# Dictionaries

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered\*, changeable and do not allow duplicates.

```
// Example
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
```

Dictionaries are written with curly brackets, and have keys and values:

```
// Example
// Create and print a dictionary:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict)
```

### Dictionary Items

Dictionary items are ordered, changeable, and does not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

```
// Example
// Print the "brand" value of the dictionary:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict["brand"])
```

### Ordered or Unordered?

As of Python version 3.7, dictionaries are *ordered*. In Python 3.6 and earlier, dictionaries are *unordered*.

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.

### Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

### Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

```
// Example
// Duplicate values will overwrite existing values:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964,
  "year": 2020
}
print(thisdict)
```

### Dictionary Length

To determine how many items a dictionary has, use the `len()` function:

```
// Example
Print the number of items in the dictionary:
print(len(thisdict))
```

### Dictionary Items - Data Types

The values in dictionary items can be of any data type:

```
// Example
// String, int, boolean, and list data types:
thisdict = {
  "brand": "Ford",
  "electric": False,
  "year": 1964,
  "colors": ["red", "white", "blue"]
}
```

### type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

`<class 'dict'>`

```
// Example
// Print the data type of a dictionary:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(type(thisdict))
```

### Python Collections (Arrays)

There are four collection data types in the Python programming language:

* [**List**](https://www.w3schools.com/python/python_lists.asp) is a collection which is ordered and changeable. Allows duplicate members.
* [**Tuple**](https://www.w3schools.com/python/python_tuples.asp) is a collection which is ordered and unchangeable. Allows duplicate members.
* [**Set**](https://www.w3schools.com/python/python_sets.asp) is a collection which is unordered, unchangeable\*, and unindexed. No duplicate members.
* **Dictionary** is a collection which is ordered\*\* and changeable. No duplicate members.

\*Set *items* are unchangeable, but you can remove and/or add items whenever you like.

\*\*As of Python version 3.7, dictionaries are *ordered*. In Python 3.6 and earlier, dictionaries are *unordered*.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

### Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

```
// Example
// Get the value of the "model" key:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
x = thisdict["model"]
```

There is also a method called `get()` that will give you the same result:

```
// Example
// Get the value of the "model" key:
x = thisdict.get("model")
```

### Get Keys

The `keys()` method will return a list of all the keys in the dictionary.

```
// Example
// Get a list of the keys:
x = thisdict.keys()
```

The list of the keys is a *view* of the dictionary, meaning that any changes done to the dictionary will be reflected in the keys list.

```
// Example
// Add a new item to the original dictionary, and see that the keys list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.keys()

print(x) #before the change

car["color"] = "white"

print(x) 
#after the change
```

### Get Values

The `values()` method will return a list of all the values in the dictionary.

```
// Example
// Get a list of the values:
x = thisdict.values()
```

The list of the values is a *view* of the dictionary, meaning that any changes done to the dictionary will be reflected in the values list.

```
// Example
// Make a change in the original dictionary, and see that the values list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.values()

print(x) #before the change

car["year"] = 2020

print(x) 
#after the change
```

```
// Example
// Add a new item to the original dictionary, and see that the values list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.values()

print(x) 
#before the change

car["color"] = "red"

print(x) 
#after the change
```

### Get Items

The `items()` method will return each item in a dictionary, as tuples in a list.

```
// Example
// Get a list of the key:value pairs
x = thisdict.items()
```

The returned list is a *view* of the items of the dictionary, meaning that any changes done to the dictionary will be reflected in the items list.

```
// Example
Make a change in the original dictionary, and see that the items list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.items()

print(x) #before the change

car["year"] = 2020

print(x) #after the change
```

```
// Example
// Add a new item to the original dictionary, and see that the items list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.items()

print(x) #before the change

car["color"] = "red"

print(x) #after the change
```

### Check if Key Exists

To determine if a specified key is present in a dictionary use the `in` keyword:

```
// Example
// Check if "model" is present in the dictionary:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
if "model" in thisdict:
  print("Yes, 'model' is one of the keys in the thisdict dictionary")
```

### Change Values

You can change the value of a specific item by referring to its key name:

```
// Example
// Change the "year" to 2018:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["year"] = 2018
```

### Update Dictionary

The `update()` method will update the dictionary with the items from the given argument.

The argument must be a dictionary, or an iterable object with key:value pairs.

```
// Example
// Update the "year" of the car by using the update() method:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"year": 2020})
```

### Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

```
// Example
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["color"] = "red"
print(thisdict)
```

### Update Dictionary

The `update()` method will update the dictionary with the items from a given argument. If the item does not exist, the item will be added.

