Set in Python (4)

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*Memo:

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

A set 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 set by indexing and slicing but can remove one or more variables themselves.

A = {10, 20, 30, 40, 50, 60}

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

A = {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: 'set' object does not support item assignment

A = {10, 20, 30, 40, 50, 60}

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

A = {10, 20, 30, 40, 50, 60}

del A

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

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

A = {10, 20, 30, 40, 50, 60}

A = list(A)

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

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

A = {10, 20, 30, 40, 50, 60}

A = list(A)

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

A = set(A)

print(A)
# {40, 50, 10}

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

A = B = C = {10, 20, 30} # Equivalent
                         # A = {10, 20, 30}
A.update({40, 50})       # B = A
B.remove(30)             # C = B
C.pop()

print(A) # {20, 40, 10}
print(B) # {20, 40, 10}
print(C) # {20, 40, 10}

The set with a frozenset can be shallow-copied and deep-copied as shown below:

<Shallow copy>:

*Memo:

• A and B refer to different outer sets and the same inner frozenset.
• is keyword can check if A and B refer to the same outer set and inner frozenset.
• set.copy(), copy.copy() and set() can shallow-copy the set with a frozenset:
  • set.copy() has no arguments.

import copy

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

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

print(A is B)
# False

A = A.pop()
B = 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 sets and inner frozensets.
• copy.deepcopy() deep-copies the set with a frozenset.
• copy.deepcopy() should be used because it's safe, deeply copying the set with a frozenset while set.copy(), copy.copy() and set() aren't safe, shallowly copying the set with a frozenset.

import copy

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

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

print(A is B)
# False

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

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

print(A is B)
# False