Python Dictionary Iteration - Complete Guide with Examples

What You Will Learn

Iterating over keys — using keys() and direct iteration
Iterating over values — using values() method
Iterating over key-value pairs — using items() method
Ordered iteration — sorting and ordered iteration
Filtering — filtering during iteration
Modifying dictionaries — safe modification during iteration
Common mistakes — and how to avoid them

Introduction to Dictionary Iteration

Iteration is the process of accessing each element in a collection sequentially. In Python, dictionaries are iterable objects, but the iteration behavior is different from lists and tuples. Since dictionaries store key-value pairs, you can iterate over keys, values, or both.

The key characteristics of dictionary iteration are:

Keys iteration — iterating over keys using keys() or directly
Values iteration — iterating over values using values()
Items iteration — iterating over key-value pairs using items()
Order preservation — Python 3.7+ preserves insertion order

💡 Key concept: Dictionaries are optimized for key lookups, not for iteration. However, iteration over dictionaries is still efficient and commonly used.

Iterating Over Keys

Accessing All Keys

You can iterate over dictionary keys using the keys() method or by iterating directly over the dictionary.

# Direct iteration (iterates over keys)
person = {"name": "Alice", "age": 25, "city": "NYC"}
for key in person:
    print(f"Key: {key}")

# Output:
# Key: name
# Key: age
# Key: city

# Using keys() method (explicit)
for key in person.keys():
    print(f"Key: {key}")

# Converting keys to a list
keys_list = list(person.keys())
print(keys_list)  # ['name', 'age', 'city']

# Iterating and accessing values
for key in person:
    print(f"{key}: {person[key]}")

# Checking keys during iteration
for key in person:
    if key == "age":
        print(f"Found age key with value: {person[key]}")

Characteristics:

Direct iteration over a dictionary iterates over keys
keys() method is explicit and readable
Order is preserved (Python 3.7+)
Useful for checking key existence

Quick Check: What does direct iteration over a dictionary return? (Answer: Keys)

Iterating Over Values

Accessing All Values

The values() method provides a view of all values in the dictionary, which can be iterated over.

# Iterating over values
person = {"name": "Alice", "age": 25, "city": "NYC"}
for value in person.values():
    print(f"Value: {value}")

# Output:
# Value: Alice
# Value: 25
# Value: NYC

# Converting values to a list
values_list = list(person.values())
print(values_list)  # ['Alice', 25, 'NYC']

# Calculating sum of numeric values
scores = {"math": 85, "science": 92, "english": 78}
total = sum(scores.values())
print(f"Total score: {total}")  # 255

# Finding maximum value
max_score = max(scores.values())
print(f"Highest score: {max_score}")  # 92

# Filtering values
for score in scores.values():
    if score >= 80:
        print(f"Good score: {score}")

Characteristics:

values() returns a view of all values
Order is preserved (Python 3.7+)
Can contain duplicate values
Useful for performing operations on values

Quick Check: Can values() contain duplicate values? (Answer: Yes, because values are not required to be unique)

Iterating Over Key-Value Pairs

Accessing Both Keys and Values

The items() method is the most commonly used method for iterating over dictionaries. It returns key-value pairs as tuples.

# Iterating over items
person = {"name": "Alice", "age": 25, "city": "NYC"}
for key, value in person.items():
    print(f"{key}: {value}")

# Output:
# name: Alice
# age: 25
# city: NYC

# Using items() in comprehensions
scores = {"math": 85, "science": 92, "english": 78}
passed = {subject: score for subject, score in scores.items() if score >= 80}
print(passed)  # {'math': 85, 'science': 92}

# Iterating and modifying values
for key, value in scores.items():
    print(f"{key}: {value + 5}")  # Adding 5 to each score

# Unpacking items
for subject, score in scores.items():
    if score >= 90:
        print(f"{subject}: Excellent!")

# Using items() with enumerate
for i, (key, value) in enumerate(scores.items()):
    print(f"{i+1}. {key}: {value}")

Characteristics:

items() returns a view of (key, value) tuples
Most common and Pythonic way to iterate
Allows unpacking in the loop
Order is preserved (Python 3.7+)

Quick Check: What does items() return when iterated? (Answer: (key, value) tuples)

Ordered Iteration

Iterating in a Specific Order

In Python 3.7+, dictionaries preserve insertion order. You can also use sorted() to iterate in a custom order.

