Learn Python and Build a Weather App

Want to learn Python and create something useful? This comprehensive guide walks you through the fundamentals of Python programming, culminating in building a realtime weather app that fetches data from an API. Even if you're a complete beginner, this article will guide you through the process. Let's get started!

Introduction

This article provides a structured learning path, including handson projects to solidify your understanding. The final project, the weather app, ties everything together, allowing you to apply your newfound skills in a practical way.

Getting Started: Downloads and Setup

We need to download two essential components:

1. Python Interpreter

The interpreter translates your Python code into machinereadable instructions.

• Visit python.org.
• Go to the downloads section and download the latest version of Python.
• Run the executable. If you're on Windows, be sure to check the "Add Python to PATH" checkbox during installation. This is crucial for running Python from the command line.

2. Integrated Development Environment (IDE)

An IDE is where you'll write and manage your Python code. Two popular choices are:

• PyCharm: Generally considered more beginnerfriendly. You can download the Community Edition (free) from JetBrains.com.
• VS Code: A versatile and widely used editor. If you already use VS Code, you can simply install the Python extension.

For this guide, we'll use PyCharm Community Edition.

Creating Your First Python Project

Let's create a new project in PyCharm:

1. Open PyCharm and create a new project.
2. You can rename your project and select a location. Keep the default Python version.
3. In the project explorer, create a new Python file (File > New > Python File).
4. Name the file main.py. Python files end with the .py extension.

Your First Lines of Code: Printing to the Console

Inside main.py, let's write a simple program that prints a message to the console.

print("I like pizza")
print("It's really good")

To run the code, click the green arrow in PyCharm. You should see "I like pizza" and "It's really good" printed in the console window.

Understanding Comments

Comments are notes you add to your code that are ignored by the Python interpreter.

# This is my first Python program
print("I like pizza")
print("It's really good")

Use comments to explain your code and make it easier to understand.

Variables: Storing Data

A variable is a container for a value. Think of it like a labeled box where you can store information.

Basic Data Types

Here are four common data types in Python:

• Strings: Text, enclosed in double or single quotes.
• Integers: Whole numbers (e.g., 10, 5, 0).
• Floats: Numbers with decimal points (e.g., 3.14, 2.5).
• Booleans: Represent True or False values (capitalized).

Example Variables

first_name = "Bro" # String
food = "Pizza" # String
age = 25 # Integer
quantity = 3 #Integer
num_of_students = 30 #Integer
price = 10.99 # Float
gpa = 3.2 # Float
distance = 5.5 # Float
is_student = True # Boolean
for_sale = True #Boolean
is_online = True #Boolean
email = "bro123@fake.com" #String

Working with Variables

You can use variables in various ways, including printing them with formatted strings (fstrings):

print(f"Hello {first_name}") # Output: Hello Bro
print(f"You like {food}") # Output: You like Pizza
print(f"Your age is {age}") # Output: Your age is 25
print(f"The price is ${price}") #Output: The price is $10.99

Using Booleans

Booleans are often used in if statements.

if is_student:
 print("You are a student")
else:
 print("You are not a student")

Type Casting: Converting Data Types

Type casting converts a variable from one data type to another.

Common Type Casting Functions

• str(): Converts to a string.
• int(): Converts to an integer.
• float(): Converts to a float.
• bool(): Converts to a boolean.

Examples

gpa = 3.2
age = 25

gpa = int(gpa) # gpa becomes 3
age = float(age) # age becomes 25.0
age = str(age) # age becomes "25.0"
name = "Bro"
name = bool(name) #name becomes True (nonempty string)

print(gpa)
print(age)
print(name)

Important Note: When converting a float to an integer, the decimal portion is truncated (removed), not rounded.

User Input: Getting Data from the User

The input() function prompts the user to enter data. The data is returned as a string.

name = input("What is your name? ")
age = input("How old are you? ")

age = int(age) # Type cast the age to an integer

print(f"Hello {name}, you are {age} years old.")

