337 lines
11 KiB
Plaintext
337 lines
11 KiB
Plaintext
PEP: 3106
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Title: Revamping dict.keys(), .values() and .items()
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Version: $Revision$
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Last-Modified: $Date$
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Author: Guido van Rossum
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Status: Accepted
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Type: Standards
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Content-Type: text/x-rst
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Created: 19-Dec-2006
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Post-History:
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Abstract
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========
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This PEP proposes to change the .keys(), .values() and .items()
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methods of the built-in dict type to return a set-like or unordered
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container object whose contents are derived of the underlying
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dictionary rather than a list which is a copy of the keys, etc.; and
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to remove the .iterkeys(), .itervalues() and .iteritems() methods.
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The approach is inspired by that taken in the Java Collections
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Framework [1]_.
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Introduction
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============
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It has long been the plan to change the .keys(), .values() and
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.items() methods of the built-in dict type to return a more
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lightweight object than a list, and to get rid of .iterkeys(),
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.itervalues() and .iteritems(). The idea is that code that currently
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(in 2.x) reads::
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for k, v in d.iteritems(): ...
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should be rewritten as::
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for k, v in d.items(): ...
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(and similar for .itervalues() and .iterkeys(), except the latter is
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redundant since we can write that loop as ``for k in d``.)
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Code that currently reads::
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a = d.keys() # assume we really want a list here
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(etc.) should be rewritten as
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a = list(d.keys())
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There are (at least) two ways to accomplish this. The original plan
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was to simply let .keys(), .values() and .items() return an iterator,
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i.e. exactly what iterkeys(), itervalues() and iteritems() return in
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Python 2.x. However, the Java Collections Framework [1]_ suggests
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that a better solution is possible: the methods return objects with
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set behavior (for .keys() and .items()) or multiset (== bag) behavior
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(for .values()) that do not contain copies of the keys, values or
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items, but rather reference the underlying dict and pull their values
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out of the dict as needed.
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The advantage of this approach is that one can still write code like
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this::
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a = d.items()
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for k, v in a: ...
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for k, v in a: ...
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Effectively, iter(d.keys()) (etc.) in Python 3.0 will do what
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d.iterkeys() (etc.) does in Python 2.x; but in most contexts we don't
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have to write the iter() call because it is implied by a for-loop.
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The objects returned by the .keys() and .items() methods behave like
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sets. The object returned by the values() method behaves like a much
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simpler unordered collection; anything more would require too much
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implementation effort for the rare use case.
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Because of the set behavior, it will be possible to check whether two
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dicts have the same keys by simply testing::
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if a.keys() == b.keys(): ...
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and similarly for .items().
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These operations are thread-safe only to the extent that using them in
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a thread-unsafe way may cause an exception but will not cause
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corruption of the internal representation.
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As in Python 2.x, mutating a dict while iterating over it using an
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iterator has an undefined effect and will in most cases raise a
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RuntimeError exception. (This is similar to the guarantees made by
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the Java Collections Framework.)
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The objects returned by .keys() and .items() are fully interoperable
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with instances of the built-in set and frozenset types; for example::
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set(d.keys()) == d.keys()
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is guaranteed to be True (except when d is being modified
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simultaneously by another thread).
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Specification
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=============
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I'm using pseudo-code to specify the semantics::
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class dict:
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# Omitting all other dict methods for brevity.
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# The .iterkeys(), .itervalues() and .iteritems() methods
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# will be removed.
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def keys(self):
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return d_keys(self)
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def items(self):
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return d_items(self)
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def values(self):
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return d_values(self)
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class d_keys:
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def __init__(self, d):
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self.__d = d
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def __len__(self):
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return len(self.__d)
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def __contains__(self, key):
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return key in self.__d
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def __iter__(self):
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for key in self.__d:
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yield key
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# The following operations should be implemented to be
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# compatible with sets; this can be done by exploiting
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# the above primitive operations:
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#
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# <, <=, ==, !=, >=, > (returning a bool)
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# &, |, ^, - (returning a new, real set object)
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#
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# as well as their method counterparts (.union(), etc.).
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#
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# To specify the semantics, we can specify x == y as:
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#
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# set(x) == set(y) if both x and y are d_keys instances
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# set(x) == y if x is a d_keys instance
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# x == set(y) if y is a d_keys instance
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#
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# and so on for all other operations.
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class d_items:
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def __init__(self, d):
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self.__d = d
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def __len__(self):
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return len(self.__d)
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def __contains__(self, (key, value)):
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return key in self.__d and self.__d[key] == value
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def __iter__(self):
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for key in self.__d:
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yield key, self.__d[key]
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# As well as the set operations mentioned for d_keys above.
