Rows of data in Python: Free Lesson

Which orders went to Leeds?

You start a job at an online shop. Your manager sends you last week's orders and asks a simple question.

"How many orders went to Leeds?"

There are 4,000 rows. Nobody reads 4,000 rows by eye. A few lines of Python can answer it in a second.

By the end of this lesson

Note

I can read and filter rows of data in Python.

Key words

Word	What it means	Example
Row	one record, such as one order
Column	one kind of value every row has	city
Dictionary	Python values looked up by a name	{"city": "Leeds"}
List of dictionaries	a table: one dictionary for each row
Filter	to keep only the rows that pass a test

The cells students fill in are left blank in this preview.

A row is a dictionary

A spreadsheet has columns across the top and one row for each order.

In Python, one row is a dictionary. Each column name is a key, and each key holds that row's value.

The whole table is a list of those dictionaries. orders[0] is the first row. orders[0]["city"] is the city of that row.

Read a row and a value

Code: python

# Each order is one row: a dictionary of column name to value.
orders = [
    {"id": 101, "city": "Leeds", "total": 24.50},
    {"id": 102, "city": "Bristol", "total": 8.99},
    {"id": 103, "city": "Leeds", "total": 61.00},
]

first = orders[0]
print("Rows in the table:", len(orders))
print("First order id:", first["id"])
print("First order city:", first["city"])

# Change the last line to print the total of the second order.
print("Second order city:", orders[1]["city"])

The same orders as a table

Python	What it gives	Value
len(orders)	how many rows	3
orders[0]	the first row	the whole order 101
orders[0]["city"]	one value in the first row	Leeds
orders[2]["total"]	one value in the third row	61.0

The cells students fill in are left blank in this preview.

A loop and an if filter rows

To filter, you look at every row in turn. A for loop does that.

Inside the loop, an if tests the row. When the test is true, you add the row to a new list with append.

When the loop ends, the new list holds only the rows you wanted. len then counts them.

Keep the orders from Leeds

Code: python

orders = [
    {"id": 101, "city": "Leeds", "total": 24.50},
    {"id": 102, "city": "Bristol", "total": 8.99},
    {"id": 103, "city": "Leeds", "total": 61.00},
    {"id": 104, "city": "Cardiff", "total": 15.25},
    {"id": 105, "city": "Leeds", "total": 9.75},
]

# Keep only the orders that pass the test, in a new list.
leeds = []
for order in orders:
    if order["city"] == "Leeds":
        leeds.append(order)

for order in leeds:
    print(order["id"], order["total"])
print("Orders from Leeds:", len(leeds))

# Change the test to keep orders with a total over 20 instead.

A common mistake

Warning

Many people think a filter deletes the rows that fail. It does not. The loop builds a new list and the original list keeps every row. So you can filter the same orders again with a different test.

Your turn: big orders

The warehouse wants every order over a set amount, so it can pack them with extra care. Write a function that filters the orders by their total. The hidden tests check your answer.