Master Python Dictionaries: Creation, Modification, and Common Operations

This article is the second part of a series on Python data structures, focusing on dictionaries (dict), one of the most flexible built‑in container types.

Characteristics of dictionaries

Mutable container that can store objects of any type.

Key‑value pairs are written as key: value and separated by commas inside curly braces {}.

Keys must be unique and immutable (e.g., strings, numbers, tuples); values can be any type.

Accessed via keys rather than positional indices.

Unordered collection, unlike lists which are ordered.

Supports nesting, allowing dictionaries within dictionaries.

Creating, modifying, adding, clearing, and deleting

dict = {"class": "5", "name": "莉莉", "年龄": "12"}
print(dict)  # {'class': '5', 'name': '莉莉', '年龄': '12'}

# Access a value
print(dict["name"])  # 莉莉

# Update a value
dict["class"] = "6"

# Add a new key‑value pair
dict["年龄"] = "7"

# Add another key‑value pair
dict["father"] = "wangxiao"

# Delete a specific key‑value pair
del dict["father"]

# Clear all contents
dict.clear()
print(dict)  # {}

# Delete the dictionary itself
del dict
print(dict)  # NameError: name 'dict' is not defined

Common dictionary operations

# Number of items
len(dict)  # 3

# Convert to printable string
str(dict)  # "{'class': '5', '年龄': '12', 'name': '莉莉'}"

# Iterate over keys and values
for key, value in dict.items():
    print(key)
    print(value)

# Get a value safely
dict.get("class")  # '5'

# Check existence of a key (Python 3)
"class" in dict  # True

# View keys, values, items
dict.keys()   # dict_keys([...])
dict.values() # dict_values([...])
dict.items()  # dict_items([...])

# Merge another dictionary
dict2 = {"mother": "feifei"}
dict.update(dict2)

# Remove a key and get its value
dict.pop("mother")  # 'feifei'

Comparison and version differences

In Python 2 the cmp() function could compare two dictionaries, but it was removed in Python 3. To compare dictionaries now, import operator and use operator.eq(dict1, dict2). The article demonstrates this change and shows how the result differs when a key’s value is altered.

The author notes that the previous article covered lists, and the next installment will explore immutable sequences (tuples), which offer additional capabilities beyond lists.