# Insertion order is preserved
person = {"name": "Alice", "age": 25, "city": "NYC"}
for key in person:
    print(key)  # name, age, city

# Iterating in sorted order (by keys)
scores = {"math": 85, "science": 92, "english": 78}
for subject in sorted(scores.keys()):
    print(f"{subject}: {scores[subject]}")
# english: 78
# math: 85
# science: 92

# Iterating in reverse order
for key in reversed(list(person.keys())):
    print(f"{key}: {person[key]}")
# city: NYC
# age: 25
# name: Alice

# Sorting by values
scores = {"math": 85, "science": 92, "english": 78}
for subject, score in sorted(scores.items(), key=lambda item: item[1]):
    print(f"{subject}: {score}")
# english: 78
# math: 85
# science: 92

# Sorting by values (descending)
for subject, score in sorted(scores.items(), key=lambda item: item[1], reverse=True):
    print(f"{subject}: {score}")
# science: 92
# math: 85
# english: 78

Characteristics:

Python 3.7+ preserves insertion order
Use sorted() for custom ordering
Use reversed() for reverse order
Sorting by values requires items() and key parameter

Quick Check: Does Python 3.7+ preserve dictionary order? (Answer: Yes — insertion order is preserved)

Filtering During Iteration

Iterating with Conditions

You can filter dictionary items during iteration using conditional statements or comprehensions.

# Using if statements
scores = {"math": 85, "science": 92, "english": 78}
for subject, score in scores.items():
    if score >= 80:
        print(f"Passing: {subject} - {score}")

# Using dictionary comprehension
passed = {subject: score for subject, score in scores.items() if score >= 80}
print(passed)  # {'math': 85, 'science': 92}

# Filtering keys
person = {"name": "Alice", "age": 25, "city": "NYC", "email": "alice@example.com"}
for key in person:
    if key.startswith('a'):
        print(f"{key}: {person[key]}")

# Filtering values
for key, value in person.items():
    if isinstance(value, str):
        print(f"String: {key} -> {value}")

# Complex filtering
for subject, score in scores.items():
    if score >= 85 and subject.startswith('s'):
        print(f"{subject}: {score}")

Methods:

Conditional statements — iterate with if conditions
Dictionary comprehension — create filtered dictionaries
Key filtering — filter by key properties
Value filtering — filter by value properties

Modifying Dictionaries During Iteration

Safe Modification Techniques

Modifying a dictionary during iteration can cause errors. Here are safe ways to do it.

# WRONG — modifies during iteration (raises RuntimeError)
scores = {"math": 85, "science": 92, "english": 78}
# for subject in scores:
#     if scores[subject] < 80:
#         del scores[subject]  # RuntimeError: dictionary changed size during iteration

# CORRECT — iterate over a copy of keys
scores = {"math": 85, "science": 92, "english": 78}
for subject in list(scores.keys()):
    if scores[subject] < 80:
        del scores[subject]
print(scores)  # {'math': 85, 'science': 92}

# CORRECT — create a new dictionary
scores = {"math": 85, "science": 92, "english": 78}
filtered = {k: v for k, v in scores.items() if v >= 80}
print(filtered)  # {'math': 85, 'science': 92}

# CORRECT — using a list of keys to remove
to_remove = [subject for subject, score in scores.items() if score < 80]
for subject in to_remove:
    del scores[subject]
print(scores)  # {'math': 85, 'science': 92}

# Updating values during iteration (safe)
scores = {"math": 85, "science": 92, "english": 78}
for subject in scores:
    scores[subject] += 5  # Safe — updating values doesn't change size
print(scores)  # {'math': 90, 'science': 97, 'english': 83}

Guidelines:

Iterate over a copy — list(dict.keys())
Use comprehension — create a new dictionary
Collect keys to remove — then remove after iteration
Updating values — safe during iteration

Quick Check: What happens if you modify a dictionary during iteration? (Answer: RuntimeError: dictionary changed size during iteration)

Common Mistakes

Watch Out For These!

