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A simple immutable dictionary

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frozendict

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Introduction

Welcome, fellow programmer!

frozendict is a simple immutable dictionary. It's fast as dict, and sometimes faster!

Unlike other similar implementations, immutability is guaranteed: you can't change the internal variables of the class, and they are all immutable objects. Reinvoking __init__ does not alter the object.

The API is the same as dict, without methods that can change the immutability. So it supports also fromkeys, unlike other implementations. Furthermore it can be pickled, unpickled and have an hash, if all values are hashable.

You can also add any dict to a frozendict using the | operator. The result is a new frozendict.

API

The API is the same of dict of Python 3.10, without the methods and operands which alter the map. Additionally, frozendict supports these methods:

__hash__()

If all the values of the frozendict are hashable, returns an hash, otherwise raises a TypeError.

set(key, value)

It returns a new frozendict. If key is already in the original frozendict, the new one will have it with the new value associated. Otherwise, the new frozendict will contain the new (key, value) item.

delete(key)

It returns a new frozendict without the item corresponding to the key. If the key is not present, a KeyError is raised.

setdefault(key[, default])

If key is already in frozendict, the object itself is returned unchanged. Otherwise, the new frozendict will contain the new (key, default) item. The parameter default defaults to None.

key([index])

It returns the key at the specified index (determined by the insertion order). If index is not passed, it defaults to 0. If the index is negative, the position will be the size of the frozendict + index

value([index])

Same as key(index), but it returns the value at the given index.

item([index])

Same as key(index), but it returns a tuple with (key, value) at the given index.

Examples

from frozendict import frozendict

fd = frozendict({"Guzzanti": "Corrado", "Hicks": "Bill"})

print(fd)
# frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill'})

print(fd["Guzzanti"])
# Corrado

fd["Brignano"]
# KeyError: 'Brignano'

len(fd)
# 2

"Guzzanti" in fd
# True

"Guzzanti" not in fd
# False

"Brignano" in fd
# False

hash(fd)
# 5833699487320513741

fd_unhashable = frozendict({1: []})
hash(fd_unhashable)
# TypeError: Not all values are hashable.

fd | {1: 2}
# frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill', 1: 2})

fd5 = frozendict(fd)
id_fd5 = id(fd5)
fd5 |= {1: 2}
fd5
# frozendict.frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill', 1: 2})
id(fd5) != id_fd5
# True

fd.set(1, 2)
# frozendict.frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill', 1: 2})

fd.set("Guzzanti", "Sabina")
# frozendict.frozendict({'Guzzanti': 'Sabina', 'Hicks': 'Bill'})

fd.delete("Guzzanti")
# frozendict.frozendict({'Hicks': 'Bill'})

fd.setdefault("Guzzanti", "Sabina")
# frozendict.frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill'})

fd.setdefault(1, 2)
# frozendict.frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill', 1: 2})

fd.key()
# 'Guzzanti'

fd.value(1)
# 'Bill'

fd.item(-1)
# (1, 2)

fd2 = fd.copy()
fd2 == fd
# True

fd3 = frozendict(fd)
fd3 == fd
# True

fd4 = frozendict({"Hicks": "Bill", "Guzzanti": "Corrado"})

print(fd4)
# frozendict({'Hicks': 'Bill', 'Guzzanti': 'Corrado'})

fd4 == fd
# True

import pickle
fd_unpickled = pickle.loads(pickle.dumps(fd))
print(fd_unpickled)
# frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill'})
fd_unpickled == fd
# True

frozendict(Guzzanti="Corrado", Hicks="Bill")
# frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill'}

frozendict((("Guzzanti", "Corrado"), ("Hicks", "Bill")))
# frozendict({'Guzzanti': 'Corrado', 'Hicks': 'Bill'})

fd.get("Guzzanti")
# 'Corrado'

print(fd.get("Brignano"))
# None

tuple(fd.keys())
# ('Guzzanti', 'Hicks')

tuple(fd.values())
# ('Corrado', 'Bill')

tuple(fd.items())
# (('Guzzanti', 'Corrado'), ('Hicks', 'Bill'))

frozendict.fromkeys(["Corrado", "Sabina"], "Guzzanti")
# frozendict({'Corrado': 'Guzzanti', 'Sabina': 'Guzzanti'})

iter(fd)
# <dict_keyiterator object at 0x7feb75c49188>

fd["Guzzanti"] = "Caterina"
# TypeError: 'frozendict' object doesn't support item assignment

