Frozenset in Python (4)

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*Memo:

• My post explains a frozenset (1).
• My post explains a frozenset (2).
• My post explains a frozenset (3).

A frozenset cannot be read by indexing and slicing and changed by indexing, slicing and a del statement as shown below:

*Memo:

• A del statement cannot remove zero or more elements from a frozenset by indexing and slicing but can remove one or more variables themselves.

A = frozenset([10, 20, 30, 40, 50, 60])

print(A[1], A[3:5])
print(A[-5], A[-3:-1])
# TypeError: 'frozenset' object is not subscriptable

A = frozenset([10, 20, 30, 40, 50, 60])

A[1] = 'X'
A[-5] = 'X'
A[3:5] = ['Y', 'Z']
A[-3:-1] = ['Y', 'Z']
A[1], A[3:5] = 'X', ['Y', 'Z']
A[-5], A[-3:-1] = 'X', ['Y', 'Z']
# TypeError: 'frozenset' object does not support item assignment

A = frozenset([10, 20, 30, 40, 50, 60])

del A[1], A[3:5]
del A[-5], A[-2:5]
# TypeError: 'frozenset' object does not support item deletion

A = frozenset([10, 20, 30, 40, 50, 60])

del A

print(A)
# NameError: name 'A' is not defined

If you really want to read a frozenset by indexing and slicing or change a frozenset by indexing, slicing and a del statement, use list() and frozenset() as shown below:

A = frozenset([10, 20, 30, 40, 50, 60])

A = list(A)

print(A[1], A[3:5])
print(A[-5], A[-3:-1])
# 10 [20, 60]

A[1] = 'X'
A[-5] = 'X'
A[3:5] = ['Y', 'Z']
A[-3:-1] = ['Y', 'Z']

A[1], A[3:5] = 'X', ['Y', 'Z']
A[-5], A[-3:-1] = 'X', ['Y', 'Z']

A = frozenset(A)

print(A)
# frozenset({'Z', 40, 50, 'Y', 30, 'X'})

A = frozenset([10, 20, 30, 40, 50, 60])

A = list(A)

del A[1], A[3:5]
# del A[-5], A[-2:5]

A = frozenset(A)

print(A)
# frozenset({40, 50, 20})

A frozenset can be continuously used through multiple variables as shown below:

A = B = C = frozenset([10, 20, 30]) # Equivalent
                                    # A = frozenset([10, 20, 30])
print(A) # frozenset({10, 20, 30})  # B = A
print(B) # frozenset({10, 20, 30})  # C = B
print(C) # frozenset({10, 20, 30})

A frozenset cannot be shallow-copied but can be deep-copied as shown below:

<Shallow copy>:

*Memo:

• A and B refer to the same outer and inner frozenset.
• is keyword can check if A and B refer to the same outer and inner frozenset.
• frozenset.copy(), copy.copy() and frozenset() cannot shallow-copy a frozenset:
  • frozenset.copy() has no arguments.

import copy

A = frozenset([frozenset([0, 1, 2])])
B = A.copy()
B = copy.copy(A)
B = frozenset(A)

print(A) # frozenset({frozenset({0, 1, 2})})
print(B) # frozenset({frozenset({0, 1, 2})})

print(A is B)
# True

A = set(A).pop()
B = set(B).pop()

print(A) # frozenset({0, 1, 2})
print(B) # frozenset({0, 1, 2})

print(A is B)
# True

<Deep copy>:

*Memo:

• A and B refer to different outer and inner frozensets.
• copy.deepcopy() deep-copies a frozenset.
• copy.deepcopy() should be used because it's safe, deeply copying a frozenset while frozenset.copy(), copy.copy() and frozenset() aren't safe, shallowly copying a frozenset.

import copy

A = frozenset([frozenset([0, 1, 2])])
B = copy.deepcopy(A)

print(A) # frozenset({frozenset({0, 1, 2})})
print(B) # frozenset({frozenset({0, 1, 2})})

print(A is B)
# False

A = set(A).pop()
B = set(B).pop()

print(A) # frozenset({0, 1, 2})
print(B) # frozenset({0, 1, 2})

print(A is B)
# False