first attribute request.
 
     """
-    try:
-        return sys.modules[fullname]
-    except KeyError:
-        pass
-
-    if "." in fullname:
-        msg = (
-            "subpackages can technically be lazily loaded, but it causes the "
-            "package to be eagerly loaded even if it is already lazily loaded."
-            "So, you probably shouldn't use subpackages with this lazy feature."
-        )
-        warnings.warn(msg, RuntimeWarning)
-
-    spec = importlib.util.find_spec(fullname)
-    if spec is None:
-        if error_on_import:
-            raise ModuleNotFoundError(f"No module named '{fullname}'")
-        else:
-            try:
-                parent = inspect.stack()[1]
-                frame_data = {
-                    "spec": fullname,
-                    "filename": parent.filename,
-                    "lineno": parent.lineno,
-                    "function": parent.function,
-                    "code_context": parent.code_context,
-                }
-                return DelayedImportErrorModule(frame_data, "DelayedImportErrorModule")
-            finally:
-                del parent
-
-    module = importlib.util.module_from_spec(spec)
-    sys.modules[fullname] = module
-
-    loader = importlib.util.LazyLoader(spec.loader)
-    loader.exec_module(module)
+    with threadlock:
+        try:
+            return sys.modules[fullname]
+        except KeyError:
+            pass
+
+        if "." in fullname:
+            msg = (
+                "subpackages can technically be lazily loaded, but it causes the "
+                "package to be eagerly loaded even if it is already lazily loaded."
+                "So, you probably shouldn't use subpackages with this lazy feature."
+            )
+            warnings.warn(msg, RuntimeWarning)
+
+        spec = importlib.util.find_spec(fullname)
+        if spec is None:
+            if error_on_import:
+                raise ModuleNotFoundError(f"No module named '{fullname}'")
+            else:
+                try:
+                    parent = inspect.stack()[1]
+                    frame_data = {
+                        "spec": fullname,
+                        "filename": parent.filename,
+                        "lineno": parent.lineno,
+                        "function": parent.function,
+                        "code_context": parent.code_context,
+                    }
+                    return DelayedImportErrorModule(
+                        frame_data, "DelayedImportErrorModule"
+                    )
+                finally:
+                    del parent
+
+        module = importlib.util.module_from_spec(spec)
+        sys.modules[fullname] = module
+
+        loader = importlib.util.LazyLoader(spec.loader)
+        loader.exec_module(module)
 
     return module
 

lazy_loader/tests/import_np_parallel.py

@@ -0,0 +1,13 @@
+import threading
+import time
+
+import lazy_loader as lazy
+
+
+def import_np():
+    time.sleep(0.5)
+    lazy.load("numpy")
+
+
+for _ in range(10):
+    threading.Thread(target=import_np).start()

lazy_loader/tests/test_lazy_loader.py

@@ -1,4 +1,6 @@
 import importlib
+import os
+import subprocess
 import sys
 import types
 
@@ -149,3 +151,14 @@ def test_stub_loading_errors(tmp_path):
 
     with pytest.raises(ValueError, match="Cannot load imports from non-existent stub"):
         lazy.attach_stub("name", "not a file")
+
+
+def test_parallel_load():
+    pytest.importorskip("numpy")
+
+    subprocess.run(
+        [
+            sys.executable,
+            os.path.join(os.path.dirname(__file__), "import_np_parallel.py"),
+        ]
+    )