Render signatures ourselves if they’re too long

Fixes  #11504

IPython/core/oinspect.py

@@ -362,7 +362,7 b' class Inspector(Colorable):'
         If any exception is generated, None is returned instead and the
         exception is suppressed."""
         try:
-            hdef = oname + str(signature(obj))
+            hdef = render_signature(signature(obj), oname)
             return cast_unicode(hdef)
         except:
             return None
@@ -1016,3 +1016,47 b' class Inspector(Colorable):'
             search_result.update(tmp_res)
 
         page.page('\n'.join(sorted(search_result)))
+
+
+def render_signature(obj, oname):
+    """
+    This was mostly taken from inspect.Signature.__str__.
+    Look there for the comments.
+    The only change is to add linebreaks when this gets too long.
+    """
+    if not isinstance(obj, inspect.Signature):
+        return oname + str(obj)
+
+    result = []
+    pos_only = False
+    kw_only = True
+    for param in self.parameters.values():
+        if param.kind == _POSITIONAL_ONLY:
+            pos_only = True
+        elif pos_only:
+            result.append('/')
+            pos_only = False
+
+        if param.kind == _VAR_POSITIONAL:
+            kw_only = False
+        elif param.kind == _KEYWORD_ONLY and kw_only:
+            result.append('*')
+            kw_only = False
+
+        result.append(str(param))
+
+    if pos_only:
+        result.append('/')
+
+    # add up name, parameters, braces (2), and commas
+    if len(oname) + sum(len(r) + 2 for r in result) > 75:
+        # This doesn’t fit behind “Signature: ” in an inspect window.
+        rendered = '(\n{})'.format(''.join('    {},\n'.format(result)))
+    else:
+        rendered = '({})'.format(', '.join(result))
+
+    if self.return_annotation is not _empty:
+        anno = formatannotation(self.return_annotation)
+        rendered += ' -> {}'.format(anno)
+
+    return rendered