Ensure set_custom_completer is working. Fixes #11272

IPython/core/completer.py

@@ -626,6 +626,8 b' class Completer(Configurable):'
         else:
             self.global_namespace = global_namespace
 
+        self.custom_matchers = []
+
         super(Completer, self).__init__(**kwargs)
 
     def complete(self, text, state):
@@ -1122,12 +1124,14 b' class IPCompleter(Completer):'
 
         if self.use_jedi:
             return [
+                *self.custom_matchers,
                 self.file_matches,
                 self.magic_matches,
                 self.dict_key_matches,
             ]
         else:
             return [
+                *self.custom_matchers,
                 self.python_matches,
                 self.file_matches,
                 self.magic_matches,

IPython/core/interactiveshell.py

@@ -2218,7 +2218,7 b' class InteractiveShell(SingletonConfigurable):'
         list where you want the completer to be inserted."""
 
         newcomp = types.MethodType(completer,self.Completer)
-        self.Completer.matchers.insert(pos,newcomp)
+        self.Completer.custom_matchers.insert(pos,newcomp)
 
     def set_completer_frame(self, frame=None):
         """Set the frame of the completer."""

IPython/core/tests/test_interactiveshell.py

@@ -998,3 +998,21 b' def test_should_run_async():'
     assert not ip.should_run_async("a = 5")
     assert ip.should_run_async("await x")
     assert ip.should_run_async("import asyncio; await asyncio.sleep(1)")
+
+
+def test_set_custom_completer():
+    num_completers = len(ip.Completer.matchers)
+
+    def foo(*args, **kwargs):
+        return "I'm a completer!"
+
+    ip.set_custom_completer(foo, 0)
+
+    # check that we've really added a new completer
+    assert len(ip.Completer.matchers) == num_completers + 1
+
+    # check that the first completer is the function we defined
+    assert ip.Completer.matchers[0]() == "I'm a completer!"
+
+    # clean up
+    ip.Completer.custom_matchers.pop()