Python dictionaries

A dictionary in Python consists of a collection of unordered, key-value pairs:

• the collection denoted by a pair of curly braces { }
• each key-value pair with
  • the key as a string or a number
  • separated by a colon
  • the value with any data type
• each value accessed via its key string using square bracket notation
  • alas, we cannot use numeric indices like in lists and tuples

vocabulary = {
  'english' : ['Hello', 'Thanks', 'Goodbye'], 
  'french' : ['Bonjour', 'Merci', 'Au revoir'],
  'german' : ['Hallo', 'Danke', 'Tschüss'],
  'swedish' : ['Hej', 'Takk', 'Hej då']
}

print(vocabulary['french'])
# ['Bonjour', 'Merci', 'Au revoir']

# dictionaries don't need to include just words 

tax_rates = {
  'taxistan' : { 
  	'federal': 0.20, 
    'provincial': 0.05
   },
  'rolimpia' : 0.24,
  'manduria' : 0.22,
  'wawaland' : 0.19
}

print(tax_rates['manduria'])
# 0.22

print(tax_rates['taxistan']['federal'])
# 0.20

print(tax_rates[0])
# KeyError

Dictionary length

Similar to strings, lists and tuples, we can also find a dictionary's length using len(dictionary) built-in method:

tax_rates = {
  'taxistan' : { 
  	'federal': 0.20, 
    'provincial': 0.05
   },
  'rolimpia' : 0.24,
  'manduria' : 0.22,
  'wawaland' : 0.19
}


print(len(tax_rates))
# 4

print(len(tax_rates['taxistan']))
# 2

Dictionary update

We can update a dictionary using the update built-in method and by passing another dictionary into the argument:

tax_rates = {
  'taxistan' : { 
  	'federal': 0.20, 
    'provincial': 0.05
   },
  'rolimpia' : 0.24,
  'manduria' : 0.22,
  'wawaland' : 0.19
}

new_tax_rates = { 
  'newyland' : 0.11,
  'wawaland' : 0.21
}

tax_rates.update(new_tax_rates)

print(tax_rates)
# {'taxistan': {'federal': 0.2, 'provincial': 0.05}, 'rolimpia': 0.24, 'manduria': 0.22, 'wawaland': 0.21, 'newyland': 0.11}

Note that if a key in the new dictionary already exists in the old dictionary, the new key will overwrite the key-value pair in the old dictionary!

Getting lists of key names and key values

By chaining the built-in keys() and values() methods to a dictionary, we can get a list of key names and key values, respectively:

tax_rates = {
  'taxistan' : { 
  	'federal': 0.20, 
    'provincial': 0.05
   },
  'rolimpia' : 0.24,
  'manduria' : 0.22,
  'wawaland' : 0.19
}

print(tax_rates.keys())
# dict_keys(['taxistan', 'rolimpia', 'manduria', 'wawaland'])

print(tax_rates.values())
# dict_values([{'federal': 0.2, 'provincial': 0.05}, 0.24, 0.22, 0.19])