Detailed Explanation of Python Dictionary Methods

Detailed Explanation of Python Dictionary Methods

Dictionary Object Methods

clear()

  • Function: Clears all key-value pairs in the dictionary
  • Example:
# Example 1: Clear an existing dictionary
d = {'a':1, 'b':2}
d.clear()
print(d)  # {}

# Example 2: Operation on an empty dictionary
d = {}
d.clear()
print(d)  # {}

# Example 3: Clear and then add new elements
d = {'x':10}
d.clear()
d['y'] = 20
print(d)  # {'y':20}

copy()

  • Function: Creates a shallow copy of the dictionary
  • Example:
# Example 1: Simple copy
d1 = {'a':1, 'b':[2,3]}
d2 = d1.copy()
d2['a'] = 10
print(d1)  # {'a':1, 'b':[2,3]}

# Example 2: Nested object impact
d1['b'].append(4)
print(d2)  # {'a':10, 'b':[2,3,4]}

# Example 3: Deep copy
dimport copy
d3 = copy.deepcopy(d1)
d1['b'].append(5)
print(d3)  # {'a':1, 'b':[2,3,4]}

fromkeys(iterable[, value])

  • Function: Creates a new dictionary from an iterable
  • Example:
# Example 1: Default value None
keys = ['a','b','c']
print(dict.fromkeys(keys))  # {'a':None, 'b':None, 'c':None}

# Example 2: Specified default value
print(dict.fromkeys(keys, 0))  # {'a':0, 'b':0, 'c':0}

# Example 3: Mutable object trap
vals = []
d = dict.fromkeys(keys, vals)
d['a'].append(1)
print(d)  # {'a':[1], 'b':[1], 'c':[1]}

get(key[, default])

  • Function: Safely retrieves the value for a key
  • Example:
# Example 1: Key exists
d = {'a':1, 'b':2}
print(d.get('a'))  # 1

# Example 2: Key does not exist
print(d.get('c', 'Not Found'))  # Not Found

# Example 3: Retrieve from nested dictionary
user = {'profile': {'name':'John'}}
print(user.get('profile', {}).get('name'))  # John

items()

  • Function: Returns a view of the dictionary’s items
  • Example:
# Example 1: Iterate over dictionary items
d = {'a':1, 'b':2}
for k,v in d.items():
    print(f'{k}:{v}')  

# Example 2: View updates dynamically
d = {'x':10}
view = d.items()
d['y'] = 20
print(list(view))  # dict_items([('x',10), ('y',20)])

# Example 3: Set operations
view1 = {'a':1, 'b':2}.items()
view2 = {'b':2, 'a':1}.items()
print(view1 == view2)  # True

keys()

  • Function: Returns a view of the dictionary’s keys
  • Example:
# Example 1: Get all keys
d = {'a':1, 'b':2}
print(list(d.keys()))  # ['a','b']

# Example 2: Check if key exists
print('a' in d.keys())  # True

# Example 3: View updates dynamically
d['c'] = 3
print(list(d.keys()))  # ['a','b','c']

pop(key[, default])

  • Function: Deletes and returns the value for the specified key
  • Example:
# Example 1: Normal deletion
d = {'a':1, 'b':2}
print(d.pop('a'))  # 1
print(d)  # {'b':2}

# Example 2: Key does not exist with default value
print(d.pop('c', 'Default Value'))  # Default Value

# Example 3: Key does not exist without default value
try:
    d.pop('c')
except KeyError as e:
    print(e)  # 'c'

popitem()

  • Function: Deletes and returns the last inserted key-value pair
  • Example:
# Example 1: Last in, first out
d = {'a':1, 'b':2}
print(d.popitem())  # ('b',2)
print(d.popitem())  # ('a',1)

# Example 2: Operation on an empty dictionary
try:
    {}.popitem() 
except KeyError:
    print('Dictionary is empty')

# Example 3: Insertion order impact
d = {}
d['x'] = 10
d['y'] = 20
print(d.popitem())  # ('y',20)

setdefault(key[, default])

  • Function: Safely sets a key-value pair
  • Example:
# Example 1: Key does not exist
d = {}
print(d.setdefault('a',1))  # 1
print(d)  # {'a':1}

# Example 2: Key exists
d = {'a':10}
print(d.setdefault('a',1))  # 10

# Example 3: Application in nested dictionaries
data = {}
for item in ['apple','banana','apple']:
    data.setdefault(item, 0)
    data[item] +=1
print(data)  # {'apple':2, 'banana':1}

update([other])

  • Function: Batch updates the dictionary
  • Example:
# Example 1: Merge dictionaries
d = {'a':1}
d.update({'b':2, 'c':3})
print(d)  # {'a':1, 'b':2, 'c':3}

