Dictionaries — data with names

The concept

A dictionary stores labelled values: student = {"name": "Asha", "marks": 72}. You read a value by its label, student["marks"], not by its position — which is exactly how a form, a JSON file from an API and a database row are shaped. A list of dictionaries, one per row, is the standard way to hold a whole table in memory.

student.get("email", "not given") returns a default instead of crashing when the key is missing, and for key, value in student.items(): walks the whole record. Counting things is its best trick: counts[name] = counts.get(name, 0) + 1 builds a frequency table in one line, which is how you answer "how many students chose each city".

student = {"name": "Asha", "marks": 72, "city": "Pune"}

print(student["marks"])                    # 72
print(student.get("email", "not given"))   # no crash on a missing key

counts = {}
for city in ["Pune", "Delhi", "Pune"]:
    counts[city] = counts.get(city, 0) + 1
print(counts)                              # {'Pune': 2, 'Delhi': 1}