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# Mutation

## Mutation

Mutable means ‘**able to be changed**’.

Immutable means ‘**constant**’. 

Example:

```python
foo = ['hi']
print(foo)
# Output: ['hi']

bar = foo
bar += ['bye']
print(foo)
# Output: ['hi', 'bye']
```

It’s not a bug. It’s **mutability in action**.

**ATENTION: Whenever you assign a variable to another variable of mutable datatype, any changes to the data are reflected by both variables!**

**The new variable is just an alias for the old variable.**

This is **only true for mutable datatypes**.

Here is a **gotcha** involving functions and mutable data types:

```python
def add_to(num, target=[]):
    target.append(num)
    return target

add_to(1)
# Output: [1]

add_to(2)
# Output: [1, 2]

add_to(3)
# Output: [1, 2, 3]
```

You might be expecting that a fresh list would be created when you call `add_to` **.**

Well again it is the mutability of lists which causes this pain.

ATENTION: In Python the **default arguments are evaluated once when the function is defined, not each time the function is called!**

**You should never define default arguments of mutable type** unless you know what you are doing. You should do something like this:

```python
def add_to(element, target=None):
    if target is None:
        target = []
    target.append(element)
    return target
```

Now whenever you call the function without the `target` argument, a new list is created. For instance:

```python
add_to(42)
# Output: [42]

add_to(42)
# Output: [42]

add_to(42)
# Output: [42
```