Building Histograms with Dictionaries

From Items to Counts

A histogram built with a dictionary records how often items occur. Each observed item becomes a dictionary key, and its value records the number of times that item has appeared. The main difficulty is the first occurrence: before an item has been counted, its key may not exist in the dictionary. Python's get() method handles that missing-key case by returning a default value.

How get() Chooses a Value

The get() method looks up a key in a dictionary. If the key exists, get() retrieves the value associated with that key. If the key is missing, get() returns the default value supplied to it. This makes get() useful when a program cannot assume that every key has already been added.

counts = {"red": 2}

existing_count = counts.get("red", 0)
missing_count = counts.get("blue", 0)

print(existing_count)
print(missing_count)

2
0

The first lookup finds the existing key red, so the stored value 2 is returned. The second lookup cannot find blue, so the default 0 is returned. The default does not represent a newly stored key by itself; it is the value produced by the lookup when the key is absent.

The Counting Idiom

counts[item] = counts.get(item, 0) + 1

Refactoring the Loop

Before get() is used, a counting loop commonly needs explicit logic for the two cases: the key is already present, or the key is missing. The get() idiom puts both cases into one assignment. The result is more concise and is described in the source material as more Pythonic and less error-prone when keys may not exist.

The two versions express the same counting idea. In the longer version, the missing-key case assigns 1 directly. In the shorter version, the missing-key case starts at 0 and then adds 1. For an existing key, both versions increase the previous count by 1.

Mistakes with Missing Keys

• Using bracket notation as though every key already exists

  The counting expression assumes that item already has a stored count, but the first occurrence may arrive before that key has been added.

  Fix: Use counts[item] = counts.get(item, 0) + 1 so a missing key starts from the default count 0.

• Forgetting to add 1 after retrieving the count

  The lookup retrieves the old count or the default, but it does not increase the total for the current occurrence.

  Fix: Add 1 before storing the result: counts[item] = counts.get(item, 0) + 1.

• Using a default other than 0 for a new count

  A new item would begin at 1 and then be increased again, so its first occurrence would be counted as 2.

  Fix: Use 0 as the default starting count, then add 1 for the current item.

Practice the Pattern

items = ["cat", "dog", "cat", "bird", "dog", "cat"]
counts = {}

for item in items:
    counts[item] = counts.get(item, 0) + 1

print(counts)

{"cat": 3, "dog": 2, "bird": 1}

Key Takeaways

1. get() returns the stored value when a dictionary key exists.
2. When the key is missing, get() returns the supplied default value.
3. The counting idiom d[c] = d.get(c, 0) + 1 handles first and later occurrences in one line.
4. A dictionary histogram maps each observed item to its occurrence count.
5. Using get() avoids a separate missing-key branch and is more Pythonic and less error-prone for this counting pattern.