Python Dictionaries

What Is A Dictionary?

A dictionary stores data as key-value pairs. A key is the label you use to look up a value.

student = {
    "name": "Asha",
    "age": 16,
    "city": "Delhi"
}

Here "name" is a key and "Asha" is its value. Dictionaries are useful when data has clear labels.

Reading Values

Use square brackets with a key to read a value.

print(student["name"])

Output:

Asha

If the key might be missing, use get().

print(student.get("grade", "Not assigned"))

The second argument is the default value to return when the key is not found.

Adding And Updating Values

Use assignment with a key to add or update a value.

student["age"] = 17
student["grade"] = "A"

If the key already exists, the value is updated. If the key does not exist, a new pair is added.

Removing Values

Use pop() to remove a key and get its value.

city = student.pop("city")
print(city)

You can also use del when you only want to delete.

del student["grade"]

Looping Through A Dictionary

Loop through keys:

for key in student:
    print(key)

Loop through values:

for value in student.values():
    print(value)

Loop through both keys and values:

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

Dictionary Keys

Keys must be unique. If the same key is written twice, the later value replaces the earlier one.

data = {"score": 80, "score": 90}
print(data["score"])

This prints 90.

Common key types include strings and numbers. Lists cannot be used as dictionary keys because lists are mutable.

Common Mistakes

A common mistake is using a missing key with square brackets.

print(student["marks"])

If "marks" does not exist, Python raises a KeyError. Use get() when missing keys are expected.

Another mistake is confusing a dictionary with a list. Lists use numeric positions. Dictionaries use meaningful keys.

Quick Summary

• A dictionary stores key-value pairs.
• Use a key to read, add, or update a value.
• Use get() to avoid errors for missing keys.
• Use items() to loop through keys and values together.
• Keys are unique.
• Missing keys with square brackets raise KeyError.

Practice Quiz

Now practice this topic with MCQs and explanations:

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