---
title: "Dictionaries"
---
# Dictionaries
> A Dictionary is a mapping object. Where sequences are indexed with a numeric index value, dictionaries are indexed using keys. These keys then have a matching value pair that is associated with a particular key. For example, with a list we can extract the object at index 0, which we may have decided is the name, whereas with dictionaries, I can instead extract the a value using a key "name". Because of how they work, dictionaries are sometimes known as associative arrays in other programming languages.
## Mapping Type
Dictionaries are created using braces `{` `}` , where each key/value pair is separated by a comma ( , ) and keys are separated from their values using a colon ( : ). In the example below, I created a dictionary with two keys: name, id.
```python title="Python - Creating a Dictionary"
# In this dictionary, I associated the name John Smith to the key "name",
# and the id number 12345 to the key "id".
myDictionary = {"name":"John Smith", "id":12345}
```
### Using a Dictionary
The keys in a dictionary can be numbers, strings, or tuples, but typically a string is used to make a key that best describes the value. Any given key may only appear once in a dictionary, so trying to set another value for a key that already exists will overwrite the previous value for that key. Alternately, attempting to access the value of a key that does not exist will throw an error, while setting a value to a key that does not exist will create a new key/value pair within the dictionary.
To access a value in a dictionary works much like accessing a value in a list; simply place brackets containing the key after the dictionary object.
```python title="Python - Accessing Values in a Dictionary"
# Creates a dictionary with three key/value pairs.
myDictionary = {'Bob': 89.9, 'Joe': 188.72, 'Sally': 21.44}
print myDictionary['Joe'] # Will print out: 188.72
# Adds a key for 'Amir', and alters the value associated with the key 'Sally'.
myDictionary['Amir'] = 45.89
myDictionary['Sally'] = 146.23
print myDictionary # Will print out the whole dictionary: {'Joe': 188.72, 'Amir': 45.89, 'Bob': 89.9, 'Sally': 146.23}
```
It is also easy to loop through all of the values of a dictionary using the keys() function. For example:
```python title="Python - Keys Function"
# The keys() function provides us with a list of keys, which we can iterate through and print out in addition to using in the value lookup.
for key in myDict.keys():
print key, myDict[key]
```
There are many use cases for dictionaries, but they are commonly used in Ignition when passing values into a Message Handler or [creating a dynamic roster](ignition-modules/alarm-notification/alarm-notification-pipelines/pipeline-blocks/notification-block.md#calculated) for alarms.
## Dictionary Functions
Dictionaries have a few functions that allow for greater control over the dictionary object and the values contained within.
|Function |Description |Example |Output |
|--|--|--|--|
|len(dictionary)|Returns the number of items in the dictionary.|myDictionary = `{"name":"John Smith", "id":12345}`print len (myDictionary)|2
|
|del dictionary[key] |Will remove the named key. |myDictionary = `{"name":"John Smith", "id":12345}`del myDictionary["id"]print myDictionary |`{'name': 'John Smith'}`|
|key in dictionary |Will return True if the dictionary has that key.Can also use "key not in dictionary" |myDictionary = `{"name":"John Smith", "id":12345}`if "name" in myDictionary: print myDictionary["name"] |John Smith
|
|dictionary.clear() |Remove all of the items in the dictionary. |myDictionary = `{"name":"John Smith", "id":12345}`myDictionary.clear()print myDictionary |`{}` |
|dictionary.keys() |Returns a list of the dictionary's keys. |myDictionary = `{"name":"John Smith", "id":12345}`print myDictionary.keys() |['name', 'id']
|
|dictionary.values() |Returns a list of the dictionary's values. |myDictionary = `{"name":"John Smith", "id":12345}`print myDictionary.values() |['John Smith', 12345]
|