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PEP 603 -- Adding a frozenmap type to collections

PEP:603
Title:Adding a frozenmap type to collections
Author:Yury Selivanov <yury at edgedb.com>
Status:Draft
Type:Standards Track
Created:12-Sep-2019
Python-Version:3.9
Post-History:12-Sep-2019

Abstract

A persistent data structure is defined as a data structure that preserves the previous version of the data when the data is modified. Such data structures are effectively immutable, as operations on them do not update the structure in-place, but instead always yield a new updated structure (see [0] for more details.)

This PEP proposes to add a new fully persistent and immutable mapping type called frozenmap to the collections module.

The bulk of frozenmap's reference implementation is already used in CPython to implement the contextvars module.

Rationale

Python has two immutable collection types: tuple and frozenset. These types can be used to represent immutable lists and sets. However, a way to represent immutable mappings does not yet exist, and this PEP proposes a frozenmap to implement an immutable mapping.

The proposed frozenmap type:

  • implements the collections.abc.Mapping protocol,
  • supports pickling, and
  • provides an API for efficient creation of "modified" versions.

The following use cases illustrate why an immutable mapping is desirable:

  • Immutable mappings are hashable which allows their use as dictionary keys or set elements.

    This hashable property permits functions decorated with @functools.lru_cache() to accept immutable mappings as arguments. Unlike an immutable mapping, passing a plain dict to such a function results in error.

  • Immutable mappings can hold complex state. Since immutable mappings can be copied by reference, transactional mutation of state can be efficiently implemented.

  • Immutable mappings can be used to safely share dictionaries across thread and asynchronous task boundaries. The immutability makes it easier to reason about threads and asynchronous tasks.

Lastly, CPython [1] already contains the main portion of the C code required for the frozenmap implementation. The C code already exists to implement the contextvars module (see PEP 567 for more details.) Exposing this C code via a public collection type drastically increases the number of users of the code. This leads to increased code quality by discovering bugs and improving performance which without a frozenmap collection would be very challenging because most programs use the contextvars module indirectly.

Specification

A new public immutable type frozenmap is added to the collections module.

Construction

frozenmap implements a dict-like construction API:

  • frozenmap() creates a new empty immutable mapping;
  • frozenmap(**kwargs) creates a mapping from **kwargs, e.g. frozenmap(x=10, y=0, z=-1)
  • frozenmap(collection) creates a mapping from the passed collection object. The passed collection object can be:
    • a dict,
    • another frozenmap,
    • an object with an items() method that is expected to return a series of key/value tuples, or
    • an iterable of key/value tuples.

Data Access

frozenmap implements the collection.abc.Mapping protocol. Therefore, getters, membership checks, and iteration work the same way that they would for a dict:

m = frozenmap(foo='bar')

assert m['foo'] == 'bar'
assert m.get('foo') == 'bar'
assert 'foo' in m

assert 'baz' not in m
assert m.get('baz', 'missing') == 'missing'

assert m == m
assert m != frozenmap()  # m is not equal to an empty frozenmap

assert len(m) == 1

# etc.

Mutation

frozenmap instances are immutable. That said, it is possible to efficiently produce mutated copies of the immutable instance.

The complexity of mutation operations is O(log N) and the resulting frozenmap copies often consume very little additional memory due to the use of structural sharing (read [6] for more details.)

frozenmap.including(key, value)

The method creates a new frozenmap copy with a new key / value pair:

m = frozenmap(foo=1)
m2 = m.including('bar', 100)

print(m)   # will print frozenmap({'foo': 1})
print(m2)  # will print frozenmap({'foo': 1, 'bar': 100})

frozenmap.excluding(key)

The method produces a copy of the frozenmap which does not include a deleted key:

m = frozenmap(foo=1, bar=100)

m2 = m.excluding('foo')

print(m)   # will print frozenmap({'foo': 1, 'bar': 100})
print(m2)  # will print frozenmap({'bar': 1})

m3 = m.excluding('spam')  # will throw a KeyError('spam')

frozenmap.union(mapping=None, **kw)

The method produces a copy of the frozenmap and adds or modifies multiple key/values for the created copy. The signature of the method matches the signature of the frozenmap constructor:

m = frozenmap(foo=1)

m2 = m.union({'spam': 'ham'})
print(m2)  # will print frozenmap({'foo': 1, 'spam': 'ham'})

m3 = m.union(foo=100, y=2)
print(m3)  # will print frozenmap({'foo': 100, 'y': 2})

print(m)   # will print frozenmap({'foo': 1})

Calling the union() method to add/replace N keys is more efficient than calling the including() method N times.

frozenmap.mutating()

The method allows efficient copying of a frozenmap instance with multiple modifications applied. This method is especially useful when the frozenmap in question contains thousands of key/value pairs and there's a need to update many of them in a performance-critical section of the code.

