Variables - Paths in the Jungle

Welcome back! Darwin here. The field journal keeps filling up: marching direction, camp coordinates, number of species. Text from print() vanishes once shown, but a program must store data and come back to it. That is what variables are for.

What Are Variables?

Imagine that during an expedition you find various items:

• A compass pointing the direction
• A map with a marked location
• A notebook with the number of discovered species

Each of these items has a name and stores a value. In Python, these are variables - like labeled jars in the specimen tent. Here is a journal entry as code:

1# Compass points north
2direction = "North"
3
4# Map shows coordinates
5latitude = -3.4653
6longitude = 29.3608
7
8# Counting discovered species
9species_count = 47
10
11# Did we find a rare species?
12rare_species_found = True

The = sign is not equality but assignment: the name on the left gets attached to the value on the right. Nothing was displayed - variables wait in memory.

Creating Variables

The syntax is always the same: a name, an equals sign, a value. Only the kind of value changes:

1# Syntax: variable_name = value
2
3# Text (strings)
4explorer_name = "Darwin"
5jungle_type = "tropical"
6
7# Whole numbers (integers)
8team_size = 5
9days_in_jungle = 14
10
11# Decimal numbers (floats)
12temperature = 32.5
13humidity = 78.3
14
15# Logical values (booleans)
16is_safe = True
17found_water = False

Important: Python automatically recognizes the variable type! You don't need to declare it. Decimals use a dot: 32,5 raises no error, it creates a pair of numbers, 32 and 5.

Variable Naming - Jungle Rules

A name is a signpost on the trail: readable for you and valid for Python.

Allowed:

A name can contain letters, digits and underscores, and words are joined with an underscore - the snake_case style. Square brackets create a list, you will meet it later:

1# Snake case (recommended in Python)
2jungle_temperature = 30
3discovered_species = ["Python", "Parrot", "Lion"]
4
5# Can contain letters, digits, underscores
6species_1 = "Python regius"
7species_2 = "Panthera leo"
8rare_species_2024 = "New species"
9
10# Can start with an underscore
11_private_notes = "Secret information"
12__hidden_treasure = "Treasure"

By convention, a leading underscore means "for internal use".

Not Allowed:

These lines raise a SyntaxError, so Python would refuse to run the program - hence the comments. The last line is the correct version:

1# Cannot start with a digit
2# 1st_species = "Error!"  # SyntaxError
3
4# Cannot contain spaces or hyphens
5# jungle temperature = 30  # SyntaxError
6# jungle-temperature = 30  # SyntaxError
7
8# Cannot use special characters
9# species! = "Error"  # SyntaxError
10# species@ = "Error"  # SyntaxError
11
12# Cannot be a Python keyword
13# if = 5  # SyntaxError
14# for = 10  # SyntaxError
15# def = "text"  # SyntaxError
16
17jungle_temperature = 30  # correct: underscore instead of a space

The hyphen is out because Python reads it as minus: jungle-temperature is a subtraction, and you cannot assign to a calculation.

Python Keywords (reserved):

Keywords have a fixed meaning in the language, so they cannot be variable names:

1# You CANNOT use these names as variables:
2# False      await      else       import     pass
3# None       break      except     in         raise
4# True       class      finally    is         return
5# and        continue   for        lambda     try
6# as         def        from       nonlocal   while
7# assert     del        global     not        with
8# async      elif       if         or         yield
9
10import keyword
11print(len(keyword.kwlist))  # 35

You can always check the list in keyword.kwlist. async and await joined in Python 3.7, so older tables miss them.

Good Naming Practices

Python accepts the name t, but in a week you won't remember its meaning. Compare the names:

1# Descriptive names - you immediately know what they store
2daily_temperature = 32
3discovered_species_count = 15
4expedition_start_date = "2024-11-24"
5
6# Bad names - unclear, short
7t = 32  # What is t?
8x = 15  # What does x mean?
9d = "2024-11-24"  # Date of what?
10
11# Snake case (Python style)
12jungle_humidity_level = 80
13
14# camelCase (popular in JavaScript and Java) - not for Python variables!
15jungleHumidityLevel = 80
16
17# Long names are OK if they're descriptive
18number_of_rare_species_discovered_in_2024 = 3  # Clear!

Both styles run, since Python doesn't check style. It is a convention from the PEP 8 guide: variables in snake_case, PascalCase (e.g. JungleExpedition) for classes. Use snake_case from day one.

