Apparently you can override what import returns

By replacing the entry in sys.modules.  pytest does this.

src/core/types.h

@@ -612,7 +612,7 @@ class BoxedClass;
 // TODO these shouldn't be here
 void setupRuntime();
 void teardownRuntime();
-BoxedModule* createAndRunModule(const std::string& name, const std::string& fn);
+Box* createAndRunModule(const std::string& name, const std::string& fn);
 BoxedModule* createModule(const std::string& name, const std::string& fn, const char* doc = NULL);
 
 // TODO where to put this

src/runtime/import.cpp

@@ -39,7 +39,7 @@ static void removeModule(const std::string& name) {
     d->d.erase(b_name);
 }
 
-BoxedModule* createAndRunModule(const std::string& name, const std::string& fn) {
+Box* createAndRunModule(const std::string& name, const std::string& fn) {
     BoxedModule* module = createModule(name, fn);
 
     AST_Module* ast = caching_parse_file(fn.c_str());
@@ -49,10 +49,14 @@ BoxedModule* createAndRunModule(const std::string& name, const std::string& fn)
         removeModule(name);
         raiseRaw(e);
     }
-    return module;
+
+    Box* r = getSysModulesDict()->getOrNull(boxString(name));
+    if (!r)
+        raiseExcHelper(ImportError, "Loaded module %.200s not found in sys.modules", name.c_str());
+    return r;
 }
 
-static BoxedModule* createAndRunModule(const std::string& name, const std::string& fn, const std::string& module_path) {
+static Box* createAndRunModule(const std::string& name, const std::string& fn, const std::string& module_path) {
     BoxedModule* module = createModule(name, fn);
 
     Box* b_path = boxStringPtr(&module_path);
@@ -69,7 +73,11 @@ static BoxedModule* createAndRunModule(const std::string& name, const std::strin
         removeModule(name);
         raiseRaw(e);
     }
-    return module;
+
+    Box* r = getSysModulesDict()->getOrNull(boxString(name));
+    if (!r)
+        raiseExcHelper(ImportError, "Loaded module %.200s not found in sys.modules", name.c_str());
+    return r;
 }
 
 #if LLVMREV < 210072

src/runtime/types.cpp

@@ -2589,18 +2589,16 @@ BoxedModule* createModule(const std::string& name, const std::string& fn, const
 
     // Surprisingly, there are times that we need to return the existing module if
     // one exists:
-    Box*& ptr = d->d[b_name];
-    if (ptr && isSubclass(ptr->cls, module_cls)) {
-        return static_cast<BoxedModule*>(ptr);
-    } else {
-        ptr = NULL;
+    Box* existing = d->getOrNull(b_name);
+    if (existing && isSubclass(existing->cls, module_cls)) {
+        return static_cast<BoxedModule*>(existing);
     }
 
     BoxedModule* module = new BoxedModule();
     moduleInit(module, boxString(name), boxString(doc ? doc : ""));
     module->giveAttr("__file__", boxString(fn));
 
-    ptr = module;
+    d->d[b_name] = module;
     return module;
 }
 

test/tests/sys_modules_replacement_target.py

+# skip-if: True
+
+import os
+import sys
+sys.modules[__name__] = os

test/tests/sys_modules_replacement_test.py

+import sys_modules_replacement_target
+
+print hasattr(sys_modules_replacement_target, "path")