Every key within a Python dictionary must be unique. Because each key acts as a unique identifier for a specific key-value pair, keys cannot be repeated. However, multiple distinct keys can map to duplicate values within the same dictionary.

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• Dictionary is a built-in data structure. A dictionary is an ordered and mutable collection of objects.
• It consists of Key-Value pairs. Each Key must be Unique.

Syntax:
```python
dictionary = {"key name": value}
```
```python
student_scores = {
    "John" : 76,
    "Peter": 95,
    "Mike": 84
}
```
We can access the Values using the Keys:
```python
print(student_scores["John"])
```
This ```python print() ``` statement would print out the value 76. It is also important to note that keys must map to a unique value. 


Dictionary 

Dictionaries have several useful built-in methods for altering dictionary values.
- Updating/adding to a dictionary:
A dictionary can be updated by passing in an iterable object with key:value pairs (usually another dictionary) into the update method. In the example below, the "apple" key is assigned the new value "green".
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
dict.update({"apple": "green"})
```
Additionally, if a key/value pair that doesn't exist in the dictionary is passed through the update method, it is added. This is shown below.
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
dict.update({"tangerine": "orange"})
```
-Removing values from a dictionary:
There are multiple ways of removing values from a dictionary. To remove a specific item, you can use the pop or del method by passing in the key name.
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
dict.pop("banana")
del dict["apple"]
```
If you want to remove the last item in the dictionary, you can use the popitem method. 
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
dict.popitem() #There are no parameters passed in, since popitem always removes the #last item in the dictionary.
```
You can also either remove every item from the dictionary using the clear method or delete the structure itself using the del method.
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
dict.clear() #dictionary is emptied, but still exists. It can be updated with new items if you want.
del dict #dictionary is actually deleted. Referencing it again will cause an error.
```
-Looping over a dictionary
Like removing items, there are multiple ways of iterating over dictionary items depending on if you want keys, values, or both. You will iterate via a for loop and either use the key method to return only keys, the values method to return only values, or the items method to return the full key:value pair.
```python
dict = {
	"banana": "yellow",
	"apple": "red",
	"grape": "purple"
}
for x in dict.keys():
	print(x)
for x in dict.values():
	print(x)
for x, y in dict.items():
	print(x, y)
```
Other methods that may be used include:
- copy()
- fromkeys()
- get()
- setdefault()

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