Dictionaries

A Python dictionary is just like those giant fat books that you don't use anymore: there is a key (the "word" you want to look up) associated with a value (the corresponding "definition" of the word or key). This key–value pair is what makes a dictionary a unique container.

Properties of dictionaries:

Keys

Immutable
- cannot change an existing key
unique
- cannot have multiple identical keys corresponding to different values
not stored an any particular order
- object access can only be by numeric index if that number is, in fact, a defined key

Values

can be any object without restriction
- values do not have to be unique or immutable



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# start with an empty dictionary, notice dictionaries are enclosed in squiggly brackets
derp = {}

key = "daniel chen"
value = 25

# you fill dictionaries, just like lists

derp = {key:value, "niels bantilan": 25}
print derp

# note dictionaries are not ordered, DO NOT assume any order when working with dictionaries

So we have an initial dictionary with key:value pairs, how do we extend it to and add more key:values?



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# derp["my_new_key"] = "my_new_value"
print derp



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derp["Vivian Peng"] = 25
print derp



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# remove entry from dictionary
derp['greg wilson'] = 85
print derp

del derp['greg wilson']
print derp



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# assigning a new value to a key that already exists will overwrite the original value
derp["daniel chen"] = 21
print derp

The main thing we are interested in, however, is using a dictionary. How do we 'look' keys up? Just like list indexing, as it turns out.



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# How old is Daniel Chen?
print derp["daniel chen"]



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# Alernatively:
print derp.get("daniel chen")

Let's look for a key that does not exist in this dictionary:



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print derp["monty python"]



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# Note the different result here, so you will handle missing keys differently.
print derp.get("monty python")

The 'in' operator is an easy way to check for a key.



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'monty python' in derp



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'daniel chen' in derp

You can also use the 'has_key' method



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derp.has_key('monty python')



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derp.has_key('daniel chen')

More dictionary stuff



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# List all keys.
print derp.keys()



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# List all values.
print derp.values()



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# Length = (number of key:value pairs)
print len(derp)



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# if you get confused about variable type, then
type(derp) # is your best friend!

Exercise

1

Load a dictionary with the following key–value pairs. Print it.

key	value
fname	your first name
lname	your last name
age	25
height	69
weight	180



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# This is one approach, where you directly fill the dictionary
my_dict = {}

my_dict = {"fname":"daniel", "lname":"chen", "age":25, "height":69, "weight":180}

print my_dict



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# This is another approach whre you dill the dictionary using variables

fname = "daniel"
lname = "chen"
age = 25
height = 69
weight = 180
my_dict = {'fname':fname, 'lname':lname, 'age':age, 'height':height, 'weight':weight}

print my_dict

2

A more advanced question: how would you store the above data in a single dictionary entry?

(You could accomplish this using a list of the above values, but a more straightforward solution is like a database: simply store the dictionary above with an appropriate key, such as a number or name.)



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#using list
hardQuestion = {1: ["daniel", "chen", 25, 69]}
print hardQuestion

print hardQuestion.get(1)[0]

# using dictionary
hardQuestion = {1: {"fname":"daniel", "lname":"chen", "age":25, "height":69}}
print hardQuestion

print hardQuestion.get(1).get("fname")

firstPerson =  hardQuestion.get(1)
firstPerson
type(firstPerson)

print firstPerson.get("fname")



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