Loop Through a Dictionary in Python

Loop Through the Keys of a Dictionary

A for loop over a dictionary goes through its keys, one at a time, in the order they were added. Inside the loop, d[key] gives you the value that goes with each key.

Think of it like a waiter reading the menu card from top to bottom: each dish name is a key, and the price next to it is the value.

123prices = {"tea": 15, "coffee": 25, "juice": 40}
for item in prices:
    print(item, "costs", prices[item])

The rules

• for key in d: gives you the keys, not the values and not the pairs.
• The order is the order the keys were added, so the output is always the same.
• Use d[key] inside the loop to get the value. The key is surely there, so no KeyError.
• Name the loop variable after what the keys are: item, name, word.

keys() says the same thing

d.keys() gives the keys too. for item in prices.keys(): works exactly like for item in prices:. The short version is more common, but some people like how clearly keys() reads:

123ages = {"Sunny": 25, "Rocky": 30}
for name in ages.keys():
    print(name)

Combine with if

An if inside the loop picks out some of the pairs, like the counting loops of chapter 10:

1234prices = {"tea": 15, "coffee": 25, "juice": 40}
for item in prices:
    if prices[item] > 20:
        print(item)

Avoid: adding or removing keys while you loop

123ages = {"Sunny": 25, "Rocky": 30}
for name in ages:
    ages["Lucky"] = 22

Python stops the loop when the dictionary grows or shrinks under it. Changing the value of an existing key is fine. To remove keys, collect them in a list first, then remove them after the loop.

for key in d: goes through the keys in order; d[key] gives each value.

Loop Over Dictionary Values with values()

The values() method gives you only the values of a dictionary, without the keys. Loop over it when the names don't matter, only the numbers or words stored under them.

Think of it like emptying your piggy bank onto the table: you count the coins, and you don't care which slot each one came from.

12345bill = {"pizza": 250, "drinks": 90, "dessert": 120}
total = 0
for cost in bill.values():
    total += cost
print("Total:", total)

That's the running total from chapter 10: start at 0 before the loop, add inside it, print after it.

sum(), min() and max() work too

You used these on lists in chapter 9. They also take values() directly:

1234marks = {"Sunny": 82, "Rocky": 67, "Lucky": 91}
print(sum(marks.values()))
print(max(marks.values()))
print(min(marks.values()))

The rules

• d.values() gives the values in the same order as the keys.
• The same value can appear more than once, unlike keys.
• in with values() checks the values: 91 in marks.values() is True.
• You can't get back to the key from a value. If you need the name too, use items(), the next lesson.

123marks = {"Sunny": 82, "Rocky": 67, "Lucky": 91}
print(91 in marks)
print(91 in marks.values())

Avoid: sum() on the dictionary itself

sum(marks) tries to add up the keys, which are strings:

12marks = {"Sunny": 82, "Rocky": 67}
print(sum(marks))

The message says Python tried to add a number and a string with +. Add .values() and it works.

Common beginner doubt: "What does print(marks.values()) show?" Something like dict_values([82, 67, 91]). It's a special view of the values, made for looping. Loop over it, or pass it to sum(), max() or min().

d.values() gives only the values: loop over it, or pass it to sum(), min() or max().

Loop Over Keys and Values with items()

The items() method gives you every pair of a dictionary: a key together with its value. In the for line, write two loop variables, and Python puts the key in the first and the value in the second.

Think of it like going through a stack of name badges with photos: on each one you see the name and the face at the same time.

123marks = {"Sunny": 82, "Rocky": 67, "Lucky": 91}
for name, mark in marks.items():
    print(name, "scored", mark)

The rules

• for key, value in d.items(): with a comma between the two names.
• The first name gets the key, the second gets the value, in every round.
• Choose names that say what they hold: name, mark or item, price, not a, b.
• The pairs come in the order the keys were added.

You'll learn more about pairs like these, called tuples, in chapter 14. For now, splitting each pair into two loop variables is all you need.

