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Mapping Type

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.

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 - 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}

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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 - 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 - 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 for alarms.


Dictionary Functions

Dictionaries have a few functions that allow for greater control over the dictionary object and the values contained within.

FunctionDescriptionExampleOutput
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]



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