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+# Python functions
+
+Define a function using the `def` keyword. Once defined, it can be called (or executed) by using the function name and a pair of brackets.
+
+```python
+def greet():
+    name = input("Enter your name: ")
+    print(f"Hello, {name}!")
+
+
+greet()
+```
+
+> Could show small callouts beside each section of the shown code sample.
+> E.g. beside `def greet()` show "Defining the function name"
+> Then beside the function body, show "Function body, will run when we execute the function".
+> Then beside `greet()`, show "Executing the function starts the function body"
+> Highlight the difference in indentation level with a callout.
+> Signal that when we have less indentation, we exit the function.
+
+
+## The `return` keyword
+
+By default, functions return `None`.
+
+Example:
+
+```python
+def greet():
+    pass  # Do nothing
+
+
+# print the function's return value
+print(greet())  # None
+```
+
+- `pass` is a keyword which means "do nothing". It's necessary since a function needs a body.
+- you can control what a function returns using the `return` keyword.
+
+Example:
+
+```python
+def greet_user():
+    name = "John Doe"
+    return f"Hello, {name}!"
+
+greeting = greet()
+print(greeting)  # John Doe
+```
+
+A function can return any value.
+
+## Arguments and parameters
+
+Functions can accept values from the caller, and return a value in their place.
+
+```py
+def greet(name):
+    return f"Hello, {name}
+
+greeting = greet("Rolf Smith")
+print(greeting)  # Hello, Rolf Smith
+```
+
+> Here we could have a diagram showing the flow of data using arrows.
+> From the string "Rolf Smith" to the function, then through the return value to the `greeting` variable.
+
+## Number of parameters
+
+A function can have any number of parameters.
+
+For example:
+
+```python
+def greet(name, age):
+    return f"{name} is {age} years old!"
+```
+
+## Named arguments
+
+If you want to be specific about what arguments you pass to parameters, use named arguments.
+
+Example:
+
+```python
+def greet(name, age):
+    return f"{name} is {age} years old!"
+
+print(greet(name="John Doe", age=31))
+```
+
+## Default parameter values
+
+Parameters can have default values:
+
+```python
+def greet(name="John Doe"):
+    return f"Hello, {name}!"
+
+
+print(greet())  # Hello, John Doe!
+print(greet("Rolf Smith"))  # Hello, Rolf Smith
+```
+
+When passing an argument to the function, the default parameter value will be overwritten by the given argument.
+
+If the function has a mix of normal and default parameters, the default parameters have to be passed last.
+
+
+```python
+def greet(age, name="John Doe"):
+    return f"{name} is {age} years old!"
+
+
+print(greet(32))
+```
+
+Incorrect:
+
+```python
+def greet(name="John Doe", age):
+    return f"{name} is {age} years old!"
+
+
+print(greet(32))
+```
+
+Read more about Python functions [here](https://www.teclado.com/30-days-of-python/python-30-day-12-functions).
+
+# Python Lambda Expressions
+
+- Lambda expressions are an alternative syntax for defining simple functions.
+
+Example: `lambda name: f"Hello, {name}"`
+
+- No name
+- Can be assigned to a variable
+- Can be difficult to read
+
+Calling a lambda inline requires multiple pairs of brackets:
+
+```python
+result = (lambda x, y: x + y)(15, 3)
+```
+
+# First Class Functions
+
+- Functions in Python are "first class citizens".
+- They can be stored in data structures, passed as arguments, and stored in variables.
+
+Example:
+
+```python
+def avg(*args):
+    return sum(args) / len(args)
+
+
+operations = {
+    "average": avg,
+    "total": sum,
+    "top": max
+}
+
+# We can also use associations to get a function from the dictionary.
+selection = "average"
+operation = operations[selection]
+operation() # This is acts the same as simply calling avg()
+```
+
+
+Read more about lambda expressions and first-class functions [here](https://www.teclado.com/30-days-of-python/python-30-day-16-lambda-expressions).