Backport PR 10743 on branch 5.x

Merge pull request  #10743 from takluyver/limit-no-completions

Limit number of completions returned

.gitignore

@@ -5,6 +5,7 b' _build'
 docs/man/*.gz
 docs/source/api/generated
 docs/source/config/options
+docs/source/config/shortcuts/*.csv
 docs/source/interactive/magics-generated.txt
 docs/gh-pages
 jupyter_notebook/notebook/static/mathjax

IPython/core/completer.py

@@ -47,6 +47,9 b" if sys.platform == 'win32':"
 else:
     PROTECTABLES = ' ()[]{}?=\\|;:\'#*"^&'
 
+# Protect against returning an enormous number of completions which the frontend
+# may have trouble processing.
+MATCHES_LIMIT = 500
 
 def has_open_quotes(s):
     """Return whether a string has open quotes.
@@ -1142,7 +1145,7 b' class IPCompleter(Completer):'
             for meth in (self.unicode_name_matches, back_latex_name_matches, back_unicode_name_matches):
                 name_text, name_matches = meth(base_text)
                 if name_text:
-                    return name_text, name_matches
+                    return name_text, name_matches[:MATCHES_LIMIT]
         
         # if text is either None or an empty string, rely on the line buffer
         if not text:
@@ -1183,6 +1186,6 b' class IPCompleter(Completer):'
         # different types of objects.  The rlcomplete() method could then
         # simply collapse the dict into a list for readline, but we'd have
         # richer completion semantics in other evironments.
-        self.matches = sorted(set(self.matches), key=completions_sorting_key)
+        self.matches = sorted(set(self.matches), key=completions_sorting_key)[:MATCHES_LIMIT]
 
         return text, self.matches