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Table of Contents

Introduction-your-first-steps-in-python

  • Welcome to Python!
  • Week 1 is all about learning the fundamental building blocks of the language.
  • We'll cover how to display information,
  • get input from a user,
  • store data in variables, and
  • organize our code into reusable blocks called functions.

Functions, Variables

Introduction to Programming

  • Programming = giving instructions to a computer
  • Computers only understand binary (0s and 1s)
  • Programming languages bridge human logic → machine code
  • Python = high-level, beginner-friendly, widely used

algorithms--pseudocode

  • Algorithm = step-by-step procedure to solve a problem
  • Pseudocode = human-readable outline of an algorithm
  • Helps bridge ideas → actual code
  • Example pseudocode for making tea: 
    1. Boil water
2. Add tea leaves
3. Steep for 5 minutes
4. Serve

Creating Code in Python

  • The very first thing you typically do in a new language is make it say something. In Python, we use the print() function for this.
  • VS Code is a text editor. In addition to editing text, you can visually browse files and run text-based commands at a terminal.
  • In the terminal, you can execute code hello.py to start coding.
  • Save scripts with .py extension
  • Write code in a text editor (e.g., VS Code, Notepad++, Notepad, Codeblocks, etc...)
  • Run code in a terminal: 
    python hello.py
  • First program: 
    print("hello, world")
  • Python is interpreted (executes line by line)

Functions

  • Function = reusable block of code
  • A function is a pre-written piece of code that performs a specific action. You "call" a function by writing its name followed by parentheses ().
  • Built-in example: print() max() min() etc....
  • An Example List of functions
  • Functions can take arguments (inputs)
  • The text you want to print, like "hello, world", is called a string (or str). A string is a sequence of characters, and you must enclose it in either double quotes (") or single quotes
  • ('). Example: 
    print("Hello, World!") #The text "hello, world" is an argument passed to the print function. print("hello, world")

Bugs

  • A bug is an error in your program. A common early bug is a SyntaxError, which means you've broken one of Python's grammar rules.
  • Bugs are a natural part of coding. These are mistakes, problems for you to solve! Don’t get discouraged!
  • Error messages can often inform you of your mistakes and provide clues on how to fix them. However, there will be many times when the interpreter is not this helpful.
#This will cause a SyntaxError because the closing quote is missing.
print("hello, world)

Strings

  • String = text between quotes
  • Examples: "hello", "123", "CS50"
  • print() can output strings
  • Multiple arguments: 
    print("hello,", "world")
  • What if you want to print a quote?
  • You can use another type of quote to enclose the string.
print('He said, "Hello!"')
  • Or you can use an escape sequence. A backslash \ tells Python that the next character has a special meaning. For example, \" represents a literal double quote.
print("He said, \"Hello!\"")

Improvements

  • This program is static, lets make it more interesting by making it interactive.
  • We can do this by using input() function, which prompts the user for text and waits for a response.
  • Example:
input("What's your name? ")
print("hello, world")
  • When you run this, the program will first ask for your name and then print "hello, world".

Comments

  • Comments = notes for humans to easily understand what the specific part of the code does and is ignored by Python
  • Written with #
  • Examples: 
    # This is a comment
print("Hello")  # inline comment
  • lets add a comment to our program it would turn into this.
#Hellooo!!!!
input("What's your name? ")
print("hello, world") #earth

Input

  • input() = get info from the user
  • Example: 
    name = input("What's your name? ")
print("Hello,", name)

Variables

  • To store the data we get from input(), we need a variable. A variable is a named container for a value. We use the equals sign (=), the assignment operator, to store a value in a variable.
  • Variables = names that store values
  • Example:
# Ask the user for their name and store it in the 'name' variable.
# Variable names in Python are traditionally written in snake_case.
name = input("What's your name? ")
# Say hello to the user
print("hello,")
print(name)
  • so we have the variable name from the input and using the variable we print it out below
  • Variables make programs reusable and flexible

Strings-and-Parameters

  • A string (or str) is a sequence of characters, like text.

  • Looking back a bit in our code, there's a visual side effect of having the result appearing in another line

  • Functions take arguments that influence their behavior. If we look at the documentation for print you’ll notice we can learn a lot about the arguments that the print function takes.

