Python Basics

Overview

Questions

• How do I write and run Python code?
• How does Python store numbers, text and collections of values?
• How do I make decisions, repeat work and reuse code?

Objectives

• Run code in a notebook and read error messages
• Use variables, numbers, strings, lists and dictionaries
• Write if statements and for loops
• Write and call your own functions

How this episode works

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This page gives a short overview of each idea. The real learning happens in the six notebooks at the end: each takes 15 to 20 minutes, covers one idea, and ends with an exercise and its solution. Work through them in order. The examples use Indiana counties, so by the end you will already be thinking about the kind of data the other modules use.

Running code and reading errors

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A notebook has text cells and code cells. Run a code cell with Shift + Enter and its output appears below it. print() shows a value; # starts a comment that Python ignores.

PYTHON

print("Hello, World!")   # text goes in quotes
print(6 * 7)             # numbers do not

Errors are normal. Read the last line of the message first: it names the problem, such as a SyntaxError (Python could not read the line, often a missing quote or bracket) or a NameError (a name Python does not know, often a typo or a skipped cell).

Variables and types

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= stores a value under a name. Every value has a type:

Type	Examples	Used for
int	177513	Whole numbers, such as counts
float	499.8	Decimal numbers, such as areas and rates
str	"Tippecanoe", "18157"	Text, including identifiers like FIPS codes
bool	True, False	Yes or no

PYTHON

population = 177513
land_sq_mi = 499.8
density = population / land_sq_mi      # 355.2 people per square mile

Numbers stored as text ("177513") cannot be used in math until you convert them with int() or float(). Real data often arrives this way: the Census API, for example, returns every value as text.

Text (strings)

---

f-strings build text from values, and string methods clean it:

PYTHON

name = "  tippecanoe county "
clean = name.strip().title()                      # "Tippecanoe County"
print(f"{clean} had {population:,} people")       # "Tippecanoe County had 177,513 people"

Lists and dictionaries

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A list holds values in order; positions start at 0. A dictionary holds key: value pairs, so you look values up by name.

PYTHON

counties = ["Marion", "Lake", "Allen"]
print(counties[0], counties[-1])                  # first and last

allen = {"name": "Allen", "fips": "18003", "unemployment": 4.3}
print(allen["unemployment"])

A dictionary of lists (one list per column) and a list of dictionaries (one dictionary per row) are both ways of describing a table. pandas, in the next episode, turns either one into a DataFrame.

Decisions and loops

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if, elif and else choose what runs. A for loop repeats a block for every item in a list. The indented lines belong to the block.

PYTHON

for d in [101.4, 548.3, 21.7]:
    if d >= 300:
        print(d, "urban")
    else:
        print(d, "rural or small town")

Functions

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A function packages a piece of work under a name so you can reuse it. return sends a result back.

PYTHON

def density(population, area):
    """Return people per unit of area."""
    return population / area

density(177513, 499.8)

Challenge

Challenge 1: Predict the output

What does each line print? Decide first, then run them in a notebook to check.

PYTHON

print(7 / 2)
print(7 // 2)
print("7" + "2")
print([10, 20, 30][1])

Show me the solution

• 3.5: / always gives a decimal number
• 3: // divides and rounds down
• 72: joining two strings, not adding numbers
• 20: positions start at 0, so [1] is the second item

Challenge

Challenge 2: Fix the bug

This code should print the total population of three counties, but it stops with an error. What is wrong?

PYTHON

populations = ["34365", "360412", "79129"]
total = 0
for p in populations:
    total = total + p
print(total)

Show me the solution

The populations are stored as text (inside quotes), and Python cannot add text to a number, so it raises a TypeError. Convert each value inside the loop with total = total + int(p), or store the numbers without quotes in the first place.

Key Points

• Run cells with Shift + Enter, in order, and read the last line of any error first
• Values have types (int, float, str, bool); convert numbers stored as text before doing math
• Lists keep values in order (from position 0); dictionaries look values up by key
• if / elif / else make decisions; for loops repeat work; functions package work for reuse

Notebooks

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Work through these in order. Each one opens in Google Colab.

Notebook	What you will do
2a. Notebooks, Cells and Printing	Run cells, print, write comments, read error messages and edit text cells
2b. Variables, Types and Math	Store values, check types, calculate a population density and convert text to numbers
2c. Strings: Working with Text	Build text with f-strings, clean messy names and split coordinates
2d. Lists and Dictionaries	Pick items from lists, store county records in dictionaries, and see how both become tables
2e. Conditions and Loops	Classify counties with if and loop over lists to build results
2f. Functions	Write reusable functions with parameters, defaults and return values