Set in Python (2)

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*Memo:

• My post explains a frozenset (1).

set() can create a set with or without an iterable as shown below:

*Memo:

• The 1st argument is iterable(Optional-Default:()-Type:Iterable):
  • Don't use iterable=.

# Empty set
print(set())
print(set(()))
# set()

print(set([0, 1, 2, 3, 4]))                      # list
print(set((0, 1, 2, 3, 4)))                      # tuple
print(set(iter([0, 1, 2, 3, 4])))                # iterator
print(set({0, 1, 2, 3, 4}))                      # set
print(set(frozenset({0, 1, 2, 3, 4})))           # frozenset
print(set(range(5)))                             # range
# {0, 1, 2, 3, 4}

print(set({'name': 'John', 'age': 36}))          # dict
print(set({'name': 'John', 'age': 36}.keys()))   # dict.keys()
# {'age', 'name'}

print(set({'name': 'John', 'age': 36}.values())) # dict.values()
# {'John', 36}

print(set({'name': 'John', 'age': 36}.items()))  # dict.items()
# {('age', 36), ('name', 'John')}

print(set('Hello'))                              # str
# {'H', 'e', 'l', 'o'}

print(set(b'Hello'))                             # bytes
print(set(bytearray(b'Hello')))                  # bytearray
# {72, 108, 101, 111}

A set comprehension can create a set as shown below:

<1D set>:

sample = {0, 1, 2, 3, 4, 5, 6, 7}

A = {x**2 for x in sample}

print(A)
# {0, 1, 4, 36, 9, 16, 49, 25}

<2D set>:

sample = {frozenset({0, 1, 2, 3}), frozenset({4, 5, 6, 7})}

A = {frozenset(y**2 for y in x) for x in sample}

print(A)
# {frozenset({16, 25, 36, 49}), frozenset({0, 1, 4, 9})}

<3D set>:

sample = {frozenset({frozenset({0, 1}), frozenset({2, 3})}),
          frozenset({frozenset({4, 5}), frozenset({6, 7})})}

A = {frozenset(frozenset(z**2 for z in y) for y in x) for x in sample}

print(A)
# {frozenset({frozenset({16, 25}), frozenset({49, 36})}),
#  frozenset({frozenset({9, 4}), frozenset({0, 1})})}

A set cannot be read or changed by indexing or slicing as shown below:

*Memo:

• A del statement can still be used to remove one or more variables themselves.

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

print(A[0], A[2:6])
# TypeError: 'set' object is not subscriptable

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

A[0] = 100
A[2:6] = [200, 300]
# TypeError: 'set' object does not support item assignment

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

del A[0], A[3:5]
# TypeError: 'set' object doesn't 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 or change a set, use list() and set() as shown below:

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

A = list(A)

print(A[0], A[2:6])
# 50 [40, 10, 60, 30]

A[0] = 100
A[2:6] = [200, 300]

A = set(A)

print(A)
# {200, 100, 20, 300}

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

A = list(A)

del A[0], A[3:5]

A = set(A)

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