Mistake 1: Modifying Dictionary During Iteration

# WRONG — raises RuntimeError
scores = {"math": 85, "science": 92, "english": 78}
# for subject in scores:
#     if scores[subject] < 80:
#         del scores[subject]  # RuntimeError

# CORRECT — iterate over a copy
for subject in list(scores.keys()):
    if scores[subject] < 80:
        del scores[subject]

Mistake 2: Forgetting .items() When Unpacking

# WRONG — iterating over keys only
person = {"name": "Alice", "age": 25}
# for key, value in person:  # ValueError: too many values to unpack

# CORRECT — use items()
for key, value in person.items():
    print(f"{key}: {value}")

Mistake 3: Assuming Order in Older Python Versions

# WRONG — order is not guaranteed in Python 3.6 and earlier
my_dict = {"a": 1, "b": 2, "c": 3}
for key in my_dict:
    print(key)  # Order may vary

# CORRECT — use OrderedDict if order is critical
from collections import OrderedDict
ordered = OrderedDict([("a", 1), ("b", 2), ("c", 3)])
for key in ordered:
    print(key)  # a, b, c

Quick Check: What is the most common mistake when iterating over dictionaries? (Answer: Modifying the dictionary during iteration)

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Python Code Editor

# Python Dictionary Iteration Practice

print("=" * 40)
print("DICTIONARY ITERATION PRACTICE")
print("=" * 40)

# Create a dictionary
person = {"name": "Alice", "age": 25, "city": "NYC", "email": "alice@example.com"}
scores = {"math": 85, "science": 92, "english": 78}

print("Person:", person)
print("Scores:", scores)

# 1. Iterating over keys
print("\n1. ITERATING OVER KEYS")
for key in person:
    print(f"  Key: {key}")

# 2. Iterating over values
print("\n2. ITERATING OVER VALUES")
for value in person.values():
    print(f"  Value: {value}")

# 3. Iterating over items
print("\n3. ITERATING OVER ITEMS")
for key, value in person.items():
    print(f"  {key}: {value}")

# 4. Ordered iteration
print("\n4. ORDERED ITERATION")
for subject in sorted(scores.keys()):
    print(f"  {subject}: {scores[subject]}")

# 5. Filtering
print("\n5. FILTERING")
for subject, score in scores.items():
    if score >= 80:
        print(f"  Passing: {subject} - {score}")

# 6. Dictionary comprehension
print("\n6. DICTIONARY COMPREHENSION")
filtered_scores = {subject: score for subject, score in scores.items() if score >= 80}
print(f"  Filtered scores: {filtered_scores}")

print("\nDictionary iteration practice complete!")

========================================
DICTIONARY ITERATION PRACTICE
========================================

Person: {'name': 'Alice', 'age': 25, 'city': 'NYC', 'email': 'alice@example.com'}
Scores: {'math': 85, 'science': 92, 'english': 78}

1. ITERATING OVER KEYS
Key: name
Key: age
Key: city
Key: email

2. ITERATING OVER VALUES
Value: Alice
Value: 25
Value: NYC
Value: alice@example.com

3. ITERATING OVER ITEMS
name: Alice
age: 25
city: NYC
email: alice@example.com

4. ORDERED ITERATION
english: 78
math: 85
science: 92

5. FILTERING
Passing: math - 85
Passing: science - 92

6. DICTIONARY COMPREHENSION
Filtered scores: {'math': 85, 'science': 92}

Dictionary iteration practice complete!

🏆

Certificate of Completion

You have completed the Python Dictionary Iteration tutorial. You understand iterating over keys, values, items, ordered iteration, filtering, and modifying dictionaries during iteration.

Quick Quiz — Test Your Knowledge

Let's see what you've learned about dictionary iteration:

1. What does direct iteration over a dictionary return?

A) Values B) Keys C) Key-value pairs D) Length

2. Which method returns key-value pairs for iteration?

A) keys() B) values() C) items() D) get()

3. Does Python 3.7+ preserve dictionary insertion order?

A) No B) Yes C) Only if ordered=True D) Never

4. What happens if you modify a dictionary during iteration?

A) Nothing B) ValueError C) RuntimeError D) TypeError

5. How do you safely remove items from a dictionary during iteration?

A) Directly remove during iteration B) Iterate over a copy of keys C) Use pop() during iteration D) It's not possible

Frequently Asked Questions

What is the most Pythonic way to iterate over a dictionary?

