6.2.3 Basic Dictionary Operations

  • Intentionally provided the incorrect value 100 for the key 'X':
In [1]:
roman_numerals = {'I': 1, 'II': 2, 'III': 3, 'V': 5, 'X': 100}
In [2]:
roman_numerals
Out[2]:
{'I': 1, 'II': 2, 'III': 3, 'V': 5, 'X': 100}

Accessing the Value Associated with a Key

In [3]:
roman_numerals['V']
Out[3]:
5

Updating the Value of an Existing Key–Value Pair

In [4]:
roman_numerals['X'] = 10
In [5]:
roman_numerals
Out[5]:
{'I': 1, 'II': 2, 'III': 3, 'V': 5, 'X': 10}

Adding a New Key–Value Pair

In [6]:
roman_numerals['L'] = 50
In [7]:
roman_numerals
Out[7]:
{'I': 1, 'II': 2, 'III': 3, 'V': 5, 'X': 10, 'L': 50}
  • String keys are case sensitive.
  • Assigning to a nonexistent key inserts a new key–value pair.

Removing a Key–Value Pair

In [8]:
del roman_numerals['III']
In [9]:
roman_numerals
Out[9]:
{'I': 1, 'II': 2, 'V': 5, 'X': 10, 'L': 50}
  • Method pop returns the value for the removed key.
In [10]:
roman_numerals.pop('X')
Out[10]:
10
In [11]:
roman_numerals
Out[11]:
{'I': 1, 'II': 2, 'V': 5, 'L': 50}

Attempting to Access a Nonexistent Key

In [12]:
roman_numerals['III']

------------------------------------------------------------------------
KeyError                               Traceback (most recent call last)
<ipython-input-12-ccd50c7f0c8b> in <module>
----> 1 roman_numerals['III']

KeyError: 'III'
  • Method get returns its argument’s corresponding value or None if the key is not found.
  • IPython does not display anything for None.
  • get with a second argument returns teh second argument if the key is not found.
In [13]:
roman_numerals.get('III')
In [14]:
roman_numerals.get('III', 'III not in dictionary')
Out[14]:
'III not in dictionary'
In [15]:
roman_numerals.get('V')
Out[15]:
5

Testing Whether a Dictionary Contains a Specified Key

In [16]:
'V' in roman_numerals
Out[16]:
True
In [17]:
'III' in roman_numerals
Out[17]:
False
In [18]:
'III' not in roman_numerals
Out[18]:
True

©1992–2020 by Pearson Education, Inc. All Rights Reserved. This content is based on Chapter 6 of the book Intro to Python for Computer Science and Data Science: Learning to Program with AI, Big Data and the Cloud.

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