Sets and Dictionaries in Python: Dictionaries (Learner Version)

Objectives

Explain the similarities and differences between sets and dictionaries.
Perform common operations on dictionaries.

Lesson

Use a dictionary to keep track of key-value pairs
Its keys are like the elements of a set: unique, unordered, and immutable
The values associated with the keys can be anything, but can only be looked up via their keys

Create a dictionary:



In [1]:

    
birthdays = {'Newton' : 1642, 'Darwin' : 1809}

Key	Value
`'Newton'`	1642
`'Darwin'`	1809

Access values by subscripting



In [2]:

    
print birthdays['Newton']



In [3]:

    
birthdays['Turing'] = 1612
print birthdays









    



{'Turing': 1612, 'Newton': 1642, 'Darwin': 1809}



In [4]:

    
birthdays['Turing'] = 1912
print birthdays









    



{'Turing': 1912, 'Newton': 1642, 'Darwin': 1809}

Can only read values that are already there



In [5]:

    
print birthdays['Nightingale']









    



---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-44ca8cabb590> in <module>()
----> 1 print birthdays['Nightingale']

KeyError: 'Nightingale'

Test for the presence of a key using in



In [6]:

    
print 'Nightingale' in birthdays









    



False



In [7]:

    
print 'Darwin' in birthdays









    



True

len reports the number of items in the dictionary
for loops over the keys in some arbitrary order



In [8]:

    
print len(birthdays)
for name in birthdays:
    print name, birthdays[name]









    



3
Turing 1912
Newton 1642
Darwin 1809

Main function to count atomic symbols in a file



In [14]:

    
def main(filename):
    counts = count_atoms(filename)
    for atom in counts:
        print atom, counts[atom]

Count atoms, one per line



In [15]:

    
def count_atoms(filename):
    '''Count unique atoms, returning a dictionary.'''

    result = {}
    with open(filename, 'r') as reader:
	for line in reader:
	    atom = line.strip()
            if atom not in result:
                result[atom] = 1
            else:
                result[atom] = result[atom] + 1
    return result



In [16]:

    
main('some_atoms.txt')









    



Na 3
Si 1
Fe 1
Pd 1

Use tuples for multi-part keys



In [17]:

    
birthdays = {
    ('Isaac', 'Newton') : 1642,
    ('Charles', 'Robert', 'Darwin') : 1809,
    ('Alan', 'Mathison', 'Turing') : 1912
}
print birthdays









    



{('Charles', 'Robert', 'Darwin'): 1809, ('Isaac', 'Newton'): 1642, ('Alan', 'Mathison', 'Turing'): 1912}

Use the keys and values methods to get lists of keys and values



In [18]:

    
all_keys = birthdays.keys()
print all_keys









    



[('Charles', 'Robert', 'Darwin'), ('Isaac', 'Newton'), ('Alan', 'Mathison', 'Turing')]



In [20]:

    
all_values = birthdays.values()
print all_values









    



[1809, 1642, 1912]

Do not loop over dict.keys(), since that creates a list of keys rather than using them directly

Key Points

Use dictionaries to store key-value pairs with distinct keys.
Create dictionaries using {k1:v1, k2:v2, ...}.
Dictionaries are mutable, i.e., they can be updated in place.
Dictionary keys must be immutable, but values can be anything.
Use tuples to store multi-part keys in dictionaries.
dict[key] refers to the dictionary entry with a particular key.
key in dict tests whether a key is in a dictionary.
len(dict) returns the number of entries in a dictionary.
A loop over a dictionary produces each key once, in arbitrary order.
dict.keys() creates a list of the keys in a dictionary.
dict.values() creates a list of the keys in a dictionary.