Expressions & Functions

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Python setup

Where to program Python?

You can complete our coursework entirely in the browser:

Screenshot of CoRise exercise Exercises can be coded in embedded Python widgets on CoRise.com


Screenshot of Google CoLab Projects will be coded in Google CoLab (Browser-based Jupyter notebooks)


...but you may still want a place to play with Python.

Python on your desktop

Download the latest Python from python.org/downloads.

Start a Python REPL:


            $ python3
            Python 3.11.0 (v3.11.0:deaf509e8f, Oct 24 2022, 14:43:23)
            >>> 2 + 2
            4

Run a Python program:


            $ python3 file.py

Python code editors

A code editor is a text editor with support for syntax highlighting, linting, refactoring, and other helpful tools.

The two most popular editors for Python:

Python in the browser

Python Building Blocks
(Values & Expressions)

Values

Programs manipulate values.

Each value has a certain data type.

Data type Example values
Integers 2 44 -3
Floats 3.14 4.5 -2.0
Booleans True False
Strings '¡hola!' 'its python time!'

Expressions (with operators)

An expression describes a computation and evaluates to a value.

Some expressions use operators:


          18 + 69
          

          6/23
          

          2 * 100
          

          2 ** 100

Expressions (with function calls)

Many expressions use function calls:


          pow(2, 100)
          

          max(50, 300)
          

          min(-1, -300)

See all built-in global functions.

Variables

Variables

Name bound to value

A variable binds a name to a value.

x = 7
Name Value

The value can be any expression:

x = 1 + 2 * 3 - 4 // 5
Name Expression

Using variables

A variable can be referenced multiple times:


          x = 10
          y = 3

          result1 = x * y
          result2 = x + y

Re-assigning variables

A particular name can only be bound to a single value.


          my_name = 'Pamela'

          my_name = my_name + 'ela'

💬 Will that code error? If not, what will my_name store?
It will not error (similar code in other languages might, however). The name my_name is now bound to the value 'Pamelaela'.

Functions

What is a function?

A function is a sequence of code that performs a particular task and can be easily reused. ♻️

We've already used functions:


            abs(-1)
            pow(2, 10)
            add(18, 69)
            mul(60, sub(5, 4))

A function takes in an input (the arguments) and returns an output (the return value).

2, 10 → pow → 1024

Why functions?

Consider this function-less code:


            greeting1 = "Hello, Jackson, how are you?";
            greeting2 = "Hello, Dr. Biden, how are you?";
            greeting3 = "Hello, Stranger, how are you?";

Functions help when code has repetition.

Function parameters/arguments help when repetitive code has variation.

Building a function

First identify the repetitive parts:


            def say_greeting():
                return "Hello, how are you?"

Then use parameters for the parts that vary:


            def say_greeting(name):
                return "Hello, " + name + ", how are you?"

Finally, test it with different arguments:


            say_greeting("Jackson")
            say_greeting("Dr. Biden")
            say_greeting("Stranger")

Function definition

The most common way to define functions is Python is the def statement.


            def <name>(<parameters>):
                return <return expression>

Example:


            def add(num1, num2):
                return num1 + num2

Once defined, we can call it:


            add(2, 2)
            add(18, 69)

Anatomy of a function definition

The first line is called the function signature, all lines after are considered the function body.


            def <name>(<parameters>):        # ← Function signature
                return <return expression>   # ← Function body
            

            def add(num1, num2):             # ← Function signature
                return num1 + num2           # ← Function body

The function body can have multiple lines:


            def add(num1, num2):             # ← Function signature
                sum = num1 + num2            # ← Function body
                return sum                   # ← Function body

Function arguments

We can pass in any expressions as arguments.


            def add(num1, num2):
                return num1 + num2
            

            x = 1
            y = 2
            add(x, y)
            

            x = 3
            add(x * x, x + x)

Return values

The return keyword returns a value to whoever calls the function (and exits the function).


            def add(num1, num2):
                return num1 + num2

            sum = add(2, 4)

Reminder: You can use function calls in expressions:


            big_sum = add(200, 412) + add(312, 256)

...and nest function calls inside function calls:


            huge_sum = add(add(200, 412), add(312, 256))

Spot the bug #1

What's wrong with this code?


            def add(num1, num2):
                return sum
                sum = num1 + num2

            sum = add(2, 4)

The code after the return statement will not be executed, that line belongs before the return.

Spot the bug #2

What's wrong with this code?


            def add():
                return num1 + num2

            sum = add(2, 4)

The function body is referring to variables that don't seem to exist. Most likely, they should be parameters in the function signature.

Spot the bug #3

What's wrong with this code?


            def add(num1, num2):
                sum = num1 + num2

            sum = add(2, 4)

The function body does not return any value. However, the code that calls it tries to use the result of the expression. It should have a return statement that returns the sum.