From 6fcb6cfb139ade1aac6dbee0b18ca72b18cbe0d2 Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Thu, 22 Aug 2019 23:44:19 -0700 Subject: [PATCH] bpo-30826: Improve control flow examples (GH-15407) --- Doc/tutorial/controlflow.rst | 28 ++++++++++++++-------------- 1 file changed, 14 insertions(+), 14 deletions(-) diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst index 92c042e26d..b7e003c550 100644 --- a/Doc/tutorial/controlflow.rst +++ b/Doc/tutorial/controlflow.rst @@ -66,20 +66,20 @@ they appear in the sequence. For example (no pun intended): window 6 defenestrate 12 -If you need to modify the sequence you are iterating over while inside the loop -(for example to duplicate selected items), it is recommended that you first -make a copy. Iterating over a sequence does not implicitly make a copy. The -slice notation makes this especially convenient:: - - >>> for w in words[:]: # Loop over a slice copy of the entire list. - ... if len(w) > 6: - ... words.insert(0, w) - ... - >>> words - ['defenestrate', 'cat', 'window', 'defenestrate'] - -With ``for w in words:``, the example would attempt to create an infinite list, -inserting ``defenestrate`` over and over again. +Code that modifies a collection while iterating over that same collection can +be tricky to get right. Instead, it is usually more straight-forward to loop +over a copy of the collection or to create a new collection:: + + # Strategy: Iterate over a copy + for user, status in users.copy().items(): + if status == 'inactive': + del users[user] + + # Strategy: Create a new collection + active_users = {} + for user, status in users.items(): + if status == 'active': + active_users[user] = status .. _tut-range: -- 2.40.0