The most Pythonic way is to use for key, value in dict.items(): when you need both keys and values. Use for key in dict: when you only need keys.

Can I modify a dictionary while iterating over it?

You cannot modify the size of a dictionary during iteration (adding or removing keys). Updating existing values is safe. To remove items, iterate over a copy of the keys: for key in list(dict.keys()):.

Does dictionary iteration preserve order?

Yes, in Python 3.7 and later, dictionaries preserve insertion order. In earlier versions, order was not guaranteed. Use OrderedDict if order is critical.

How do I iterate over a dictionary in sorted order?

Use sorted(): for key in sorted(dict.keys()): for keys, or for key, value in sorted(dict.items()): for both keys and values.

What is the difference between keys(), values(), and items()?

keys() returns only keys, values() returns only values, and items() returns key-value pairs as tuples. All return view objects that reflect changes to the dictionary.

How do I filter a dictionary during iteration?

Use dictionary comprehension: {k: v for k, v in dict.items() if condition}, or use a loop with an if statement and collect items to keep.

Where to Go From Here

After mastering dictionary iteration, consider exploring these related topics:

Previous: Dictionary Methods Next: Formatting Dictionaries →

Python

Python Dictionary: Iteration (Looping)

Introduction to Dictionary Iteration

Iterating Over Keys

Accessing All Keys

Iterating Over Values

Accessing All Values

Iterating Over Key-Value Pairs

Accessing Both Keys and Values

Ordered Iteration

Iterating in a Specific Order

Filtering During Iteration

Iterating with Conditions

Modifying Dictionaries During Iteration

Safe Modification Techniques

Common Mistakes

Watch Out For These!

Mistake 1: Modifying Dictionary During Iteration

Mistake 2: Forgetting .items() When Unpacking

Mistake 3: Assuming Order in Older Python Versions

Interactive Editor

Certificate of Completion

Quick Quiz — Test Your Knowledge

Frequently Asked Questions

What is the most Pythonic way to iterate over a dictionary?

Can I modify a dictionary while iterating over it?

Does dictionary iteration preserve order?

How do I iterate over a dictionary in sorted order?

What is the difference between keys(), values(), and items()?

How do I filter a dictionary during iteration?

Where to Go From Here

Formatting Dictionaries

Nested Dictionaries

Dictionary Comprehension

Python

Python Dictionary: Iteration (Looping)

Introduction to Dictionary Iteration

Iterating Over Keys

Accessing All Keys

Iterating Over Values

Accessing All Values

Iterating Over Key-Value Pairs

Accessing Both Keys and Values

Ordered Iteration

Iterating in a Specific Order

Filtering During Iteration

Iterating with Conditions

Modifying Dictionaries During Iteration

Safe Modification Techniques

Common Mistakes

Watch Out For These!

Mistake 1: Modifying Dictionary During Iteration

Mistake 2: Forgetting .items() When Unpacking

Mistake 3: Assuming Order in Older Python Versions

Interactive Editor

Certificate of Completion

Quick Quiz — Test Your Knowledge

Frequently Asked Questions

What is the most Pythonic way to iterate over a dictionary? ▼

Can I modify a dictionary while iterating over it? ▼

Does dictionary iteration preserve order? ▼

How do I iterate over a dictionary in sorted order? ▼

What is the difference between keys(), values(), and items()? ▼

How do I filter a dictionary during iteration? ▼

Where to Go From Here

Formatting Dictionaries

Nested Dictionaries

Dictionary Comprehension

What is the most Pythonic way to iterate over a dictionary?

Can I modify a dictionary while iterating over it?

Does dictionary iteration preserve order?

How do I iterate over a dictionary in sorted order?

What is the difference between keys(), values(), and items()?

How do I filter a dictionary during iteration?