Neat output with f-strings

Pair items() with the f-string widths from chapter 12, and you get a tidy table:

123menu = {"tea": 15, "coffee": 25, "sandwich": 60}
for item, price in menu.items():
    print(f"{item:<10}{price:>4}")

Find the best key

With the key and value side by side, you can remember who had the top value, the way you found the largest number in chapter 10:

12345678marks = {"Sunny": 82, "Rocky": 67, "Lucky": 91}
best_name = ""
best_mark = -1
for name, mark in marks.items():
    if mark > best_mark:
        best_name = name
        best_mark = mark
print("Top:", best_name, best_mark)

Avoid: two variables without items()

Looping over the dictionary itself gives only keys, and one key can't be split into two names:

123ages = {"Sunny": 25, "Rocky": 30}
for name, age in ages:
    print(name, age)

The fix is to add .items() after ages.

for key, value in d.items(): gives you each key and its value together.

Count Things with a Dictionary

A dictionary makes a perfect counter: each thing you count is a key, and its value is how many times you've seen it. One loop counts every different thing at once.

Think of it like a tally sheet at an election: each name gets its own row, and every vote adds one mark to that row.

12345votes = ["Sunny", "Rocky", "Sunny", "Lucky", "Sunny", "Rocky"]
count = {}
for name in votes:
    count[name] = count.get(name, 0) + 1
print(count)

How the counting line works

count[name] = count.get(name, 0) + 1 does it all in one line:

1. count.get(name, 0) reads the count so far, or 0 if the name is new.
2. + 1 adds this vote.
3. count[name] = … stores the new count, adding the key if it's new.

The same with in

If the get() line looks too clever, in and if from this chapter do the same job:

1234567count = {}
for ch in "banana":
    if ch in count:
        count[ch] += 1
    else:
        count[ch] = 1
print(count)

The keys appear in the order each letter is first seen: b, then a, then n.

Print the counts nicely

Once counting is done, loop over items() to show the result:

12345count = {}
for word in "the cat and the dog and the bird".split():
    count[word] = count.get(word, 0) + 1
for word, n in count.items():
    print(word, n)

split() from chapter 12 cuts the sentence into a list of words. Then each word is counted.

Avoid: += on a new key

count[name] += 1 on its own fails the first time a name appears, with a KeyError, because there's no old value yet (topic 3). Use get(name, 0) + 1, or the in check.

Count with d[key] = d.get(key, 0) + 1: new keys start at 0, old ones go up by one.

Lists of Dictionaries and Dictionaries of Lists

Lists and dictionaries can hold each other. A list of dictionaries stores many records with the same keys, like a table. A dictionary of lists stores several values under each key.

Think of it like a class register: the register is a list, and each student's row is a dictionary with a name and a mark.

A list of dictionaries

123456students = [
    {"name": "Sunny", "mark": 82},
    {"name": "Rocky", "mark": 67},
]
for student in students:
    print(student["name"], "got", student["mark"])

Each round of the loop gets one whole dictionary in student. Then student["name"] reads a key from it.

Two steps to reach a value

Read from the outside in: first the list index, then the key.

1234567students = [
    {"name": "Sunny", "mark": 82},
    {"name": "Rocky", "mark": 67},
]
print(students[1]["name"])
students[0]["mark"] = 85
print(students[0])

students[1] is Rocky's dictionary, and ["name"] reads his name from it.

A dictionary of lists

Here each key holds a list, so one student can have many marks:

1234marks = {"Sunny": [80, 92], "Rocky": [70, 64, 75]}
marks["Sunny"].append(88)
for name, scores in marks.items():
    print(name, scores, "best:", max(scores))

marks["Sunny"] is a list, so every list tool from chapter 9 works on it: append(), max(), len().

Which one?

Common beginner doubt: "How deep can this go?" As deep as you like, but keep it to two levels while you're learning. If you need three pairs of brackets to reach a value, your data might be easier as two simpler pieces.

Read nested data from the outside in: students[0]["name"], marks["Sunny"][1].

Practice: Looping

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 117.Python Program to Print a Menu from a DictionaryEasySolve
2. 118.Python Program to Count Each Letter in a WordMediumSolve
3. 119.Python Program to Manage a Shop InventoryHardSolve