Python Mini-Course

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Lesson 16Dictionaries

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

1. Describe the characteristics of the dictionary
   data structure in Python
2. Perform basic operations with dictionaries
   including creation, copying, updating, and
   traversing
3. Use dictionaries in functions

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The dictionary data structure

• In Python, a dictionary is mapping between a set
  of indices (keys) and a set of values
• The items in a dictionary are key-value pairs

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The dictionary data structure

• Keys can be any Python data type
• Because keys are used for indexing, they should
  be immutable
• Values can be any Python data type
• Values can be mutable or immutable

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Creating a dictionary

• eng2sp dict()
• print eng2sp
• eng2sp'one' 'uno'
• print eng2sp
• eng2sp'two' 'dos'
• print eng2sp

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Creating a dictionary

• eng2sp 'one' 'uno', 'two' 'dos',
• 'three' 'tres'
• print eng2sp
• In general, the order of items in a dictionary is
  unpredictable
• Dictionaries are indexed by keys, not integers

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Dictionary indexing

• print eng2sp'three'
• print eng2sp'five'
• If the index is not a key in the dictionary,
  Python raises an exception

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Dictionary indexing

• if 'five' in eng2sp
• print eng2sp'five'
• print eng2sp.get('five')

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The in operator

• Note that the in operator works differently for
  dictionaries than for other sequences
• For offset indexed sequences (strings, lists,
  tuples), x in y checks to see whether x is an
  item in the sequence
• For dictionaries, x in y checks to see whether x
  is a key in the dictionary

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Keys and values

• The keys method returns a list of the keys in a
  dictionary
• print eng2sp.keys()
• The values method returns a list of the values
• print eng2sp.values()

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Keys and values

• The items method returns a list of tuple pairs
  of the key-value pairs in a dictionary
• print eng2sp.items()

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Example histogram.py

• def histogram(seq)
• d dict()
• for element in seq
• if element not in d
• delement 1
• else
• delement 1
• return d
• h histogram('brontosaurus')
• print h

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Example histogram2.py

• Add the following code to histogram.py
• def print_hist(hist)
• for key in hist
• print key, histkey
• h histogram('brontosaurus')
• print_hist(h)

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Example histogram2.py

• Change the print_hist function
• def print_hist(hist)
• for key, value in hist
• print key, value
• h histogram('brontosaurus')
• print_hist(h)

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Sorting the keys

• Change the print_hist function
• def print_hist(hist)
• keys hist.keys()
• keys.sort()
• for key in keys
• print key, histkey
• h histogram('brontosaurus')
• print_hist(h)

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Using lists as values invert.py

• Add the following code to histogram.py
• def invert_dict(d)
• inv dict()
• for key in d
• val dkey
• if val not in inv
• invval key
• else
• invval.append(key)
• return inv

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Using lists as values invert.py

• Add the following code to histogram.py
• hist histogram('parrot')
• print hist
• inverted invert_dict(hist)
• print inverted

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Using tuples as keys troupe.py

• troupe ('Cleese', 'John') 1,2,3,
• ('Chapman', 'Graham') 4,5,6,
• ('Idle', 'Eric') 7,8,9,
• ('Jones', 'Terry') 10,11,12,
• ('Gilliam', 'Terry')
  13,14,15,16,17,18,
• ('Palin', 'Michael') 19,20
• for last, first in troupe
• print first, last, troupelast, first

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Next session

• Handling program errors (exceptions)
• Reading and writing data files
• Sharing data with other programs (Excel, SAS,
  etc.)

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Suggested exercises

• Exercise 12.4
• The Case Study in chapter 13