Dictionary in Python (4)

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*Memo:

• My post explains a dictionary (1).
• My post explains a dictionary (2) and the dictionary with keying and copy.
• My post explains dictionary functions (1).

get() can get a value from the dictionary by a key as shown below:

*Memo:

• The 1st argument is key(Required-Type:Any):
  • Don't use key=.
• The 2nd argument is default(Optional-Default:None-Type:Any) to return it if key doesn't exist:
  • Don't use default=.

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

print(v.get('name'))
# John

print(v.get('gender'))
# None

print(v.get('gender', "Doesn't exist!"))
# Doesn't exist!

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}

print(v.get('person2'))
# {'name': 'Anna', 'age': 24}

print(v['person2'].get('name'))
# Anna

update() can update the dictionary 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 **other(Optional-Default:{}-Type:Any):
  • Don't use any keywords like **other=, other=, etc.

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

v.update({'name':'Emily', 'gender':'Female'})
v.update([('name', 'Emily'), ('gender', 'Female')])
v.update(name='Emily', gender='Female')
v.update({'name':'Emily'}, gender='Female')
v.update([('name', 'Emily')], gender='Female')
v |= {'name':'Emily', 'gender':'Female'} 
v |= [('name', 'Emily'), ('gender', 'Female')]

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

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}

v.update({'person1':{'name':'Emily', 'gender':'Female'}})
v.update([('person1', {'name':'Emily', 'gender':'Female'})])
v.update(person1={'name':'Emily', 'gender':'Female'})

v['person2'].update({'name':'Tom', 'gender':'Male'})
v['person2'].update([('name', 'Tom'), ('gender', 'Male')])
v['person2'].update(name='Tom', gender='Male')

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

pop() can remove a pair of a key and value by key from the dictionary, throwing the value as shown below:

*Memo:

• The 1st argument is key(Required-Type:Hashable).
• The 2nd argument is default(Optional-Type:Any) to return its value if key doesn't exist:
  • It has no default value.
  • Don't use default=.
• If key doesn't exist and default isn't set, error occurs.

v = {'fname':'John', 'lname':'Smith', 'age':36}

print(v.pop('fname')) # John
print(v)              # {'lname': 'Smith', 'age': 36}

print(v.pop('age')) # 36
print(v)            # {'lname': 'Smith'}

print(v.pop('gender', "Doesn't exist!")) # Doesn't exist!
print(v)                                 # {'lname': 'Smith'}

print(v.pop('gender'))
# KeyError: 'gender'

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}

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

print(v['person2'].pop('name')) # Anna
print(v)                        # {'person2': {'age': 24}}

popitem() can remove the last pair of a key and value from the dictionary, throwing a tuple of the key and value as shown below:

*Memo:

• It has no arguments.
• Error occurs if the dictionary is empty.

v = {'name':'John', 'age':36, 'gender':'Male'}

print(v.popitem()) # ('gender', 'Male')
print(v)           # {'name': 'John', 'age': 36}

print(v.popitem()) # ('age', 36)
print(v)           # {'name': 'John'}

print(v.popitem()) # ('name', 'John')
print(v)           # {}

print(v.popitem())
# KeyError: 'popitem(): dictionary is empty'

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}

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

print(v['person1'].popitem()) # ('age', 36)
print(v)                      # {'person1': {'name': 'John'}}

print(v['person1'].popitem()) # ('name', 'John')
print(v)                      # {'person1': {}}

print(v['person1'].popitem()) # ('name', 'John')
# KeyError: 'popitem(): dictionary is empty'

clear() can remove all pairs of a key and value from the dictionary as shown below:

*Memo:

• It has no arguments.

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

v.clear()
print(v)
# {}

v = {'person1':{'name':'John', 'age':36},
     'person2':{'name':'Anna', 'age':24}}

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

v.clear()
print(v)
# {}

fromkeys() can create a dictionary with pairs of a key and default value as shown below:

*Memo:

• The 1st argument is iterable(Required-Type:Iterable):
  • Don't use iterable=.
• The 2nd argument is value(Optional-Default:None-Type:Any or NoneType) to set its default value with the keys of iterable:
  • Don't use value=.

print(dict.fromkeys(['a', 'b', 'c']))
print(dict.fromkeys(['a', 'b', 'c'], None))
# {'a': None, 'b': None, 'c': None}

print(dict.fromkeys(['a', 'b', 'c'], 0))
# {'a': 0, 'b': 0, 'c': 0}

setdefault() can set a pair of a key and default value to the dictionary if the key doesn't exist in the dictionary as shown below:

*Memo:

• The 1st argument is key(Required-Type:Hashable):
  • Don't use key=.
• The 2nd argument is default(Optional-Default:None-Type:Any or NoneType) to set it with key:
  • Don't use default=.

v = {'name':'John'}

v.setdefault('age')
v.setdefault('age', None)
print(v)
# {'name': 'John', 'age': None}

v.setdefault('gender', 'Male')
print(v)
# {'name': 'John', 'age': None, 'gender': 'Male'}

v.setdefault('name', 'Emily')
v.setdefault('age', '36')
v.setdefault('gender', 'Female')

print(v)
# {'name': 'John', 'age': None, 'gender': 'Male'}

v = {'person1':{'name':'John'}}

v['person1'].setdefault('age')
print(v)
# {'person1': {'name': 'John', 'age': None}}

v.setdefault('person2')
print(v)
# {'person1': {'name': 'John', 'age': None}, 'person2': None}

v.setdefault('person3', {'name':'Tom'})
print(v)
# {'person1': {'name': 'John', 'age': None},
#  'person2': None,
#  'person3': {'name': 'Tom'}}

v['person3'].setdefault('age', '18')
print(v)
# {'person1': {'name': 'John', 'age': None},
#  'person2': None,
#  'person3': {'name': 'Tom', 'age': '18'}}

v['person1'].setdefault('age', 36)
v.setdefault('person2', {'name':'Anna'})
v['person3'].setdefault('name', 'David')

print(v)
# {'person1': {'name': 'John', 'age': None},
#  'person2': None,
#  'person3': {'name': 'Tom', 'age': '18'}}