Calculating the Area of a Rectangle (Exercise)

length = float(input("Enter the length: "))
width = float(input("Enter the width: "))

area = length * width

print(f"The area is {area} cm²")

Creating a Shopping Cart Program (Exercise)

item = input("What item would you like to buy? ")
price = float(input("What is the price? $"))
quantity = int(input("How many would you like? "))

total = price * quantity

print(f"You have bought {quantity} x {item}s")
print(f"Your total is ${total}")

Mad Libs Game (Project)

Let's create a fun Mad Libs game!

adj1 = input("Enter an adjective: ")
noun1 = input("Enter a noun: ")
adj2 = input("Enter an adjective: ")
verb1 = input("Enter a verb ending in ing: ")
adj3 = input("Enter an adjective: ")

print(f"Today I went to a {adj1} zoo.")
print(f"In an exhibit, I saw a {noun1}.")
print(f"The {noun1} was {adj2} and {verb1}.")
print(f"I was {adj3}.")

Math Operators and Functions

Python provides a range of math operators and functions.

Basic Arithmetic Operators

• +: Addition
• : Subtraction
• *: Multiplication
• /: Division
• **: Exponentiation (power)
• %: Modulus (remainder)

Augmented Assignment Operators

Shorthand for combining an operation with assignment.

• +=: x += 1 is equivalent to x = x + 1
• =: x = 2 is equivalent to x = x 2
• *=: x *= 3 is equivalent to x = x * 3
• /=: x /= 2 is equivalent to x = x / 2
• %=: x %= 3 is equivalent to x = x % 3
• **=: x **= 2 is equivalent to x = x ** 2

Builtin Math Functions

• round(x): Rounds a number to the nearest integer.
• abs(x): Returns the absolute value of a number.
• pow(x, y): Returns x raised to the power of y.
• max(x, y, z): Returns the maximum value.
• min(x, y, z): Returns the minimum value.

Math Module Constants and Functions

To use these, you must first import math.

• math.pi: The value of pi.
• math.e: The value of Euler's number (e).
• math.sqrt(x): The square root of x.
• math.ceil(x): Rounds x up to the nearest integer.
• math.floor(x): Rounds x down to the nearest integer.

Calculating the Circumference of a Circle (Exercise)

import math

radius = float(input("Enter the radius of a circle: "))

circumference = 2 * math.pi * radius

print(f"The circumference is {round(circumference, 2)} cm")

Calculating the Area of a Circle (Exercise)

import math

radius = float(input("Enter the radius of a circle: "))

area = math.pi * radius ** 2

print(f"The area of the circle is {round(area, 2)} cm²")

Finding the Hypotenuse of a Right Triangle (Exercise)

import math

a = float(input("Enter side a: "))
b = float(input("Enter side b: "))

c = math.sqrt(a**2 + b**2)

print(f"Side C equals {c}")

String Formatting

Let's improve how we display strings using string formatting. You'll learn if statements, loops, and how to accept and validate input. Here's a brief explanation.

If Statements: Making Decisions

if statements allow you to execute code based on conditions.

age = int(input("Enter your age: "))

if age >= 100:
 print("You are too old to sign up.")
elif age >= 18:
 print("You are now signed up.")
elif age < 0:
 print("You haven't been born yet.")
else:
 print("You must be 18+ to sign up.")

Key Concepts

• if: Checks a condition.
• elif: Checks another condition if the previous if or elif conditions were false.
• else: Executes if none of the if or elif conditions are true.
• Comparison Operators: == (equal to), != (not equal to), > (greater than), < (less than), >= (greater than or equal to), <= (less than or equal to).

Examples

response = input("Would you like food? (y/n): ")

if response == "y":
 print("Have some food!")
else:
 print("No food for you.")

name = input("Enter your name: ")

if name == "":
 print("You did not type in your name.")
else:
 print(f"Hello, {name}")

Using Booleans with If Statements

for_sale = True

if for_sale:
 print("This item is for sale.")
else:
 print("This item is not for sale.")

Simple Python Calculator (Project)

operator = input("Enter an operator (+, , *, /): ")
num1 = float(input("Enter the first number: "))
num2 = float(input("Enter the second number: "))

if operator == "+":
 result = num1 + num2
elif operator == "":
 result = num1  num2
elif operator == "*":
 result = num1 * num2
elif operator == "/":
 result = num1 / num2
else:
 print("That is not a valid operator.")
 result = None # Added to prevent error if operator is invalid

if result is not None:
 print(f"Result: {round(result, 3)}")

Weight Converter Program (Project)

weight = float(input("Enter your weight: "))
unit = input("Kilograms or Pounds? (K/L): ").upper()

if unit == "K":
 weight = weight * 2.205
 unit = "lbs"
elif unit == "L":
 weight = weight / 2.205
 unit = "kgs"
else:
 print("Invalid unit.")
 exit() # Stop execution if the unit is invalid

print(f"Your weight is {round(weight, 1)} {unit}")

Temperature Conversion Program (Project)

unit = input("Is this temperature in Celsius or Fahrenheit (C/F)? ").upper()
temp = float(input("Enter the temperature: "))

if unit == "C":
 temp = (9 * temp / 5) + 32
 unit = "°F"
elif unit == "F":
 temp = (temp  32) * 5 / 9
 unit = "°C"
else:
 print("Invalid unit.")
 exit()

print(f"The temperature is {round(temp, 1)} {unit}")

Logical Operators: Combining Conditions

Logical operators allow you to evaluate multiple conditions.

