freeCodeCamp/guide/english/python/functions/index.md

---
title: Functions
---
## Functions

A function allows you to define a reusable block of code that can be executed many times within your program. 

Functions allow you to create more modular and [DRY](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself) solutions to complex problems.

While Python already provides many built-in functions such as `print()` and `len()`, you can also define your own functions to use within your projects.

One of the great advantages of using functions in your code is that it reduces the overall number of lines of code in your project.

### Syntax

In Python, a function definition has the following features:

1. The keyword `def`
2. A function name
3. Parentheses '()', and within paranthesis input parameters,although the input parameters are optional
4. A colon ':'
5. Some block of code to execute (that must be indented relative to the definition statement)
6. A return statement (optional)
7. [docstring](https://github.com/freeCodeCamp/freeCodeCamp/blob/master/guide/english/python/docstring/index.md) (optional)

```python
# a function with no parameters or returned values
def sayHello():
  print("Hello!")

sayHello()  # calls the function, 'Hello!' is printed to the console

# a function with a parameter
def helloWithName(name):
  print("Hello " + name + "!")

helloWithName("Ada")  # calls the function, 'Hello Ada!' is printed to the console

# a function with multiple parameters with a return statement
def multiply(val1, val2):
  return val1 * val2

print(multiply(3, 5))  # prints 15 to the console

# a function with a sample docstring, multiple parameters and a return statement
def addValues(val1, val2): 
  """ Returns the sum of two numbers """
  return val1 + val2

print(addValues(4, 7))  # prints 11 to the console
  
```

Functions are blocks of code that can be reused simply by calling the function. This enables simple, elegant code reuse without explicitly re-writing sections of code. This makes code more readable, easily debugged, and error-proof. 

Functions in Python are created using the `def` keyword, followed by a function name and function parameters inside parentheses.

A function always returns a value,The `return` keyword is used by the function to return a value, if you don't want to return any value, the default value `None` will be returned. 

The function name is used to call the function, passing the needed parameters inside parentheses.

```python
# this is a basic sum function
def sum(a, b):
  return a + b

result = sum(1, 2)
# result = 3
```
It's important to note that if there is any code within the function after the `return` value, it can't be reached.

```python
def sum(a, b):
  return a + b
  print("This print action will never be reached")
```

You can define default values for the parameters so that Python will interpret those values as the default values if none are given.

```python
def sum(a, b=3):
  return a + b

result = sum(1)
# result = 4
```

You can pass the parameters in a desired order, using the name of the parameter.

```python
result = sum(b=2, a=2)
# result = 4
```

However, it is not possible to pass a keyword argument before a non-keyword one

```Python
result = sum(3, b=2)
#result = 5
result2 = sum(b=2, 3)
#Will raise SyntaxError
```

Functions are also objects, so you can assign them to a variable, and use that variable like a function.

```python
s = sum
result = s(1, 2)
# result = 3
```

### Notes

- If a function definition includes parameters, you must provide the same number of parameters when you call the function.

  ```python
  print(multiply(3))  # TypeError: multiply() takes exactly 2 arguments (0 given)

  print(multiply('a', 5))  # 'aaaaa' printed to the console

  print(multiply('a', 'b'))  # TypeError: Python can't multiply two strings
  ```

- The block of code that the function will run includes all statements indented within said function.

  ```python
  def myFunc():
    print('this will print')
    print('so will this')

  x = 7
  # the assignment of x is not a part of the function since it is not indented
  ```

- Variables defined within a function only exist within the scope of that function.

  ```python
  def double(num):
    x = num * 2
    return x

  print(x)  # error - x is not defined
  print(double(4))  # prints 8
  ```

- Python interprets the function block only when the function is called and not when the function is defined. So even if the function definition block contains some sort of error, the python interpreter will only reveal this when the function is called. 

### Recursion
Using functions in our programs can help to explicitly handle the execution of specific type of algorithms.
One useful method is recursion.  Recursion may not only save time but can explictly handle the code and execute the given task.

```python
def factorial(n):
   if n==0:
     return 1
   else:
     return  n*factorial(n-1)
```
In this example we are computing factorial of a given number without using any loops. Whenever we use recursion we must define a base case.  The remaining cases can recursively call the `factorial` function with a number as the argument without the need for loop statements to perform the itertion.

**Walkthrough**

We need to find the factorial of 5 (`5!`)

1. First we call `factorial(5)`
2. `n` is not equal to `0` so the flow jumps to the else statement
3. In the else statment, we return the product of `n` and the `factorial` function with `n-1` as an argument.
4. This continues until `n` is equal to `0`, our base case, where we return `1`.
5. The final return value will be the product of all values between `1` and `5` (e.g. `1 * 2 * 3 * 4 * 5`) or `120`.

### More Information:
- [Python 3 Docs: Defining Functions](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
- [Python 3 Docs: Docstring Conventions](https://www.python.org/dev/peps/pep-0257/)