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Python

Python is a free programming language with short, readable code. People use it to clean and analyze data, make charts, automate tasks and build AI agents.

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In sales_feb.csv, one amount is saved as "1,250.00", in quotes and with a comma. With all three files in the folder, what does python3 total.py print?

total.py
import csv, globtotal = 0for path in glob.glob("sales_*.csv"):    with open(path) as f:        for row in csv.DictReader(f):            total += float(row["amount"])print(f"Total: {total:,.2f}")
Answer the question to see the other views.

Same script, different folder, different answer. Example data.

Why data people use it: it's free, it runs on any computer, and almost every data tool has a Python package: pandas for tables, and others for charts, files, APIs and AI agents.

Where it runs: in a script you run from the terminal, in a Jupyter notebook, or in Google Colab with nothing to install.

How to start: install Python 3, make a virtual environment for each project and add packages with pip. Or ask Claude Code to write your first script with you.

How it's used

Decimals in binary

Terminal

  1. $ python3 -c "print(0.1 + 0.2)"
  2. marked: 0.30000000000000004Computers store decimals in binary, so they're rarely exact. Excel does too; it hides it by showing 15 digits.
  3. $ python3 -c "print(round(0.1 + 0.2, 2))"
  4. 0.3Round money before you compare it, or use the decimal module.
Python does exactly what you ask, by the computer's rules: add two prices and check with == that they match the total, and a correct bill can come out False.

When to look closer

A line outside the loop

add.py

  1. total = 0
  2. for row in rows:
  3. amount = float(row["amount"])
  4. total += amount
  5. print(total)

Line 4 isn't indented, so it sits outside the loop. rows holds 30 sales. What does total end up as?

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Related terms

  • pandas

    pandas is a Python package for working with tables of data, called DataFrames: loading them, filtering rows, picking columns and summarizing them.

  • Script

    A script is a short program, saved in a text file, that does one task from start to finish every time you run it, the same way each time. It doesn't think or guess: it reads what you give it, does its steps and hands back a result or an error.

  • Jupyter notebook

    A Jupyter notebook is a file (ending in .ipynb) that mixes code, its results and notes in cells. You run one cell at a time and see its output right below it.

  • Google Colab

    Google Colab is a free Google service that runs Jupyter notebooks in your browser, with nothing to install. pandas and Altair come already installed.

The whole glossary