From 9265dd72e5ec1cfa5fcdb5be8ebffe1d9994bd4b Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Sun, 8 Apr 2018 08:44:20 -0700
Subject: [PATCH] Add a prepend() recipe to teach a chain() idiom (GH-6415)

---
 Doc/library/itertools.rst  | 5 +++++
 Lib/test/test_itertools.py | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index a5a5356a9a1..959424ff914 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -688,6 +688,11 @@ which incur interpreter overhead.
        "Return first n items of the iterable as a list"
        return list(islice(iterable, n))
 
+   def prepend(value, iterator):
+       "Prepend a single value in front of an iterator"
+       # prepend(1, [2, 3, 4]) -> 1 2 3 4
+       return chain([value], iterator)
+
    def tabulate(function, start=0):
        "Return function(0), function(1), ..."
        return map(function, count(start))
diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index effd7f0e21b..cbbb4c4f71d 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -2198,6 +2198,11 @@ Samuele
 ...     "Return first n items of the iterable as a list"
 ...     return list(islice(iterable, n))
 
+>>> def prepend(value, iterator):
+...     "Prepend a single value in front of an iterator"
+...     # prepend(1, [2, 3, 4]) -> 1 2 3 4
+...     return chain([value], iterator)
+
 >>> def enumerate(iterable, start=0):
 ...     return zip(count(start), iterable)
 
@@ -2350,6 +2355,9 @@ perform as purported.
 >>> take(10, count())
 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
+>>> list(prepend(1, [2, 3, 4]))
+[1, 2, 3, 4]
+
 >>> list(enumerate('abc'))
 [(0, 'a'), (1, 'b'), (2, 'c')]