Python CSV Files

1Python CSV: overview

A CSV file stores a table as plain text — one row per line, values separated by commas — and Python's csv module reads and writes it correctly, including tricky cases such as commas inside quoted values.

Before this lesson: Python Dictionaries, Python File Handling

2Python CSV: diagram

Quick reference: Python CSV

• How to Read a CSV File in Python — Use the `csv` module: `with open("data.csv", newline="", encoding="utf-8") as f:` then `csv.DictReader(f)` gi…

4Python CSV: syntax

import csv

with open("data.csv", newline="") as f:
    for row in csv.reader(f):           # row = list of strings
        ...

with open("data.csv", newline="") as f:
    for row in csv.DictReader(f):       # row = dict keyed by the header
        ...

with open("out.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "score"])  # one row
    writer.writerows(rows)              # many rows

5Python CSV: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

import csv

with open("sales.csv", "w", newline="") as f:
    f.write("product,qty,price\n")
    f.write("Pen,10,1.50\n")
    f.write("Notebook,4,3.25\n")
    f.write("Bag,1,24.00\n")

grand_total = 0
with open("sales.csv", newline="") as f:
    for row in csv.DictReader(f):
        qty = int(row["qty"])
        price = float(row["price"])
        line_total = qty * price
        grand_total += line_total
        print(f"{row['product']:<10}{qty:>3} x {price:5.2f} = {line_total:6.2f}")

print(f"Grand total: {grand_total:.2f}")

Expected outputPen        10 x  1.50 =  15.00
Notebook    4 x  3.25 =  13.00
Bag         1 x 24.00 =  24.00
Grand total: 52.00

7Python CSV: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1The csv module is part of the standard library.
2. Line 3-7We create sales.csv first, because the browser's file system starts empty. On your own computer the file would usually already exist.
3. Line 9A running total starts at 0.
4. Line 10The file is opened for reading with newline="", as the csv module expects.
5. Line 11DictReader reads the header line automatically, then gives each data row as a dict such as {'product': 'Pen', 'qty': '10', 'price': '1.50'}.
6. Line 12-13The values are strings, so they are converted: int() for the quantity and float() for the price.
7. Line 14-15The row's total is calculated and added to the grand total.
8. Line 16Format codes line the columns up: <10 pads the name to 10 characters, >3 right-aligns the quantity, and .2f shows two decimals.
9. Line 18After the loop, the grand total of all rows is printed: 15 + 13 + 24 = 52.

8Python CSV: common mistakes

import csv
import io

data = io.StringIO("item,qty\napples,3\npears,5\n")
total = 0
for row in csv.DictReader(data):
    total += row["qty"]

import csv
import io

data = io.StringIO("item,qty\napples,3\npears,5\n")
total = 0
for row in csv.DictReader(data):
    total += int(row["qty"])
print(total)

line = '"Smith, Jane",Leeds,42'
name, city, age = line.split(",")

import csv

line = '"Smith, Jane",Leeds,42'
name, city, age = next(csv.reader([line]))
print(name, "|", city, "|", age)

import csv
import io

data = io.StringIO("name,score\nAsha,82\nBen,67\n")
total = 0
for row in csv.reader(data):
    total += int(row[1])

import csv
import io

data = io.StringIO("name,score\nAsha,82\nBen,67\n")
reader = csv.reader(data)
next(reader)
total = 0
for row in reader:
    total += int(row[1])
print(total)

9Python CSV: exercises

Write your answer, then press Check. The checks run in your browser.

import csv
import io

def column_total(csv_text, column):
    reader = csv.DictReader(io.StringIO(csv_text))
    total = 0
    # loop over the rows and add float(row[column]) to total
    return total

orders = "item,qty,price\nTea,2,4.50\nCake,1,3.25\n"
print(column_total(orders, "price"))

import csv
import io

def column_total(csv_text, column):
    reader = csv.DictReader(io.StringIO(csv_text))
    total = 0
    for row in reader:
        total += float(row[column])
    return total

orders = "item,qty,price\nTea,2,4.50\nCake,1,3.25\n"
print(column_total(orders, "price"))

import csv
import io

text = 'name,tags\nAsha,"python,sql"\nBen,excel\n'
rows = list(csv.reader(io.StringIO(text)))
print(len(rows))
print(rows[1])
print(rows[2][1])

3
['Asha', 'python,sql']
excel

2 more exercises → 4 more examples

Further reading

• json
• csv
• re
• sqlite3
• logging
• unittest
• Requests documentation
• Installing packages

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Related:
Python DictionariesPython File HandlingWorking with JSON in PythonPandas Basics

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