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Super Kai (Kazuya Ito)
Super Kai (Kazuya Ito)

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Dictionary in Python (2)

#python #dictionary #curlybraces #datastructure

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*Memos:

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

dict() can create a dictionary with or without a dictonary as shown below:

*Memos:

  • The 1st argument is iterable(Optional-Type:iterable). *Don't use iterable=.
  • The 2nd or the later arguments are **kwarg(Optional). *Don't use any keywords like **kwarg=, kwarg=, **kwargs=, kwargs=, etc. 
print(dict()) # []  print(dict({'name':'John', 'age':36})) # Dictionary print(dict([('name', 'John'), ('age', 36)])) print(dict({'name':'John'}, age=36)) print(dict([('name', 'John')], age=36)) print(dict(name='John', age=36)) # {'name': 'John', 'age': 36}
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dict() cannot create a dictionary with a list, tuple, set, iterator, string or range() as shown below:

v = ['a', 'b', 'c', 'd', 'e'] print(dict(v)) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
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v = ('a', 'b', 'c', 'd', 'e') print(dict(v)) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
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v = {'a', 'b', 'c', 'd', 'e'} print(dict(v)) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
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v = iter(['a', 'b', 'c', 'd', 'e']) # Iterator  print(dict(v)) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
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v = 'Hello' # String  print(dict(v)) # ValueError: dictionary update sequence element #0 has length 1; 2 is required
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v = range(5) print(dict(v)) # TypeError: cannot convert dictionary update sequence element #0 to a sequence
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Be careful, a huge dictionary gets MemoryError as shown below:

v = {x:x for x in range(100000000)} # MemoryError
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You can access and change a dictionary by keying as shown below. *Keying can be done with one or more [key]:

v = {'name':'John', 'age':36} # 1D dictionary v = dict(name='John', age=36) v = dict([('name', 'John'), ('age', 36)]) print(v['name'], v['age']) # John 36  print(v[0]) # KeyError: 0 print(v[0:2]) # KeyError: slice(0, 2, None)  v['name'] = 'David' v['gender'] = 'Male' print(v) # {'name': 'David', 'age': 36, 'gender': 'Male'}
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v = {'person1':{'name':'John', 'age':36}, # 2D dictionary  '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'] = 'David' v['person2']['gender'] = 'Female' v['person3'] = {'name':'Tom', 'age':18, 'gender':'Male'} print(v) # {'person1': {'name': 'David', 'age': 36}, # 'person2': {'name': 'Anna', 'age': 24, 'gender': 'Female'}, # 'person3': {'name': 'Tom', 'age': 18, 'gender': 'Male'}}
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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'}
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The variables v1 and v2 refer to the same dictionary unless copied as shown below:

*Memos:

  • is keyword can check if v1 and v2 refer to the same dictionary.
  • copy() can do shallow copy. *There are no arguments.
  • deepcopy() can do deep copy. *There are no arguments.
  • deepcopy() should be used because it's safe, doing copy deeply while copy() isn't safe, doing copy shallowly. 
from copy import deepcopy v1 = {'name':'John', 'age':36} v2 = v1 # v2 refers to the same dictionary as v1.  v2['name'] = 'David' # Changes the same dictionary as v1.  # ↓↓↓↓↓↓↓ print(v1) # {'name': 'David', 'age': 36} print(v2) # {'name': 'David', 'age': 36}  # ↑↑↑↑↑↑↑ print(v1 is v2) # True  v2 = v1.copy() # v2 refers to the different dictionary from v1. v2 = deepcopy(v1) v2['name'] = 'Anna' # Changes the different dictionary from v1.  # ↓↓↓↓↓↓↓ print(v1) # {'name': 'David', 'age': 36} print(v2) # {'name': 'Anna', 'age': 36}  # ↑↑↑↑↑↑ print(v1 is v2) # False
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