• or: If at least one condition is true, the entire statement is true.
• and: Both conditions must be true for the entire statement to be true.
• not: Inverts a condition (True becomes False, False becomes True).

OR Example

temp = 36
is_raining = False

if temp > 35 or temp < 0 or is_raining:
 print("Outdoor event is canceled.")
else:
 print("Outdoor event is still scheduled.")

AND Example

temp = 30
is_sunny = True

if temp >= 28 and is_sunny:
 print("It is hot outside and it is sunny.")

NOT Example

is_sunny = False

if not is_sunny:
 print("It is cloudy.")

Conditional Expressions: OneLine If Statements

Conditional expressions are a concise way to write ifelse statements on a single line. They work like a ternary operator.

Syntax

return X if condition else return Y

Examples

num = 5
print("Positive" if num > 0 else "Negative") # Output: Positive

result = "Even" if num % 2 == 0 else "Odd"
print(result) # Output: Odd

a = 6
b = 7

max_num = a if a > b else b
print(max_num) # Output: 7

age = 25
status = "Adult" if age >= 18 else "Child"
print(status) # Output: Adult

temperature = 30
weather = "Hot" if temperature > 20 else "Cold"
print(weather) #Output: Hot

user_role = "admin"
access_level = "Full Access" if user_role == "admin" else "Limited Access"
print(access_level) #Output: Full Access

String Methods: Useful Tools for Text Manipulation

Python strings come with a variety of builtin methods.

Common String Methods

• len(string): Returns the length of the string.
• string.find(substring): Returns the index of the first occurrence of a substring. Returns 1 if not found.
• string.rfind(substring): Returns the index of the last occurrence of a substring. Returns 1 if not found.
• string.capitalize(): Capitalizes the first letter of the string.
• string.upper(): Converts the string to uppercase.
• string.lower(): Converts the string to lowercase.
• string.isdigit(): Returns True if the string contains only digits.
• string.isalpha(): Returns True if the string contains only alphabetical characters.
• string.count(substring): Returns the number of occurrences of a substring.
• string.replace(old, new): Replaces all occurrences of a substring with a new substring.

Validating User Input (Exercise)

username = input("Enter a username: ")

if len(username) > 12:
 print("Your username can't be more than 12 characters.")
elif " " in username:
 print("Your username can't contain spaces.")
elif not username.isalpha():
 print("Your username can't contain numbers.")
else:
 print(f"Welcome, {username}!")

String Indexing: Accessing Characters

String indexing allows you to access individual characters or slices of a string using square brackets.

Basic Syntax

string[start:end:step]

• start: The starting index (inclusive). Defaults to 0.
• end: The ending index (exclusive). Defaults to the end of the string.
• step: The step size. Defaults to 1.

Examples

credit_card = "1234567890123456"

print(credit_card[0]) # Output: 1
print(credit_card[:4]) # Output: 1234
print(credit_card[5:9]) # Output: 5678
print(credit_card[5:]) # Output: 567890123456
print(credit_card[1]) # Output: 6 (last character)
print(credit_card[::2]) # Output: 13579135 (every second character)
print(credit_card[::1]) # Output: 6543210987654321 (reversed string)

last_digits = credit_card[4:]
print(f"XXXXXXXXXXXX{last_digits}") #Output: XXXXXXXXXXXX3456

credit_card = credit_card[::1] #Reverse the string
print(credit_card)

Format Specifiers

Format specifiers, used within fstrings, allow you to format values in specific ways.

Common Format Specifiers

• :.2f: Two decimal places (float).
• :10: Allocate 10 spaces.
• :010: Zeropad to 10 digits.
• :<: Leftjustify.
• :>: Rightjustify.
• :^: Centeralign.
• :+: Show plus sign for positive numbers.
• : : Show a space for positive numbers.
• :,: Thousand separator.

