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| -rw-r--r-- | Doc/library/collections.rst | 13 | ||||
| -rw-r--r-- | Lib/collections/__init__.py | 4 |
2 files changed, 14 insertions, 3 deletions
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst index b8a717d883..b97bc425de 100644 --- a/Doc/library/collections.rst +++ b/Doc/library/collections.rst @@ -271,7 +271,7 @@ For example:: .. versionadded:: 3.1 .. versionchanged:: 3.7 As a :class:`dict` subclass, :class:`Counter` - Inherited the capability to remember insertion order. Math operations + inherited the capability to remember insertion order. Math operations on *Counter* objects also preserve order. Results are ordered according to when an element is first encountered in the left operand and then by the order encountered in the right operand. @@ -366,19 +366,26 @@ Several mathematical operations are provided for combining :class:`Counter` objects to produce multisets (counters that have counts greater than zero). Addition and subtraction combine counters by adding or subtracting the counts of corresponding elements. Intersection and union return the minimum and -maximum of corresponding counts. Each operation can accept inputs with signed +maximum of corresponding counts. Equality and inclusion compare +corresponding counts. Each operation can accept inputs with signed counts, but the output will exclude results with counts of zero or less. +.. doctest:: + >>> c = Counter(a=3, b=1) >>> d = Counter(a=1, b=2) >>> c + d # add two counters together: c[x] + d[x] Counter({'a': 4, 'b': 3}) >>> c - d # subtract (keeping only positive counts) Counter({'a': 2}) - >>> c & d # intersection: min(c[x], d[x]) # doctest: +SKIP + >>> c & d # intersection: min(c[x], d[x]) Counter({'a': 1, 'b': 1}) >>> c | d # union: max(c[x], d[x]) Counter({'a': 3, 'b': 2}) + >>> c == d # equality: c[x] == d[x] + False + >>> c <= d # inclusion: c[x] <= d[x] + False Unary addition and subtraction are shortcuts for adding an empty counter or subtracting from an empty counter. diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py index fa8b30985a..264435cb41 100644 --- a/Lib/collections/__init__.py +++ b/Lib/collections/__init__.py @@ -736,6 +736,10 @@ class Counter(dict): # To strip negative and zero counts, add-in an empty counter: # c += Counter() # + # Results are ordered according to when an element is first + # encountered in the left operand and then by the order + # encountered in the right operand. + # # When the multiplicities are all zero or one, multiset operations # are guaranteed to be equivalent to the corresponding operations # for regular sets. |