Building

You can build frozendict directly from the code, using

python3 setup.py bdist_wheel

The C Extension is not optional by default. You can build the pure py package using the py custom argument:

python3 setup.py py bdist_wheel

Benchmarks

Some benchmarks between dict and frozendict[1]:

################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(d)`;         Size:    5; Keys: str; Type:       dict; Time: 8.02e-08; Sigma: 4e-09
Name: `constructor(d)`;         Size:    5; Keys: str; Type: frozendict; Time: 8.81e-08; Sigma: 3e-09
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(d)`;         Size:    5; Keys: int; Type:       dict; Time: 7.96e-08; Sigma: 5e-09
Name: `constructor(d)`;         Size:    5; Keys: int; Type: frozendict; Time: 8.97e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(d)`;         Size: 1000; Keys: str; Type:       dict; Time: 6.38e-06; Sigma: 9e-08
Name: `constructor(d)`;         Size: 1000; Keys: str; Type: frozendict; Time: 6.21e-06; Sigma: 2e-07
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(d)`;         Size: 1000; Keys: int; Type:       dict; Time: 3.49e-06; Sigma: 3e-07
Name: `constructor(d)`;         Size: 1000; Keys: int; Type: frozendict; Time: 3.48e-06; Sigma: 2e-07
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(kwargs)`;    Size:    5; Keys: str; Type:       dict; Time: 2.40e-07; Sigma: 1e-09
Name: `constructor(kwargs)`;    Size:    5; Keys: str; Type: frozendict; Time: 2.48e-07; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(kwargs)`;    Size: 1000; Keys: str; Type:       dict; Time: 4.80e-05; Sigma: 1e-06
Name: `constructor(kwargs)`;    Size: 1000; Keys: str; Type: frozendict; Time: 2.90e-05; Sigma: 7e-07
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(seq2)`;      Size:    5; Keys: str; Type:       dict; Time: 2.01e-07; Sigma: 9e-10
Name: `constructor(seq2)`;      Size:    5; Keys: str; Type: frozendict; Time: 2.50e-07; Sigma: 1e-09
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(seq2)`;      Size:    5; Keys: int; Type:       dict; Time: 2.18e-07; Sigma: 2e-09
Name: `constructor(seq2)`;      Size:    5; Keys: int; Type: frozendict; Time: 2.73e-07; Sigma: 1e-09
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(seq2)`;      Size: 1000; Keys: str; Type:       dict; Time: 4.29e-05; Sigma: 6e-07
Name: `constructor(seq2)`;      Size: 1000; Keys: str; Type: frozendict; Time: 4.33e-05; Sigma: 6e-07
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(seq2)`;      Size: 1000; Keys: int; Type:       dict; Time: 3.04e-05; Sigma: 4e-07
Name: `constructor(seq2)`;      Size: 1000; Keys: int; Type: frozendict; Time: 3.45e-05; Sigma: 4e-07
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(o)`;         Size:    5; Keys: str; Type:       dict; Time: 7.93e-08; Sigma: 3e-09
Name: `constructor(o)`;         Size:    5; Keys: str; Type: frozendict; Time: 2.41e-08; Sigma: 6e-10
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(o)`;         Size:    5; Keys: int; Type:       dict; Time: 7.94e-08; Sigma: 5e-09
Name: `constructor(o)`;         Size:    5; Keys: int; Type: frozendict; Time: 2.41e-08; Sigma: 6e-10
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(o)`;         Size: 1000; Keys: str; Type:       dict; Time: 6.18e-06; Sigma: 3e-07
Name: `constructor(o)`;         Size: 1000; Keys: str; Type: frozendict; Time: 2.41e-08; Sigma: 6e-10
////////////////////////////////////////////////////////////////////////////////
Name: `constructor(o)`;         Size: 1000; Keys: int; Type:       dict; Time: 3.47e-06; Sigma: 2e-07
Name: `constructor(o)`;         Size: 1000; Keys: int; Type: frozendict; Time: 2.41e-08; Sigma: 6e-10
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `o.copy()`;               Size:    5; Keys: str; Type:       dict; Time: 7.28e-08; Sigma: 2e-09
Name: `o.copy()`;               Size:    5; Keys: str; Type: frozendict; Time: 3.18e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o.copy()`;               Size:    5; Keys: int; Type:       dict; Time: 7.21e-08; Sigma: 4e-09