Displaying Variables

print() accepts a variable like text, and an f-string, a string with the letter f before the quote, puts values straight into a sentence:

1explorer_name = "Darwin"
2expedition_day = 5
3
4# Simple print
5print(explorer_name)  # Darwin
6print(expedition_day)  # 5
7
8# Print with text
9print("Explorer:", explorer_name)  # Explorer: Darwin
10
11# F-string (the best way!) - Python 3.6+
12print(f"Hello, {explorer_name}!")
13# Hello, Darwin!
14
15print(f"Expedition day: {expedition_day}")
16# Expedition day: 5
17
18# Complex f-strings
19species_found = 12
20days = 3
21print(f"We discovered {species_found} species in {days} days!")
22# We discovered 12 species in 3 days!
23
24print(f"Temperature: {30}°C")  # Temperature: 30°C

Python evaluates everything in the braces {}: a variable, a calculation, even a bare number, as in the last line. Without the f, the braces are printed literally. A comma in print() works too, but I recommend f-strings: they read better.

Changing Variable Values

Variables can change their value (that's why they're called "variables"!). Each day brings discoveries, and we update the same entry:

1# Day 1 of the expedition
2species_count = 5
3print(f"Day 1: {species_count} species")  # Day 1: 5 species
4
5# Day 2 - we found more
6species_count = 12
7print(f"Day 2: {species_count} species")  # Day 2: 12 species
8
9# Day 3 - adding new discoveries
10species_count = species_count + 7  # 12 + 7 = 19
11print(f"Day 3: {species_count} species")  # Day 3: 19 species

Each assignment discards the old value. In the last step Python first computes the right side (12 + 7), then attaches the result to the name. A bare species_count + 7 without = would change nothing.

Multiple Variables at Once

Python allows clever assignments: several variables in one line, a shared starting value and swapping places:

1# Assigning multiple variables in one line
2x, y, z = 10, 20, 30
3print(f"x={x}, y={y}, z={z}")  # x=10, y=20, z=30
4
5# Same value for multiple variables
6a = b = c = 0
7print(f"a={a}, b={b}, c={c}")  # a=0, b=0, c=0
8
9# Swapping values
10first = "A"
11second = "B"
12print(f"Before: first={first}, second={second}")  # Before: first=A, second=B
13
14first, second = second, first  # Magic swap!
15print(f"After: first={first}, second={second}")  # After: first=B, second=A

During the swap Python reads both values first, then assigns them crosswise, so no helper variable is needed. The number of names must match the number of values: x, y = 1, 2, 3 is a ValueError.

Input - Collecting Data from the User

User data comes through input(): it shows a question, waits for a keyboard answer and hands it to the program:

1# Simple question
2name = input("What's your name? ")
3print(f"Hello, {name}!")
4
5# Input ALWAYS returns a string (text)!
6age_text = input("How old are you? ")
7print(type(age_text))  # <class 'str'>
8
9# Converting to a number
10age = int(age_text)  # Change string → int
11print(f"Next year you'll be {age + 1} years old")

IMPORTANT: input() always returns a string! A typed 25 is the text "25", and "25" + 1 ends with a TypeError. Only a conversion gives you a number: int() or float().

Practical Example - Expedition Journal

Let's combine everything into an evening journal program: it asks for data, converts the numbers and prints a report:

1# Program: Explorer's Journal
2print("=== Expedition Journal ===")
3
4# Collecting data
5explorer_name = input("Explorer name: ")
6location = input("Location: ")
7species_found = int(input("How many species did you discover today? "))
8temperature = float(input("Temperature (°C): "))
9notes = input("Notes for the day: ")
10
11# Displaying the report
12print("\n--- DAILY REPORT ---")
13print(f"Explorer: {explorer_name}")
14print(f"Location: {location}")
15print(f"Discovered species: {species_found}")
16print(f"Temperature: {temperature}°C")
17print(f"Notes: {notes}")
18print("--- END OF REPORT ---")

In int(input(...)) the answer goes straight into the conversion, so species_found is a number. The location field stays a string, because we don't calculate with it.

Practical Exercise

Try to create a program on your own that:

1. Asks for your name
2. Asks for your favorite animal
3. Asks for your favorite number
4. Displays: "Hello {name}! Your favorite animal is {animal} and your favorite number is {number}!"

1# Task: Create a program that collects user data
2
3# TODO: Ask for the name
4name = input("What is your name? ")
5
6# TODO: Ask for favorite animal
7animal = input("Favorite animal: ")
8
9# TODO: Ask for favorite number
10number = input("Favorite number: ")
11
12# TODO: Display a message using an f-string
13print(f"Hello {name}! Your favorite animal is {animal} and your favorite number is {number}!")

Summary

Before moving on, make sure that:

• You can create a variable and change its value
• You understand naming rules
• You display variables with print() and f-strings
• You know the difference between input() (string) and int(), float()

In the next lesson Darwin will show you data types - how to classify discoveries in the jungle!

Remember: a variable is a labeled jar in the camp tent - name on the label, value inside, contents swappable at any time.