# Variables

A variable is used to store data that will be used by the program. This data can be a number, a string, a Boolean, a list or some other data type. Every variable has a name which can consist of letters, numbers, and the underscore character `_`.

The equal sign `=` is used to assign a value to a variable. After the initial assignment is made, the value of a variable can be updated to new values as needed.

```python
# These are all valid variable names and assignment
user_name = "@sonnynomnom"
user_id = 100
new_user = False

# A variable's value can be changed after assignment
points = 100
points = 120
```

## Plus Equals

Python offers a shorthand for updating variables. When you have a number saved in a variable and want to add to the current value of the variable, you can use the `+=` (plus-equals) operator.

```python
# First we have a variable with a number saved
number_of_miles_hiked = 12

# Then we need to update that variable
# Let's say we hike another two miles today
number_of_miles_hiked += 2

# The new value is the old value
# Plus the number after the plus-equals
print(number_of_miles_hiked)
# Prints 14
```

The plus-equals operator also can be used for string concatenation, like so:

```python
hike_caption = "What an amazing time to walk through nature!"

# Almost forgot the hashtags!
hike_caption += " #nofilter"
hike_caption += " #blessed"
```

The `+=` operator allows **adding more than just a single value at a time.** Anything on the right-hand side of the `+=` will be evaluated, and then it will be added to the variable which is then updated to that new value.

```python
# Plus Equals with a single value:
miles = 80
miles += 20 # miles is now 100

# Plus Equals with more than one value:
snow_level = 10
snow_level += 20 + 10 + 40 # snow_level is now 80
```

## The Scope of Variables

In Python, a variable defined inside a function is called a **local variable**. It cannot be used outside of the scope of the function, and attempting to do so without defining the variable outside of the function will cause an error.

A variable that is defined outside of a function is called a **global variable**. It can be accessed inside the body of a function.

```python
a = 5

def f1():
  a = 2
  print(a)
  
print(a)   # Will print 5
f1()       # Will print 2
```

In the example, the variable `a` is defined both inside and outside of the function. When the function `f1()` is implemented, `a` is printed as `2` because it is locally defined to be so. However, when printing `a` outside of the function, `a` is printed as `5` because it is implemented outside of the scope of the function.


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