Master Python Dictionaries: Creation, Access, and Manipulation Techniques

Introduction

Dictionary is a built‑in Python data structure that provides a semantic way to store structured data and is widely used in development.

Dictionary Overview

1. Dictionaries are created with curly braces {} or the dict function and can be nested.

2. They consist of key‑value pairs.

3. Keys and values are separated by a colon :, and pairs are separated by commas ,.

4. Keys must be unique, while values may repeat.

5. Keys should not be Chinese or other non‑ASCII characters, following common conventions.

Creating Dictionaries

Dictionaries can be created using curly braces {} or the dict function.

1. Using curly braces

emp = {'name':'张三', 'age':22, 'sex':'男'}
print(emp)
# {'name': '张三', 'age': 22, 'sex': '男'}
print(type(emp))
# <class 'dict'>

2-1. Using dict function

The dict function creates a dictionary without needing quotes around the keys.

emp = dict(name='张三', age=22, sex='男')
print(emp)
# {'name': '张三', 'age': 22, 'sex': '男'}

2-2. Using fromkeys method

The fromkeys method together with dict creates a dictionary with keys from a sequence.

emp = dict.fromkeys(['name','age','sex'])
print(emp)
# {'name': None, 'age': None, 'sex': None}

If a default value is needed, pass it as the second argument:

emp = dict.fromkeys(['name','age','sex'], 'N/A')
print(emp)
# {'name': 'N/A', 'age': 'N/A', 'sex': 'N/A'}

Accessing Dictionary Values

Two ways to retrieve values.

1. Bracket notation

emp = {'name':'张三', 'age':22, 'sex':'男'}
print(emp['name'])
# 张三

Note: Accessing a non‑existent key raises KeyError.

2. get method

emp = {'name':'张三', 'age':22, 'sex':'男'}
v = emp.get('name')
print(v)  # 张三
v = emp.get('dept', '其他部门')
print(v)  # 其他部门

Dictionary Write Operations

Update, add, and delete entries.

Update

Python follows “update if exists, otherwise add”.

1. Update a value

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
emp['dept'] = '推广部'
print(emp)
# {'name': '张三', 'age': 22, 'sex': '男', 'dept': '推广部'}

2. Bulk update with update()

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
emp.update(age=18, dept='推广部')
print(emp)
# {'name': '张三', 'age': 18, 'sex': '男', 'dept': '推广部'}

Add

Adding follows the same “update if exists, otherwise add” rule.

1. Add a new key

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
emp['job'] = '销售'
print(emp)
# {'name': '张三', 'age': 22, 'sex': '男', 'dept': '研发部', 'job': '销售'}

2. Bulk add with update()

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
emp.update(dept='推广部', job='推广员')
print(emp)
# {'name': '张三', 'age': 22, 'sex': '男', 'dept': '推广部', 'job': '推广员'}

Delete

1. pop() removes a specific key and returns its value

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
dept = emp.pop('dept')
print(emp)   # {'name': '张三', 'age': 22, 'sex': '男'}
print(dept)  # 研发部

2. popitem() removes and returns the last key‑value pair

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
kv = emp.popitem()
print(emp)  # {'name': '张三', 'age': 22, 'sex': '男'}
print(kv)   # ('dept', '研发部')

3. clear() empties the dictionary

emp = {'name':'张三', 'age':22, 'sex':'男', 'dept':'研发部'}
emp.clear()
print(emp)  # {}