Name: `o.copy()`;               Size:    5; Keys: int; Type: frozendict; Time: 3.32e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o.copy()`;               Size: 1000; Keys: str; Type:       dict; Time: 6.16e-06; Sigma: 3e-07
Name: `o.copy()`;               Size: 1000; Keys: str; Type: frozendict; Time: 3.18e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o.copy()`;               Size: 1000; Keys: int; Type:       dict; Time: 3.46e-06; Sigma: 1e-07
Name: `o.copy()`;               Size: 1000; Keys: int; Type: frozendict; Time: 3.18e-08; Sigma: 2e-09
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `o == o`;                 Size:    5; Keys: str; Type:       dict; Time: 7.23e-08; Sigma: 8e-10
Name: `o == o`;                 Size:    5; Keys: str; Type: frozendict; Time: 2.44e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o == o`;                 Size:    5; Keys: int; Type:       dict; Time: 7.30e-08; Sigma: 1e-09
Name: `o == o`;                 Size:    5; Keys: int; Type: frozendict; Time: 2.44e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o == o`;                 Size: 1000; Keys: str; Type:       dict; Time: 1.38e-05; Sigma: 1e-07
Name: `o == o`;                 Size: 1000; Keys: str; Type: frozendict; Time: 2.44e-08; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `o == o`;                 Size: 1000; Keys: int; Type:       dict; Time: 1.05e-05; Sigma: 7e-08
Name: `o == o`;                 Size: 1000; Keys: int; Type: frozendict; Time: 2.44e-08; Sigma: 2e-09
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o`;             Size:    5; Keys: str; Type:       dict; Time: 7.33e-08; Sigma: 2e-09
Name: `for x in o`;             Size:    5; Keys: str; Type: frozendict; Time: 6.70e-08; Sigma: 1e-09
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o`;             Size:    5; Keys: int; Type:       dict; Time: 7.33e-08; Sigma: 2e-09
Name: `for x in o`;             Size:    5; Keys: int; Type: frozendict; Time: 6.70e-08; Sigma: 1e-09
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o`;             Size: 1000; Keys: str; Type:       dict; Time: 8.84e-06; Sigma: 5e-08
Name: `for x in o`;             Size: 1000; Keys: str; Type: frozendict; Time: 7.06e-06; Sigma: 6e-08
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o`;             Size: 1000; Keys: int; Type:       dict; Time: 8.67e-06; Sigma: 7e-08
Name: `for x in o`;             Size: 1000; Keys: int; Type: frozendict; Time: 6.94e-06; Sigma: 3e-08
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.values()`;    Size:    5; Keys: str; Type:       dict; Time: 7.28e-08; Sigma: 9e-10
Name: `for x in o.values()`;    Size:    5; Keys: str; Type: frozendict; Time: 6.48e-08; Sigma: 8e-10
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.values()`;    Size:    5; Keys: int; Type:       dict; Time: 7.25e-08; Sigma: 1e-09
Name: `for x in o.values()`;    Size:    5; Keys: int; Type: frozendict; Time: 6.45e-08; Sigma: 1e-09
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.values()`;    Size: 1000; Keys: str; Type:       dict; Time: 9.06e-06; Sigma: 5e-07
Name: `for x in o.values()`;    Size: 1000; Keys: str; Type: frozendict; Time: 7.04e-06; Sigma: 4e-08
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.values()`;    Size: 1000; Keys: int; Type:       dict; Time: 9.53e-06; Sigma: 3e-08
Name: `for x in o.values()`;    Size: 1000; Keys: int; Type: frozendict; Time: 6.97e-06; Sigma: 3e-08
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.items()`;     Size:    5; Keys: str; Type:       dict; Time: 1.13e-07; Sigma: 3e-09
Name: `for x in o.items()`;     Size:    5; Keys: str; Type: frozendict; Time: 1.16e-07; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.items()`;     Size:    5; Keys: int; Type:       dict; Time: 1.14e-07; Sigma: 3e-09