The frozenmap.mutating() method returns a mutable dict-like copy of the frozenmap object: an instance of collections.FrozenMapCopy.

The FrozenMapCopy objects:

  • are copy-on-write views of the data of frozenmap instances they were created from;
  • are mutable, although any mutations on them do not affect the frozenmap instances they were created from;
  • can be passed to the frozenmap constructor; creating a frozenmap from a FrozenMapCopy object is an O(1) operation;
  • have O(log N) complexity for get/set operations; creating them is an O(1) operation;
  • have a FrozenMapCopy.close() method that prevents any further access/mutation of the data;
  • can be used as a context manager.

The below example illustrates how mutating() can be used with a context manager:

numbers = frozenmap((i, i ** 2) for i in range(1_000_000))

with numbers.mutating() as copy:
    for i in numbers:
        if not (numbers[i] % 997):
            del copy[i]

    numbers_without_997_multiples = frozenmap(copy)

    # at this point, *numbers* still has 1_000_000 key/values, and
    # *numbers_without_997_multiples* is a copy of *numbers* without
    # values that are multiples of 997.

    for i in numbers:
        if not (numbers[i] % 593):
            del copy[i]

    numbers_without_593_multiples = frozenmap(copy)

    print(copy[10])  # will print 100.

print(copy[10])  # This will throw a ValueError as *copy*
                 # has been closed when the "with" block
                 # was executed.

Iteration

As frozenmap implements the standard collections.abc.Mapping protocol, so all expected methods of iteration are supported:

assert list(m) == ['foo']
assert list(m.items()) == [('foo', 'bar')]
assert list(m.keys()) == ['foo']
assert list(m.values()) == ['bar']

Iteration in frozenmap, unlike in dict, does not preserve the insertion order.

Hashing

frozenmap instances can be hashable just like tuple objects:

hash(frozenmap(foo='bar'))  # works
hash(frozenmap(foo=[]))     # will throw an error

Typing

It is possible to use the standard typing notation for frozenmaps:

m: frozenmap[str, int] = frozenmap()

Implementation

The proposed frozenmap immutable type uses a Hash Array Mapped Trie (HAMT) data structure. Functional programming languages, like Clojure, use HAMT to efficiently implement immutable hash tables, vectors, and sets.

HAMT

The key design contract of HAMT is the guarantee of a predictable value when given the hash of a key. For a pair of key and value, the hash of the key can be used to determine the location of value in the hash map tree.

Immutable mappings implemented with HAMT have O(log N) performance for set() and get() operations. This efficiency is possible because mutation operations only affect one branch of the tree, making it possible to reuse non-mutated branches, and, therefore, avoiding copying of unmodified data.

Read more about HAMT in [5]. The CPython implementation [1] has a fairly detailed description of the algorithm as well.

Performance

pep-0603-hamt_vs_dict.png

Figure 1. Benchmark code can be found here: [3].

The above chart demonstrates that:

  • frozenmap implemented with HAMT displays near O(1) performance for all benchmarked dictionary sizes.
  • dict.copy() becomes less efficient when using around 100-200 items.
pep-0550-lookup_hamt.png

Figure 2. Benchmark code can be found here: [4].

Figure 2 compares the lookup costs of dict versus a HAMT-based immutable mapping. HAMT lookup time is ~30% slower than Python dict lookups on average. This performance difference exists since traversing a shallow tree is less efficient than lookup in a flat continuous array.

Further to that, quoting [6]: "[using HAMT] means that in practice while insertions, deletions, and lookups into a persistent hash array mapped trie have a computational complexity of O(log n), for most applications they are effectively constant time, as it would require an extremely large number of entries to make any operation take more than a dozen steps."

Design Considerations

Why "frozenmap" and not "FrozenMap"

The lower-case "frozenmap" resonates well with the frozenset built-in as well as with types like collections.defaultdict.

Why "frozenmap" and not "frozendict"

"Dict" has a very specific meaning in Python:

  • a dict is a concrete implementation of abc.MutableMapping with O(1) get and set operations (frozenmap has O(log N) complexity);
  • Python dicts preserve insertion order.

The proposed frozenmap does not have these mentioned properties. Instead, frozenmap has an O(log N) cost of set/get operations, and it only implements the abc.Mapping protocol.

Implementation

The full implementation of the proposed frozenmap type is available at [2]. The package includes C and pure Python implementations of the type.

See also the HAMT collection implementation as part of the CPython project tree here: [1].

Acknowledgments

I thank Carol Willing, Ɓukasz Langa, Larry Hastings, and Guido van Rossum for their feedback, ideas, edits, and discussions around this PEP.

Source: https://github.com/python/peps/blob/master/pep-0603.rst