Python - (Dictionary|Map)

One dimension

An empty dict is created

• with the accolade without content

myEmptyDict = {}

• or with the dict function

myEmptyDict = dict()

A dict with values can be initialized:

• with the dict function

myDict = dict(a=1,b=2,c=3)

• or directly with the accolades syntax:

myDict = {'a':1,'b':2,'c':3}

You can also construct a dictionary using a comprehension.

>>> { (x,y):x*y for x in [1,2] for y in [1,2] }
{(1, 2): 2, (1, 1): 1, (2, 1): 2, (2, 2): 4}
>>> { key:value for (key,value) in [(3,2),(4,0),(100,1)] }
{100: 1, 3: 2, 4: 0}

Multi-dimensional

new_dic = {}
new_dic[1] = {}
new_dic[1][2] = 5

Access the value of a key

ie Indexing.

print d['key1']

1

>>> {'key1' : 1, 'key2' : 2, 'key3' : 3}['key1']
1

Get the keys and/of values

prices = {
"banana": 4,
"apple": 2,
"orange": 1.5,
"pear": 3,
}
>>> prices.keys()
dict_keys(['orange', 'apple', 'pear', 'banana'])
>>> prices.values()
dict_values([1.5, 2, 3, 4])

Get its length

The length len() of a dictionary is the number of key-value pairs it has.

print len(dict_name)

3

Print every pair in the dictionary

print myDict.items() # 

Add a new entry or set a new value

A new key-value pair in a dictionary is created by assigning a new key.

dict_name[new_key] = new_value

A new value is associated to a key value with the same syntax:

dict_name[existing_key] = new_value

Put a list inside a dictionary

You can put lists inside dictionaries.

dict_name['myListKey'] = ['Element1', 'Element2', 'Element3']

Delete an entry

Items can be removed from a dictionary with the del command:

del dict_name[key_name]

It will remove the key key_name and its associated value from the dictionary.

Check if a key exist

if key in d:
        d[key] += 1
    else:
        d[key] = 1

In a comprehension:

>>> myDict = {'Nico':40}
>>> 'Gerard' in myDict
False
>>> myDict['Gerard'] if 'Gerard' in myDict else 'Not Present'
'Not Present'

Key

for key in dict_name: 
    print dict_name[key]  

key and value

When looping through dictionaries, the key and corresponding value can be retrieved at the same time using:

• Python 3 See Dictviews

map = {1:1,2:2}
for k, v in iter(map.items()):
    print(k,':',v)

• Python 2 using the iteritems() method.

knights = {'gallahad': 'the pure', 'robin': 'the brave'}
for k, v in knights.iteritems():
    print k, v

gallahad the pure
robin the brave

Get the value in a list of dictionary

>>> dlist = [{'James':'Sean', 'director':'Terence'}, {'James':'Roger','director':'Lewis'}, {'James':'Pierce', 'director':'Roger'}]
>>> k = 'James'
>>> [ i[k] for i in dlist]
['Sean', 'Roger', 'Pierce']

Iterate over Key or values

>>> [2*x for x in {4:'a',3:'b'}.keys() ]
[6, 8]
>>> [x for x in {4:'a', 3:'b'}.values()]
['b', 'a']

Iterate over the pairs (key, value)

• On the pair

>>> prices = {"banana": 4, "apple": 2, "orange": 1.5, "pear": 3 }
>>> [myPair for myPair in prices.items()]
[('orange', 1.5), ('apple', 2), ('pear', 3), ('banana', 4)]

• On the value of the pairs with unpacking because the items are tuple:

>>> [fruit+' have a price of '+str(price) for (fruit,price) in prices.items()]
['orange have a price of 1.5', 'apple have a price of 2', 'pear have a price of 3', 'banana have a price of 4']

Test presence of a pair otherwise default value

>>> dlist = [{'Bilbo':'Ian','Frodo':'Elijah'},{'Bilbo':'Martin','Thorin':'Richard'}]
>>> k = 'Frodo'
>>> [ i[k] if k in i else 'NOT PRESENT' for i in dlist]
['Elijah', 'NOT PRESENT']