  • the print function automatically includes the argument end='\n'. This \n indicates that the print function will automatically create a line break when run.

  • However, we can technically provide an argument for end ourselves such that a new line is not created

  • Modifying the code like this

# Ask the user for their name
name = input("What's your name? ")
print("hello,", end="")
print(name)

By providing end="" we are overwriting the default value of end such that it never creates a new line after this first print statement. Providing the name as “David”, the output in the terminal window will be hello, David.

  • A more elegant way to print multiple items is to pass them as separate arguments to the print() function. print() will automatically put a space between them. The placeholder for an argument in a function's definition is called a parameter.
name = input("What's your name? ")

# Here, "hello," and the variable 'name' are two separate arguments.
print("hello,", name)

A-small-problem-with-quotation marks

If your string needs to contain a quote, you can either enclose the string with the other type of quote or use a backslash (\) to escape the inner quote.

# Using single quotes to enclose a string with double quotes
print('He said, "Hello!"')

# Using a backslash to escape the inner double quotes
print("She said, \"Hi!\"")

Formatting-Strings-(f-Strings)

The best way to embed variables directly into strings is with f-strings (formatted strings). Simply prefix the string with an f and place your variables inside curly braces {}.

name = input("What's your name? ")

# This f-string embeds the value of 'name' directly.
print(f"hello, {name}")

More-on-Strings

Strings come with built-in functions called methods. You call them using dot notation (variable.method()). You can also chain methods together to perform multiple actions in one line.

  • str.strip(): Removes leading and trailing whitespace.
# Ask the user for their name
name = input("What's your name? ")

# Remove whitespace from the str
name = name.strip()

# Print the output
print(f"hello, {name}")
  • str.title(): Capitalizes the first letter of each word.
# Ask the user for their name
name = input("What's your name? ")

# Remove whitespace from the str
name = name.strip()

# Capitalize the first letter of each word
name = name.title()

# Print the output
print(f"hello, {name}")
  • Notice that you can modify your code to be more efficient:
# Chaining methods: input is received, whitespace is stripped,
# and then the result is title-cased.
name = input("What's your name? ").strip().title()

print(f"hello, {name}")
  • You can learn more about strings in Python’s documentation on str

Integers-or-int

  • In Python, an integer is referred to as an int.
Operator Description Example
+ (Addition) Adds two numbers together. 5 + 3 = 8
- (Subtraction) Finds the difference between two numbers. 10 - 4 = 6
***** (Multiplication) Multiplies two numbers. 7 * 6 = 42
/ (Division) Divides one number by another. 20 / 5 = 4
% (Modulo) Returns the remainder of a division. 10 % 3 = 1 (because 10 divided by 3 is 3 with a remainder of 1)
An integer (int) is a whole number. Since input() always gives back a string, you must convert it to an int if you want to do math. This conversion is called type casting. 
# Cast the string from input() into an integer
x = int(input("What's x? "))
y = int(input("What's y? "))

# Now we can perform mathematical operations
print(x + y)

Using what we learned earlier we can make this more interactive using input()

x = input("What's x? ")
y = input("What's y? ")

z = x + y

print(z)

Running it gives us a wrong output. Why might this be?

Prior, we have seen how the + sign concatenates two strings. Because your input from your keyboard on your computer comes into the interpreter as text, it is treated as a str string. We, therefore, need to convert this input from a string to an integer. We can do so as follows:

x = input("What's x? ")
y = input("What's y? ")

z = int(x) + int(y)

print(z)

We can further improve our program as follows:

x = int(input("What's x? "))
y = int(input("What's y? "))

print(x + y)

This shows that you can run functions on functions although the inner function run first, then the other one runs after. First the input function is run, Then the int function.

You can always learn more about it in Python's documentation of  int.

Readability-Wins

Regardless of what approach you take to a programming task, remember that your code must be readable. You should use comments to give yourself and others clues about what your code is doing. Further, you should create code in a way that is readable.

While you can nest functions like print(int(input("x: ")) + int(input("y: "))), it can be hard to read. Writing clear, step-by-step code is often better than writing the most compact code possible. Readability wins.