The argument must be a dictionary, or an iterable object with key:value pairs.

```
// Example
// Add a color item to the dictionary by using the update() method:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"color": "red"})
```

### Removing Items

There are several methods to remove items from a dictionary:

```
// Example
// The pop() method removes the item with the specified key name:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.pop("model")
print(thisdict)
```

```
// Some code popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.popitem()
print(thisdict)
```

```
// Example
// The del keyword removes the item with the specified key name:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
del thisdict["model"]
print(thisdict)
```

```
// Example
// The del keyword can also delete the dictionary completely:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
del thisdict
print(thisdict) #this will cause an error because "thisdict" no longer exists.

```

```
// Some codeExample
The clear() method empties the dictionary:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.clear()
print(thisdict)
```

### Loop Through a Dictionary

You can loop through a dictionary by using a `for` loop.

When looping through a dictionary, the return value are the *keys* of the dictionary, but there are methods to return the *values* as well.

```
// Example
// Print all key names in the dictionary, one by one:
for x in thisdict:
  print(x)
```

```
// Example
// Print all values in the dictionary, one by one:
for x in thisdict:
  print(thisdict[x])
```

```
// Example
// You can also use the values() method to return values of a dictionary:
for x in thisdict.values():
  print(x)
```

```
// Example
You can use the keys() method to return the keys of a dictionary:
for x in thisdict.keys():
  print(x)
```

```
// Example
// Loop through both keys and values, by using the items() method:
for x, y in thisdict.items():
  print(x, y)

```

### Copy a Dictionary

You cannot copy a dictionary simply by typing `dict2 = dict1`, because: `dict2` will only be a *reference* to `dict1`, and changes made in `dict1` will automatically also be made in `dict2`.

There are ways to make a copy, one way is to use the built-in Dictionary method `copy()`.

```
// Example
Make a copy of a dictionary with the copy() method:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
mydict = thisdict.copy()
print(mydict)
```

Another way to make a copy is to use the built-in function dict().

```
// Example
/ Make a copy of a dictionary with the dict() function:
thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
mydict = dict(thisdict)
print(mydict)

```

### Nested Dictionaries

A dictionary can contain dictionaries, this is called nested dictionaries.

```
// Example
Create a dictionary that contain three dictionaries:
myfamily = {
  "child1" : {
    "name" : "Emil",
    "year" : 2004
  },
  "child2" : {
    "name" : "Tobias",
    "year" : 2007
  },
  "child3" : {
    "name" : "Linus",
    "year" : 2011
  }
}
```

Or, if you want to add three dictionaries into a new dictionary:

```
// Example
// Create three dictionaries, then create one dictionary that will contain the other three dictionaries:
child1 = {
  "name" : "Emil",
  "year" : 2004
}
child2 = {
  "name" : "Tobias",
  "year" : 2007
}
child3 = {
  "name" : "Linus",
  "year" : 2011
}

myfamily = {
  "child1" : child1,
  "child2" : child2,
  "child3" : child3
}
```

### Dictionary Methods

Python has a set of built-in methods that you can use on dictionaries.

| Method                                                                         | Description                                                                                                 |
| ------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------- |
| [clear()](https://www.w3schools.com/python/ref_dictionary_clear.asp)           | Removes all the elements from the dictionary                                                                |
| [copy()](https://www.w3schools.com/python/ref_dictionary_copy.asp)             | Returns a copy of the dictionary                                                                            |
| [fromkeys()](https://www.w3schools.com/python/ref_dictionary_fromkeys.asp)     | Returns a dictionary with the specified keys and value                                                      |
| [get()](https://www.w3schools.com/python/ref_dictionary_get.asp)               | Returns the value of the specified key                                                                      |
| [items()](https://www.w3schools.com/python/ref_dictionary_items.asp)           | Returns a list containing a tuple for each key value pair                                                   |
| [keys()](https://www.w3schools.com/python/ref_dictionary_keys.asp)             | Returns a list containing the dictionary's keys                                                             |
| [pop()](https://www.w3schools.com/python/ref_dictionary_pop.asp)               | Removes the element with the specified key                                                                  |
| [popitem()](https://www.w3schools.com/python/ref_dictionary_popitem.asp)       | Removes the last inserted key-value pair                                                                    |
| [setdefault()](https://www.w3schools.com/python/ref_dictionary_setdefault.asp) | Returns the value of the specified key. If the key does not exist: insert the key, with the specified value |
| [update()](https://www.w3schools.com/python/ref_dictionary_update.asp)         | Updates the dictionary with the specified key-value pairs                                                   |
| [values()](https://www.w3schools.com/python/ref_dictionary_values.asp)         | <p>Returns a list of all the values in the dictionary<br></p>                                               |


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