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Dictionary Methods

Introduction

Python dictionaries come with several built-in methods that make iterating and safely accessing data much cleaner. Mastering these methods will make your interview code faster to write and less prone to bugs.

Safely Accessing Values: get()

Instead of checking if a key exists before accessing it, you can use the get() method.

If the key exists, get() returns the value. If it doesn't, it returns None (or a default value you specify), avoiding a KeyError.

counts = {"a": 1, "b": 2}

# Avoids KeyError, returns None
print(counts.get("c")) 

# Returns a default value of 0 if "c" is missing
print(counts.get("c", 0)) # 0

When to use get()

Use get(key, default) when incrementing counters or building frequency maps manually.

# Increment count of char, defaulting to 0 if not seen yet
counts[char] = counts.get(char, 0) + 1

Iterating Over Dictionaries

By default, iterating over a dictionary yields its keys.

person = {"name": "Alice", "age": 25}

for key in person:
    print(key) 
# "name"
# "age"

1. keys()

Returns a view object of the dictionary's keys. Rarely needed in loops since iterating the dictionary directly does the same thing, but useful for set operations.

print(person.keys()) # dict_keys(['name', 'age'])

2. values()

Returns a view object of the dictionary's values.

for val in person.values():
    print(val)
# "Alice"
# 25

3. items() (Most Important)

Returns a view object yielding (key, value) tuples. Always use this when you need both the key and the value in a loop.

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

Common Interview Mistake: Manual Value Lookup

Many candidates iterate over the keys and manually look up the value. This is un-Pythonic and technically requires an extra hash table lookup per iteration.

Avoid: 

for key in person:
    value = person[key] # Extra O(1) lookup
    print(key, value)

Prefer: 

for key, value in person.items():
    print(key, value)

Safely Updating Values: setdefault()

If you are building a dictionary where values are lists (like an adjacency list for a graph), you often need to initialize the list before appending to it.

graph = {}

# The verbose way
if "A" not in graph:
    graph["A"] = []
graph["A"].append("B")

You can do this in one line using setdefault(). It checks if the key exists; if not, it sets it to the default value, and then returns the value.

graph = {}

# The concise way
graph.setdefault("A", []).append("B")
(Note: While setdefault() is useful, collections.defaultdict is usually preferred in interviews. We will cover defaultdict in the Collections section).

Summary

  • Use get(key, default) to safely retrieve values without KeyErrors.
  • Use items() to iterate over both keys and values simultaneously.
  • Use setdefault(key, default) to initialize complex values like lists or sets before modifying them.