Examples

price1 = 3.14159
price2 = 987.65
price3 = 12.34

print(f"Price 1: ${price1:.2f}") # Output: Price 1: $3.14
print(f"Price 2: ${price2:+10,.2f}") # Output: Price 2: + 987.65
print(f"Price 3: ${price3:<10,.2f}") # Output: Price 3: $12.34 

While Loops: Repeating Code Until a Condition is False

while loops execute a block of code as long as a condition remains true. Make sure to have an exit strategy to avoid infinite loops!

Basic Syntax

while condition:
 # Code to execute

Example

name = ""

while name == "":
 name = input("Enter your name: ")
 if name == "":
 print("You did not enter your name.")

print(f"Hello, {name}!")

Compound Interest Calculator (Project)

principal = 0
rate = 0
time = 0

while True:
 try:
 principal = float(input("Enter the principal amount: "))
 if principal < 0:
 print("Principal can't be less than zero.")
 else:
 break # Exit the loop if the input is valid
 except ValueError:
 print("Invalid input. Please enter a number.")

# Similar while loops for rate and time (omitted for brevity)

total = principal * (1 + rate / 100) ** time
print(f"Balance after {time} years: ${total:.2f}")

For Loops: Iterating Over Sequences

for loops execute a block of code a fixed number of times, iterating over a sequence like a range, string, or list.

Basic Syntax

for item in sequence:
 # Code to execute

Examples

for x in range(1, 11):
 print(x, end=" ") # Output: 1 2 3 4 5 6 7 8 9 10

for char in "Hello":
 print(char)

Continue and Break Keywords

• continue: Skips the current iteration and proceeds to the next one.
• break: Exits the loop entirely.

Countdown Timer (Project)

import time

my_time = int(input("Enter the time in seconds: "))

for x in range(my_time, 0, 1):
 seconds = x % 60
 minutes = int(x / 60) % 60
 hours = int(x / 3600)

 print(f"{hours:02}:{minutes:02}:{seconds:02}", end="\r")
 time.sleep(1)

print("Times Up!")

Nested Loops

Nested loops are loops within loops. They're useful for working with twodimensional data structures.

rows = int(input("Enter the number of rows: "))
cols = int(input("Enter the number of cols: "))
symbol = input("Enter a symbol to use: ")

for i in range(rows):
 for j in range(cols):
 print(symbol, end="")
 print()

Collections: Organizing Data

Collections are single variables used to store multiple values. They allow you to group and manage related data.

Common Collection Types

• Lists: Ordered, changeable, and allow duplicates (use square brackets []).
• Sets: Unordered, immutable, and do not allow duplicates (use curly braces {}).
• Tuples: Ordered, unchangeable, and allow duplicates (use parentheses ()). Tuples are generally faster than lists.
• Dictionaries: Keyvalue pairs (use curly braces {}), ordered and changeable.

Quiz Game (Project)

Here's how we would construct the barebones logic for the final quiz game before moving into the actual construction of the UI elements:

questions = (
 "How many elements are in the periodic table? : ",
 "Which animal lays the largest eggs? : ",
 "What is the most abundant gas in Earth's atmosphere? : ",
 "How many bones are in the human body? : ",
 "Which planet in the solar system is the hottest? : "
)

options = (
 ("A. 116", "B. 117", "C. 118", "D. 119"),
 ("A. Whale", "B. Crocodile", "C. Elephant", "D. Ostrich"),
 ("A. Hydrogen", "B. Oxygen", "C. Nitrogen", "D. CarbonDioxide"),
 ("A. 206", "B. 207", "C. 208", "D. 209"),
 ("A. Mercury", "B. Venus", "C. Earth", "D. Mars")
)

answers = ["C", "D", "C", "A", "B"]
guesses = []
score = 0
question_num = 0

for question in questions:
 print("")
 print(question)
 for option in options[question_num]:
 print(option)
 guess = input("Enter (A, B, C, D): ").upper()
 guesses.append(guess)
 if guess == answers[question_num]:
 score += 1
 print("CORRECT!")
 else:
 print("INCORRECT!")
 print(f"The answer was: {answers[question_num]}")
 question_num += 1

print("")
print(" RESULTS ")
print("")

print("answers: ", end="")
for answer in answers:
 print(answer, end=" ")
print()

print("guesses: ", end="")
for guess in guesses:
 print(guess, end=" ")
print()

score = int(score/len(questions) * 100)
print(f"Your score is: {score}%")

print("BYE!")