freeCodeCamp/client/src/pages/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.

### Syntax

In Python, a function definition has the following features:

1. The keyword `def`
2. a function name
3. input parameters (optional)
4. some block of code to execute
5. a return statement (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
```

Functions are blocks of code that can be reused simply by calling the function. This enables simple, elegent code reuse without explicitly re-writing sections of code. This makes code both more readable, makes for easier debugging, and limits typing errors. 

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

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 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
```

You can define default values for the parameters, that way Python will interpretate that the value of that parameter is the default one if none is given.

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

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

You can pass the parameters in the order you want, 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 the 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
  ```

### More Information:
- <a href='https://docs.python.org/3/tutorial/controlflow.html#defining-functions' target='_blank' rel='nofollow'>Python 3 Docs: Defining Functions</a>