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| -rw-r--r-- | Doc/ref/ref5.tex | 21 |
1 files changed, 12 insertions, 9 deletions
diff --git a/Doc/ref/ref5.tex b/Doc/ref/ref5.tex index 5002f4d..be672b7 100644 --- a/Doc/ref/ref5.tex +++ b/Doc/ref/ref5.tex @@ -191,10 +191,10 @@ When a comma-separated list of expressions is supplied, its elements are evaluated from left to right and placed into the list object in that order. When a list comprehension is supplied, it consists of a single expression followed by at least one \keyword{for} clause and zero or -more \keyword{for} or \keyword{if} clauses. In this +more \keyword{for} or \keyword{if} clauses. In this case, the elements of the new list are those that would be produced by considering each of the \keyword{for} or \keyword{if} clauses a block, -nesting from +nesting from left to right, and evaluating the expression to produce a list element each time the innermost block is reached. \obindex{list} @@ -815,13 +815,16 @@ Tuples and lists are compared lexicographically using comparison of corresponding items. \item -Mappings (dictionaries) are compared through lexicographic -comparison of their sorted (key, value) lists.\footnote{ -This is expensive since it requires sorting the keys first, -but it is about the only sensible definition. An earlier version of -Python compared dictionaries by identity only, but this caused -surprises because people expected to be able to test a dictionary for -emptiness by comparing it to \code{\{\}}.} +Mappings (dictionaries) compare equal if and only if their sorted +(key, value) lists compare equal.\footnote{The implementation computes + this efficiently, without constructing lists or sorting.} +Outcomes other than equality are resolved consistently, but are not +otherwise defined.\footnote{Earlier versions of Python used\ + lexicographic comparison of the sorted (key, value) lists, but this + was very expensive for the common case of comparing for equality. An + even earlier version of Python compared dictionaries by identity only, + but this caused surprises because people expected to be able to test + a dictionary for emptiness by comparing it to \code{\{\}}.} \item Most other types compare unequal unless they are the same object; |