Name: `for x in o.items()`;     Size:    5; Keys: int; Type: frozendict; Time: 1.17e-07; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.items()`;     Size: 1000; Keys: str; Type:       dict; Time: 1.53e-05; Sigma: 3e-07
Name: `for x in o.items()`;     Size: 1000; Keys: str; Type: frozendict; Time: 1.53e-05; Sigma: 4e-07
////////////////////////////////////////////////////////////////////////////////
Name: `for x in o.items()`;     Size: 1000; Keys: int; Type:       dict; Time: 1.53e-05; Sigma: 3e-07
Name: `for x in o.items()`;     Size: 1000; Keys: int; Type: frozendict; Time: 1.55e-05; Sigma: 4e-07
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.dumps(o)`;        Size:    5; Keys: str; Type:       dict; Time: 6.82e-07; Sigma: 2e-08
Name: `pickle.dumps(o)`;        Size:    5; Keys: str; Type: frozendict; Time: 2.86e-06; Sigma: 1e-07
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.dumps(o)`;        Size:    5; Keys: int; Type:       dict; Time: 4.77e-07; Sigma: 2e-08
Name: `pickle.dumps(o)`;        Size:    5; Keys: int; Type: frozendict; Time: 2.72e-06; Sigma: 8e-08
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.dumps(o)`;        Size: 1000; Keys: str; Type:       dict; Time: 1.24e-04; Sigma: 4e-06
Name: `pickle.dumps(o)`;        Size: 1000; Keys: str; Type: frozendict; Time: 1.92e-04; Sigma: 5e-06
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.dumps(o)`;        Size: 1000; Keys: int; Type:       dict; Time: 2.81e-05; Sigma: 6e-07
Name: `pickle.dumps(o)`;        Size: 1000; Keys: int; Type: frozendict; Time: 7.37e-05; Sigma: 1e-06
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.loads(dump)`;     Size:    5; Keys: str; Type:       dict; Time: 9.08e-07; Sigma: 6e-09
Name: `pickle.loads(dump)`;     Size:    5; Keys: str; Type: frozendict; Time: 1.79e-06; Sigma: 9e-08
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.loads(dump)`;     Size:    5; Keys: int; Type:       dict; Time: 4.46e-07; Sigma: 6e-09
Name: `pickle.loads(dump)`;     Size:    5; Keys: int; Type: frozendict; Time: 1.32e-06; Sigma: 7e-08
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.loads(dump)`;     Size: 1000; Keys: str; Type:       dict; Time: 1.57e-04; Sigma: 8e-06
Name: `pickle.loads(dump)`;     Size: 1000; Keys: str; Type: frozendict; Time: 1.69e-04; Sigma: 7e-06
////////////////////////////////////////////////////////////////////////////////
Name: `pickle.loads(dump)`;     Size: 1000; Keys: int; Type:       dict; Time: 5.97e-05; Sigma: 5e-06
Name: `pickle.loads(dump)`;     Size: 1000; Keys: int; Type: frozendict; Time: 6.68e-05; Sigma: 2e-06
################################################################################
////////////////////////////////////////////////////////////////////////////////
Name: `class.fromkeys()`;       Size:    5; Keys: str; Type:       dict; Time: 1.88e-07; Sigma: 1e-09
Name: `class.fromkeys()`;       Size:    5; Keys: str; Type: frozendict; Time: 2.22e-07; Sigma: 7e-09
////////////////////////////////////////////////////////////////////////////////
Name: `class.fromkeys()`;       Size:    5; Keys: int; Type:       dict; Time: 2.08e-07; Sigma: 6e-09
Name: `class.fromkeys()`;       Size:    5; Keys: int; Type: frozendict; Time: 2.44e-07; Sigma: 2e-09
////////////////////////////////////////////////////////////////////////////////
Name: `class.fromkeys()`;       Size: 1000; Keys: str; Type:       dict; Time: 4.05e-05; Sigma: 4e-06
Name: `class.fromkeys()`;       Size: 1000; Keys: str; Type: frozendict; Time: 3.84e-05; Sigma: 5e-07
////////////////////////////////////////////////////////////////////////////////
Name: `class.fromkeys()`;       Size: 1000; Keys: int; Type:       dict; Time: 2.93e-05; Sigma: 7e-07
Name: `class.fromkeys()`;       Size: 1000; Keys: int; Type: frozendict; Time: 3.08e-05; Sigma: 2e-06
################################################################################

[1] Benchmarks done under Linux 64 bit, Python 3.10.2, using the C Extension.

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