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Dictionaries

Dictionaries

Dictionary Internals

  • keys in any single dictionary must be unique
  • duplicate values are allowed
  • interally
    • maintains pairs of references to key objects and value objects
    • key objects must be immutable strings, numbers, and tuples(?)
    • value objects may be mutable
  • should never rely on order of items in dictionary (it's basically random, and may even vary between different runs of same program)

dict() constructor

  • can convert other types to dictionaries
names_and_ages = [('Alice', 32), ('Bob', 48), ('Charlie', 28), ('Daniel', 33)]
d = dict(names_and_ages)
d #=> {'Alice', 32, 'Bob', 48, 'Charlie', 28, 'Daniel', 33}

phonetic = dict(a='alfa', b='bravo', c='charlie', d='delta', e='echo', f='foxtrot')
phonetic #=> {'a': 'alfa', 'b': 'bravo', 'c': 'charlie', 'd': 'delta', 'e': 'echo', 'f': 'foxtrot'}

basic_dict = dict(foo='bar', baz='beep')
'foo' in basic_dict #=> True
del basic_dict['foo']

Copying Dictionaries

💡 Important 💡 as with lists, dictionary copying is shallow by default

  • two means of copying
    • using copy method
    • pass existing dictionary to dict() constructor (more common)
d = dict(goldenrod=0xDAA520, indigo=0x4B0082, seashell=0xFFF5EE)
e.copy()
e #=> {'goldenrod': 0xDAA520, 'indigo': 0x4B0082, 'seashell': 0xFFF5EE}

f = dict(e)
f #=> {'goldenrod': 0xDAA520, 'indigo': 0x4B0082, 'seashell': 0xFFF5EE}

dict.update()

  • add entries from one dictionary into another
  • call on dictionary that is to be updated
  • if argument update includes keys which already exist in target dictionary, values associated with those keys will be replaced in target with corresponding values from source
f #=> {'goldenrod': 0xDAA520, 'indigo': 0x4B0082, 'seashell': 0xFFF5EE}
g = dict(wheat=0xF5DEB3, khaki=0xF0E68C, crimson=0xDC143C)
f.update(g)
f #=> {'goldenrod': 0xDAA520, 'indigo': 0x4B0082, 'seashell': 0xFFF5EE, 'wheat': 0xF5DEB3, 'khaki': 0xF0E68C, 'crimson': 0xDC143C}

### common keys between target and source dictionaries
stocks = {'GOOG': 891, 'AAPL': 416, 'IBM': 194}
stocks.update({'GOOG': 894, 'YHOO': 25})
stocks #=> {'GOOG': 894, 'AAPL': 416, 'IBM': 194, 'YHOO': 25}

Dictionary Iteration

  • dictionaries yield next key on each iteration
  • values can be retrieved using square-bracket operator
  • again, order should not be relied upon here
colors = dict(aquamarine='#7FFFD4', burlywood='#DEB887',
            chartreuse='#7FFF00', cornflower='#6495ED',
            firebrick='#B22222', honeydew='#F0FFF0',
            maroon='#B03060', sienna='#A0522D')
for key in colors:
    print(f"{key} => {colors[key]}")

#=> aquamarine => '#7FFFD4'
#=> burlywood => '#DEB887'
#=> chartreuse => '#7FFF00'
#=> cornflower => '#6495ED'
#=> firebrick => '#B22222'
#=> honeydew => '#F0FFF0'
#=> maroon => '#B03060'
#=> sienna => '#A0522D'


### using `values` method
# provides iterable view of dictionaries values without them being copied
for value in colors.values():
    print(value)

#=> #7FFFD4
#=> #DEB887
#=> #7FFF00
#=> #6495ED
#=> #B22222
#=> #F0FFF0
#=> #B03060
#=> #A0522D


### using `keys` method
for key in colors.keys():
    print(key)

#=> aquamarine
#=> burlywood
#=> chartreuse
#=> cornflower
#=> firebrick
#=> honeydew
#=> maroon
#=> sienna

dict.items()

  • iterates over keys and values in tandem
  • yields (key, value) tuple on each iteration
# NOTE: this uses tuple unpacking
for key, value in colors.items():
    print(f"{key} => {value}")

#=> aquamarine => '#7FFFD4'
#=> burlywood => '#DEB887'
#=> chartreuse => '#7FFF00'
#=> cornflower => '#6495ED'
#=> firebrick => '#B22222'
#=> honeydew => '#F0FFF0'
#=> maroon => '#B03060'
#=> sienna => '#A0522D'

Other(?)

  • membership tests using in and not in work on keys
  • remove elements using del
### `in` and `not in`
symbols = dict(usd='$', gbp='£', nzd='$', eur='€', jpy='¥', nok='kr', hhg='Pu')
symbols #=> {'usd': '$', 'gbp': '£', 'nzd': '$' 'eur': '€', 'jpy': '¥', 'nok': 'kr', 'hhg': 'Pu'}

'usd' in symbols #=> True
'mkd' not in symbols #=> True


### del
z = {'H': 1, 'Tc': 43, 'Xe': 54, 'Fy': 137, 'Rf': 104, 'Fm': 100}
del z['Fy']
z #=> {'H': 1, 'Tc': 43, 'Xe': 54, 'Rf': 104, 'Fm': 100}

### modifying
m = {'H': [1, 2, 3],
    'He': [3, 4],
    'Li': [6, 7],
    'Be': [7, 9, 10],
    'B': [10, 11],
    'C': [11, 12, 13, 14]}

m['H'] += [4, 5, 6, 7]
m #=> {'H': [1, 2, 3, 4, 5, 6, 7], 'He': [3, 4], 'Li': [6, 7], 'Be': [7, 9, 10], 'B': [10, 11], 'C': [11, 12, 13, 14]}}

### pretty-printing using `pprint` module from Python standard library
from pprint import pprint as pp # NOTE: withou `as pp`, `pprint` function would mask `pprint` module
m #=> {'H': [1, 2, 3, 4, 5, 6, 7], 'He': [3, 4], 'Li': [6, 7], 'Be': [7, 9, 10], 'B': [10, 11], 'C': [11, 12, 13, 14]}}
pp(m)

Common/Useful Methods

get()

Official Python docs for get() 
basic_dict = dict(foo='bar', baz='beep')
basic_dict.get('foo')
#=> 'bar'
basic_dict.get('meow', 'DEFAULT')
#=> 'DEFAULT'

counts_by_letter = {'a': 1, 'b': 2}
counts_by_letter['c'] = counts_by_letter.get('c', 0) + 1
counts_by_letter
#=> {'a': 1, 'b': 2, 'c': 1}

pretty-printing using pprint module from Python standard library

from pprint import pprint as pp
pp(basic_dict)
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