Dictionary in Python (2)

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*Memo:

• My post explains a dictionary (1).
• My post explains dictionary functions (1).
• My post explains dictionary functions (2).

dict() can create a dictionary with dict, keyword arguments(kwargs) and a 2D iterable(list, tuple, set, frozenset and iterator) as shown below:

*Memo:

• The 1st argument is mapping or iterable(Optional-Type:Mapping or Iterable):
  • Don't use mapping= or iterable=.
• The 2nd arguments are **kwargs(Optional-Default:{}-Type:Any):
  • Don't use any keywords like **kwargs=, kwargs=, etc.

# Empty dict
print(dict())
print(dict({}))
# {}

print(dict({'name':'John', 'age':36}))       # dict
print(dict([('name', 'John'), ('age', 36)])) # list(tuple)
print(dict(name='John', age=36))             # kwargs
print(dict({'name':'John'}, age=36))         # dict & kwargs
print(dict([('name', 'John')], age=36))      # list(tuple) & kwargs
print(dict([['name', 'John'], ['age', 36]])) # list(list)
print(dict((('name', 'John'), ('age', 36)))) # tuple(tuple)
print(dict({frozenset({'name', 'John'}),     # frozenset(set)
frozenset({'age', 36})}))
print(dict(iter([iter(['name', 'John']),     # iter(iter)
                 iter(['age', 36])])))
# {'name': 'John', 'age': 36}

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

dict() can also create a dictionary with an empty 1D iterable as shown below:

print(dict([]))               # list
print(dict(()))               # tuple
print(dict(set()))            # set
print(dict(frozenset(set()))) # frozenset
print(dict({}))               # dict
print(dict(iter([])))         # iterator
print(dict(''))               # str
print(dict(b''))              # bytes
print(dict(bytearray(b'')))   # bytearray
print(dict(range(0)))         # range
# {}

dict() cannot create a dictionary with only one unempty iterable except dict as shown below:

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

print(dict(['A', 'B', 'C']))            # list
print(dict(('A', 'B', 'C')))            # tuple
print(dict({'A', 'B', 'C'}))            # set
print(dict(frozenset({'A', 'B', 'C'}))) # frozenset
print(dict(iter(['A', 'B', 'C'])))      # iterator
print(dict('Hello World'))              # str
# ValueError: dictionary update sequence element #0 has length 1; 2 is required

print(dict(b'Hello World'))             # bytes
print(dict(bytearray(b'Hello World')))  # bytearray
print(dict(range(100)))                 # range
# TypeError: cannot convert dictionary update sequence element #0 to a sequence

A dictionary(dict) comprehension can create a dictionary as shown below:

v = {x:x**2 for x in range(6)}

print(v)
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Be careful, a huge dictionary gets MemoryError as shown below:

v = {x:x for x in range(100000000)}
# MemoryError

A dictionary can be read or changed by keying as shown below:

*Memo:

• Keying can be done with one or more [key].
• A del statement can remove one or more pairs of a key and value from a dictionary by keying and can remove one or more variables themselves.

1D dictionary:

v = {'name':'John', 'age':36}
v = dict(name='John', age=36)
v = dict([('name', 'John'), ('age', 36)])

print(v['name'], v['age'])
# John 36

v['name'] = 'Emily'
v['gender'] = 'Female'

print(v)
# {'name': 'Emily', 'age': 36, 'gender': 'Female'}

del v['name'], v['gender']

print(v)
# {'age': 36}

del v

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

2D dictionary:

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}
v = dict(person1=dict(name='John', age=36),
         person2=dict(name='Anna', age=24))
v = dict([('person1', dict([('name', 'John'), ('age', 36)])),
          ('person2', dict([('name', 'Anna'), ('age', 24)]))])

print(v['person1'], v['person2'])
# {'name': 'John', 'age': 36} {'name': 'Anna', 'age': 24}

print(v['person1']['name'], v['person1']['age'],
      v['person2']['name'], v['person2']['age'])
# John 36 Anna 24

v['person1']['name'] = 'Emily'
v['person2']['gender'] = 'Female'
v['person3'] = {'name':'Tom', 'age':18, 'gender':'Male'}

print(v)
# {'person1': {'name': 'Emily', 'age': 36},
#  'person2': {'name': 'Anna', 'age': 24, 'gender': 'Female'},
#  'person3': {'name': 'Tom', 'age': 18, 'gender': 'Male'}}

del v['person1']
del v['person3']['name'], v['person3']['gender']

print(v)
# {'person2': {'name': 'Anna', 'age': 24, 'gender': 'Female'},
#  'person3': {'age': 18}}

A dictionary can be used like a list by indexing but not by slicing properly as shown below:

v = {0:'apple', 1:'orange', 2:'kiwi'}

print(v[0], v[1], v[2])
# Apple Orange Kiwi

v[0] = 'APPLE'
v[1] = 'ORANGE'
v[2] = 'KIWI'

print(v)
# {0: 'APPLE', 1: 'ORANGE', 2: 'KIWI'}

v[0:2] = ['banana', 'peach']

print(v)
# {0: 'APPLE', 1: 'ORANGE', 2: 'KIWI', slice(0, 2, None): ['banana', 'peach']}

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

v1 = v2 = v3 = {'name':'John', 'age':36, 'gender':'Male'}
                        # Equivalent
                        # v1 = {'name':'John', 'age':36, 'gender':'Male'}
                        # v2 = v1
                        # v3 = v2
v1['name'] = 'Anna'
v2['age'] = 24
v3['gender'] = 'Female'

print(v1) # {'name': 'Anna', 'age': 24, 'gender': 'Female'}
print(v2) # {'name': 'Anna', 'age': 24, 'gender': 'Female'}
print(v3) # {'name': 'Anna', 'age': 24, 'gender': 'Female'}

The variables v1 and v2 refer to the same dictionary unless copied as shown below:

*Memo:

• is keyword or is and not keyword can check if v1 and v2 refer or don't refer to the same dictionary respectively.
• dict.copy(), copy.copy() and dict() do shallow copy:
  • dict.copy() has no arguments.
• deepcopy() does deep copy.
• copy.deepcopy() should be used because it's safe, doing copy deeply while dict.copy(), copy.copy() and dict() aren't safe, doing copy shallowly.

import copy

v1 = {'name':'John', 'age':36}

v2 = v1 # v2 refers to the same dictionary as v1.

v2['name'] = 'Emily' # Changes the same dictionary as v1.
                   # ↓↓↓↓↓↓↓
print(v1) # {'name': 'Emily', 'age': 36}
print(v2) # {'name': 'Emily', 'age': 36}
                   # ↑↑↑↑↑↑↑
print(v1 is v2, v1 is not v2)
# True False

v2 = v1.copy() # v2 refers to the different dictionary from v1.
v2 = copy.copy(v1) 
v2 = copy.deepcopy(v1)
v2 = dict(v1)

v2['name'] = 'Anna' # Changes the different dictionary from v1.
                   # ↓↓↓↓↓↓↓
print(v1) # {'name': 'Emily', 'age': 36}
print(v2) # {'name': 'Anna', 'age': 36}
                   # ↑↑↑↑↑↑
print(v1 is v2, v1 is not v2)
# False True