# Example 2: List of key-value pairs
d.update([('d',4), ('e',5)])
print(d)  # {'a':1, 'b':2, 'c':3, 'd':4, 'e':5}

# Example 3: Keyword arguments
d.update(f=6, g=7)
print(d)  # {..., 'f':6, 'g':7}

values()

  • Function: Returns a view of the dictionary’s values
  • Example:
# Example 1: Get all values
d = {'a':1, 'b':2}
print(list(d.values()))  # [1,2]

# Example 2: Check if value exists
print(2 in d.values())  # True

# Example 3: View updates dynamically
d['c'] = 3
print(list(d.values()))  # [1,2,3]

Adding Elements

  • Function: Adds key-value pairs to the dictionary
  • Example:
# Example 1: Direct assignment
colors = {}
colors['red'] = '#FF0000'
print(colors)  # {'red': '#FF0000'}

# Example 2: Merge using update method
info = {'name': 'Alice'}
info.update({'age': 25, 'city': 'Beijing'})
print(info)  # {'name': 'Alice', 'age': 25, 'city': 'Beijing'}

# Example 3: Automatically add using setdefault
data = {}
data.setdefault('counter', 0)
data['counter'] += 1
print(data)  # {'counter': 1}

Modifying Elements

  • Function: Updates the value of an existing key
  • Example:
# Example 1: Direct modification
inventory = {'apples': 10}
inventory['apples'] = 15
print(inventory)  # {'apples': 15}

# Example 2: Batch update
config = {'debug': False, 'log_level': 'info'}
config.update({'debug': True, 'timeout': 30})
print(config)  # {'debug': True, 'log_level': 'info', 'timeout': 30}

# Example 3: Conditional modification
temperatures = {'room1': 18, 'room2': 22}
for room in temperatures:
    if temperatures[room] > 20:
        temperatures[room] -= 2
print(temperatures)  # {'room1': 18, 'room2': 20}

Iteration Operations

  • Function: Access all elements in the dictionary
  • Example:
# Example 1: Iterate over keys
student = {'name': 'Bob', 'age': 20, 'major': 'CS'}
for key in student:
    print(key)

# Example 2: Iterate over values
for value in student.values():
    print(value)

# Example 3: Iterate over key-value pairs
for k, v in student.items():
    print(f'{k}: {v}')

Dictionary Comprehensions

  • Function: Quickly generate dictionary expressions
  • Example:
# Example 1: Simple comprehension
squares = {x: x**2 for x in range(5)}
print(squares)  # {0:0, 1:1, 2:4, 3:9, 4:16}

# Example 2: Conditional filtering
even_squares = {k:v for k,v in squares.items() if v%2 ==0}
print(even_squares)  # {0:0, 2:4, 4:16}

# Example 3: Key-value transformation
d = {'a':1, 'b':2}
flipped = {v:k for k,v in d.items()}
print(flipped)  # {1:'a', 2:'b'}

Ordered Dictionary (OrderedDict)

from collections import OrderedDict

# Example 1: Maintain order
od = OrderedDict()
od['z'] = 3
od['a'] = 1
print(list(od.keys()))  # ['z','a']

# Example 2: Order operations
od.move_to_end('z')
print(list(od.keys()))  # ['a','z']

# Example 3: Equality check
d1 = {'a':1, 'b':2}
d2 = {'b':2, 'a':1}
print(d1 == d2)  # True
print(OrderedDict(d1) == OrderedDict(d2))  # False

Dictionary Merge Operator

# Example 1: Basic merge
d1 = {'a':1, 'b':2}
d2 = {'b':3, 'c':4}
print(d1 | d2)  # {'a':1, 'b':3, 'c':4}

# Example 2: Chained merge
d3 = {'d':5}
print(d1 | d2 | d3)  # {..., 'd':5}

# Example 3: In-place update
d1 |= d2
print(d1)  # {'a':1, 'b':3, 'c':4}

Dictionary View Set Operations

d1 = {'a':1, 'b':2}
d2 = {'b':2, 'c':3}

# Example 1: Key intersection
common_keys = d1.keys() & d2.keys()
print(common_keys)  # {'b'}

# Example 2: Item union
all_items = d1.items() | d2.items()
print(all_items)  # {('a',1), ('b',2), ('b',2), ('c',3)}

# Example 3: Value difference
unique_values = d1.values() - d2.values()
print(unique_values)  # {1}

Dictionary and JSON Conversion

import json

# Example 1: Dictionary to JSON
data = {'name':'Alice', 'age':30, 'skills':['Python','SQL']}
json_str = json.dumps(data, indent=2)
print(json_str)  

# Example 2: JSON to dictionary
restored = json.loads(json_str)
print(restored['skills'])  # ['Python','SQL']

# Example 3: Special type handling
from datetime import datetime
data = {'time': datetime.now()}

class DateTimeEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        return super().default(obj)

print(json.dumps(data, cls=DateTimeEncoder))

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