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# However the specifications suggested there will not work if
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# the values aren't hashable. Fortunately, the operations can
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# still be implemented efficiently. For example, this is how
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# intersection can be specified:
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def __and__(self, other):
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if isinstance(other, (set, frozenset, d_keys)):
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result = set()
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for item in other:
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if item in self:
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result.add(item)
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return result
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if not isinstance(other, d_items):
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return NotImplemented
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d = {}
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if len(other) < len(self):
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self, other = other, self
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for item in self:
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if item in other:
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key, value = item
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d[key] = value
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return d.items()
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# And here is equality:
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def __eq__(self, other):
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if isinstance(other, (set, frozenset, d_keys)):
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if len(self) != len(other):
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return False
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for item in other:
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if item not in self:
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return False
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return True
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if not isinstance(other, d_items):
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return NotImplemented
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# XXX We could also just compare the underlying dicts...
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if len(self) != len(other):
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return False
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for item in self:
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if item not in other:
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return False
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return True
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def __ne__(self, other):
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# XXX Perhaps object.__ne__() should be defined this way.
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result = self.__eq__(other)
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if result is not NotImplemented:
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result = not result
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return result
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class d_values:
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def __init__(self, d):
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self.__d = d
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def __len__(self):
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return len(self.__d)
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def __contains__(self, value):
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# This is slow, and it's what "x in y" uses as a fallback
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# if __contains__ is not defined; but I'd rather make it
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# explicit that it is supported.
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for v in self:
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if v == value:
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return True
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return False
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def __iter__(self):
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for key in self.__d:
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yield self.__d[key]
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def __eq__(self, other):
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if not isinstance(other, d_values):
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return NotImplemented
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if len(self) != len(other):
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return False
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# XXX Sometimes this could be optimized, but these are the
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# semantics: we can't depend on the values to be hashable
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# or comparable.
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olist = list(other)
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for x in self:
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try:
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olist.remove(x)
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except ValueError:
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return False
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assert olist == []
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return True
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def __ne__(self, other):
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result = self.__eq__(other)
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if result is not NotImplemented:
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result = not result
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return result
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Notes:
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The view objects are not directly mutable, but don't implement
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__hash__(); their value can change if the underlying dict is mutated.
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The only requirements on the underlying dict are that it implements
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__getitem__(), __contains__(), __iter__(), and __len__(0.
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We don't implement .copy() -- the presence of a .copy()
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method suggests that the copy has the same type as the original, but
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that's not feasible without copying the underlying dict. If you want
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a copy of a specific type, like list or set, you can just pass one
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of the above to the list() or set() constructor.
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The specification implies that the order in which items
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are returned by .keys(), .values() and .items() is the same (just as
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it was in Python 2.x), because the order is all derived from the dict
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iterator (which is presumably arbitrary but stable as long as a dict
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isn't modified). This can be expressed by the following invariant::
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list(d.items()) == list(zip(d.keys(), d.values()))
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Open Issues
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===========
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Do we need more of a motivation? I would think that being able to do
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set operations on keys and items without having to copy them should
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speak for itself.
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I've left out the implementation of various set operations. These
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could still present small surprises.
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It would be okay if multiple calls to d.keys() (etc.) returned the
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same object, since the object's only state is the dict to which it
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refers. Is this worth having extra slots in the dict object for?
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Should that be a weak reference or should the d_keys (etc.) object
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live forever once created? Strawman: probably not worth the extra
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slots in every dict.
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Should d_keys, d_values and d_items have a public instance variable or
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method through which one can retrieve the underlying dict? Strawman:
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yes (but what should it be called?).
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I'm soliciting better names than d_keys, d_values and d_items. These
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classes could be public so that their implementations could be reused
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by the .keys(), .values() and .items() methods of other mappings. Or
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should they?
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Should the d_keys, d_values and d_items classes be reusable?
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Strawman: yes.
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Should they be subclassable? Strawman: yes (but see below).
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A particularly nasty issue is whether operations that are specified in
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terms of other operations (e.g. .discard()) must really be implemented
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in terms of those other operations; this may appear irrelevant but it
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becomes relevant if these classes are ever subclassed. Historically,
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Python has a really poor track record of specifying the semantics of
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highly optimized built-in types clearly in such cases; my strawman is
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to continue that trend. Subclassing may still be useful to *add* new
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methods, for example.
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I'll leave the decisions (especially about naming) up to whoever
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submits a working implementation.
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References
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==========
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.. [1] Java Collections Framework
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http://java.sun.com/docs/books/tutorial/collections/index.html
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