Float-Basics--More-on-Floats

A float is a number with a decimal point. Just like with integers, you can cast strings to floats using the float() function.

This is in contrast to an int (integer), which can only represent whole numbers. When you get input from a user using the input() function, it returns a string by default, even if the user types in a number. Therefore, to perform mathematical operations with decimals, you must explicitly convert the string to a float using float().

Now, when you perform math with floats, you can sometimes end up with what's called floating-point imprecision. It's a quirk of how computers store and calculate numbers. You might divide 1 by 10 and get 0.09999999999999999. That's not what we want

The built-in round() function can be used to round a float to a specified number of decimal places. You can also format numbers directly inside f-strings.

But what if you want to format that number for display, maybe to show it as currency? That's when you use an f-string, or "formatted string literal." It's a way of embedding expressions inside string literals, and it gives you a lot of control over formatting.

Consider this line print(f"${z:,.2f}")

  • The f tells Python it's a formatted string.

  • The $ is just a literal character.

  • The {} is where you put your variable, z.

  • And after the colon, you have a format specifier.

    • The , adds a comma as a thousands separator.

    • The .2f tells Python to format the number as a float with exactly two digits after the decimal point.

x = float(input("What's x? "))
y = float(input("What's y? "))

# Round the sum of x and y to two decimal places
z = round(x + y, 2)
print(z) # e.g., 6.91

# An f-string can format this for currency, adding comma separators
# and ensuring two decimal places.
print(f"${z:,.2f}") # e.g., $1,000.00

def

To keep code organized and reusable, we can define our own functions using the def keyword. It's a convention to put the main logic of your program inside a function called main.

put your main program logic inside a function called main(). Why? Because it keeps your code organized and prevents everything from running all at once when you import other files.

def main():
    name = input("What's your name? ")
    # This is a call to our custom 'hello' function
    hello(name)

def hello(to):
    print(f"hello, {to}")

# This line starts the program by calling the main function
main()

First, we define a function main(). Inside it, we ask the user for their name, and then we call our second custom function, hello(), and pass it the name variable.

Next, we define hello(). Notice it has a parameter, to, inside the parentheses. This is a placeholder for a value that the function will receive. When main() calls hello(name), it passes the value of the name variable into hello(), where it gets assigned to the parameter to. The function then uses this value to print the greeting.

Finally, the very last line, main(), is what actually starts the program. It calls the main function, and that function then handles everything else. It's a great way to structure your code for clarity and reusability.

When the code calls hello(name) from inside the main function, the value stored in the name variable is passed to the hello function. The hello function then receives this value and assigns it to its own internal variable, to. This allows the function to use that value to complete its task, which is to print "hello, " followed by the person's name.

Returning-Values

Functions can send data back to the code that called them using the return keyword. This is incredibly powerful because it allows you to get a result from a function and store it in a variable.

def main():
    x = int(input("What's x? "))
    # Call the square function and print the value it returns
    print("x squared is", square(x))

# This function takes a number 'n' as a parameter and returns its square
def square(n):
    return n * n

main()

The square function takes a parameter n. Inside the function, it calculates n * n. The return keyword then takes that calculated value and sends it back out of the function.

A step by step breakdown of this would be

  1. The program starts by calling main().

  2. Inside main(), the user is prompted to enter a number, which is stored in the variable x. Let's say the user enters 5.

  3. The next line is print("x squared is", square(x)). Before print can do its job, it needs to get a value from the square(x) part.

  4. Python calls the square function, passing the value of x (which is 5) as the argument for the parameter n.s

  5. Inside square, n * n is calculated (5 * 5 = 25).

  6. The return keyword sends the value 25 back to where the function was called.

  7. Now, the original line effectively becomes print("x squared is", 25). The square(x) part has been replaced by the value it returned.

  8. The program then prints the final output: "x squared is 25".

By using return, we can keep our code clean and modular. The square function does one thing—calculate a square—and then gives the result back to main, which handles the printing. This separation of concerns is a core principle of good programming.

Summing Up

In Week 1,

  • you've learned the absolute essentials of Python: using functions like print() and input(), storing data in variables, understanding data types (str, int, float), and organizing your code by defining your own functions with def and return. You now have the foundational tools to start building more complex programs.

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