Python Dictionaries Reference

Construction
Getting Data
Safer Access With Get
Adding and Changing Data
String Substitution With a Dictionary
Default Elements
Set Operations On Keys and Items
Dictionary Comprehensions

1. Construction



In [1]:

    
# create an empty dictionary 
empty_dict = {}
empty_dict = dict()

# create a dictionary with content
family = {'dad':'homer', 'mom':'marge', 'size':6}
faimly = dict(dad='homer', mom='marge', size=6)
family









    Out[1]:





{'dad': 'homer', 'mom': 'marge', 'size': 6}



In [1]:

    
# convert a liist of tuples into a dictionary
list_of_tuples = [('dad','homer'), ('mom','marge'), ('size',6)]
family=dict(list_of_tuples)
family









    Out[1]:





{'dad': 'homer', 'mom': 'marge', 'size': 6}



In [3]:

    
# creating a dictionary from a sequence of keys (no values created)
seq = ('name', 'age', 'sex')
d = dict.fromkeys(seq)
d









    Out[3]:





{'age': None, 'name': None, 'sex': None}



In [7]:

    
# creating a dictionary from a sequence of keys and a value to use as the default value
d = dict.fromkeys(seq, 10)
d









    Out[7]:





{'age': 10, 'name': 10, 'sex': 10}

2. Getting Data



In [3]:

    
# pass a key ti return its value
family['dad']









    Out[3]:





'homer'



In [4]:

    
# return number of key-value pairs
len(family)









    Out[4]:





3



In [5]:

    
# check if key exists in dictionary
'mom' in family









    Out[5]:





True



In [6]:

    
# only keys, not values, are checked
'marge' in family









    Out[6]:





False



In [7]:

    
# returns an iterable view of keys
family.keys()









    Out[7]:





dict_keys(['dad', 'mom', 'size'])



In [8]:

    
# returns an iterable view of values
family.values()









    Out[8]:





dict_values(['homer', 'marge', 6])



In [16]:

    
# returns an iterable view of key-value pairs
family.items()









    Out[16]:





dict_items([('mom', 'marge'), ('size', 6), ('kids', ['bart', 'lisa'])])

3. Safer Access With Get



In [17]:

    
# equivalent to a dictionary lookup
family.get('mom')









    Out[17]:





'marge'



In [18]:

    
# this would throw an error since the key does not exist
# family['grandma']



In [20]:

    
# return None if not found
family.get('grandma')



In [22]:

    
# provide a default return value if not found
family.get('grandma', 'not found')









    Out[22]:





'not found'



In [23]:

    
# accesss an element within a list inside a dictionary
family['kids'][0]









    Out[23]:





'bart'

4. Adding and Changing Data



In [11]:

    
# edit an existing entry
family['cat'] = 'snowball'
family









    Out[11]:





{'cat': 'snowball', 'dad': 'homer', 'mom': 'marge', 'size': 6}



In [12]:

    
# edit an existing entry
family['cat'] = 'snowball ii'
family









    Out[12]:





{'cat': 'snowball ii', 'dad': 'homer', 'mom': 'marge', 'size': 6}



In [13]:

    
# delete an entry
del family['cat']
family









    Out[13]:





{'dad': 'homer', 'mom': 'marge', 'size': 6}



In [14]:

    
# dictionary value can be a list
family['kids']=['bart','lisa']
family









    Out[14]:





{'dad': 'homer', 'kids': ['bart', 'lisa'], 'mom': 'marge', 'size': 6}



In [15]:

    
# remove an entry and return the value
family.pop('dad')









    Out[15]:





'homer'



In [24]:

    
# add multiple-entries
family.update({'baby':'maggie', 'mom':'Marge', 'grandpa':'abe'})
family









    Out[24]:





{'baby': 'maggie',
 'grandpa': 'abe',
 'kids': ['bart', 'lisa'],
 'mom': 'Marge',
 'size': 6}



In [25]:

    
family['kids'].remove('lisa')
family









    Out[25]:





{'baby': 'maggie',
 'grandpa': 'abe',
 'kids': ['bart'],
 'mom': 'Marge',
 'size': 6}

5. String Substitution With a Dictionary



In [28]:

    
# replaces %(baby)s with the result of looking up 'baby' in family
'youngest child is %(baby)s' % family









    Out[28]:





'youngest child is maggie'

6. Default Elements

Ways to get a default element add to the dictionary on first access



In [29]:

    
# using defaultdict 
from collections import defaultdict

d=defaultdict(list)
d['a'].append(1)
d['a'].append(2)
d['b'].append(3)
d









    Out[29]:





defaultdict(list, {'a': [1, 2], 'b': [3]})



In [1]:

    
# using setdefault
d={}
d.setdefault('a',[]).append(1)
d.setdefault('a',[]).append(2)
d.setdefault('b',[]).append(3)
d









    Out[1]:





{'a': [1, 2], 'b': [3]}

7. Set Operations On Keys and Items



In [2]:

    
a = {
'x' : 1,
'y' : 2,
'z' : 3
}

b = {
'w' : 10,
'x' : 11,
'y' : 2
}

# Find keys in common
a.keys() & b.keys() # { 'x', 'y' }









    Out[2]:





{('y', 2)}



In [3]:

    
# Find keys in a that are not in b
a.keys() - b.keys() # { 'z' }









    Out[3]:





{'z'}



In [4]:

    
# Find (key,value) pairs in common
a.items() & b.items() # { ('y', 2) }









    Out[4]:





{('y', 2)}

These kinds of operations can also be used to alter or filter dictionary contents. For example, suppose you want to make a new dictionary with selected keys removed. Here is some sample code using a dictionary comprehension:



In [5]:

    
{key:a[key] for key in a.keys() - {'z', 'w'}}









    Out[5]:





{'x': 1, 'y': 2}

8. Dictionary Comprehensions



In [1]:

    
prices = {
'ACME': 45.23,
'AAPL': 612.78,
'IBM': 205.55,
'HPQ': 37.20,
'FB': 10.75
}

# Make a dictionary of all prices over 200
{ key:value for key, value in prices.items() if value > 200 }









    Out[1]:





{'AAPL': 612.78, 'IBM': 205.55}



In [2]:

    
# Make a dictionary of tech stocks
tech_names = { 'AAPL', 'IBM', 'HPQ', 'MSFT' }
{ key:value for key,value in prices.items() if key in tech_names }









    Out[2]:





{'AAPL': 612.78, 'HPQ': 37.2, 'IBM': 205.55}