Overview

Teaching: 10 min
Exercises: 0 min

Questions

• What do sequences, sets, and dictionaries have in common?

• How do they differ?

• When should I use each type?

Objectives

• Explain what it means for a data type to be iterable.

• Understand the difference between ordered and unordered types.

• Explain how a mutable data type differs from an immutable data type.

• Understand when to use one iterable type versus another.

What is an iterable data type?

What is iteration?

• The types we just learned (strings, lists, tuples, sets, and dictionaries) are all collections of things.
• Iteration is going through a collection item-by-item

Example

• Translating a collection of DNA sequences into protein sequences
• Get the atomic number of every element in a molecule
• Calculate the mean of 3 measurements at every timepoint

What are the iterable data types?

• Collection types are often called iterable
• As we’ve seen, Python has 5 basic iterable data types
  • Strings
  • Tuples
  • Lists
  • Sets
  • Dictionaries
• These data types have different features and are used for different types of data
• It’s helpful to consider whether these types are ordered or unordered and whether they are mutable or immutable.

Ordered vs. Unordered Types

Try running the following:

my_list = list('abc')
my_set = set('abc')

print('my_list = ', my_list)
print('my_set = ', my_set)

What do you notice about the result of printing my_list and my_set?

Solution

my_list = ['a', 'b', 'c']
my_set = {'b', 'c', 'a'}

• Ordered types maintain the order in which they were created
• Unordered types do not maintain this order

Challenge

• Is unordered the same as random?
• Will the same thing print each time?

print(set('abc'))
print(set('bca'))
print(set('cba'))

Solution

{'b', 'c', 'a'}
{'b', 'c', 'a'}
{'b', 'c', 'a'}

Unordered means you can’t depend on any particular order

• The order may change when you upgrade python
• It may also change the next time you run your program

Mutable vs Immutable Types

• Mutability is a tricky subject and the cause of many python bugs
• Some types can be modified “in place”, that is the original object is changed instead of creating a new object
• If an object can be modified in place it is mutable; otherwise it is immutable

Example

Run the following code. What’s the difference?

my_list = ['a', 'b', 'c']
also_my_list = my_list

my_string = 'abc'
also_my_string = my_string

my_list.extend(['d', 'e', 'f'])
my_string = my_string + "def"

print('my_list = ', my_list)
print('also_my_list = ', also_my_list)

print('my_string = ', my_string)
print('also_my_string = ', also_my_string)

Solution

• Modifying my_list also modifies also_my_list
• Modifying my_string does not change also_my_string

my_list =  ['a', 'b', 'c', 'd', 'e', 'f']
also_my_list =  ['a', 'b', 'c', 'd', 'e', 'f']
my_string =  abcdef
also_my_string =  abc

Iterables have lengths

• The len() function gives the number of things in a collection

print(len("abc"))
print(len([1, 2, 3, 4]))

3
4

• Numbers do not have a length

print(len(52))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-f769e8e8097d> in <module>()
----> 1 print(len(52))

TypeError: object of type 'int' has no len()

Choosing the best data structure

• Best Practice: Write programs for people and not for computers!
• The best choice of data structure depends on how you will use it
• Focus on clarity before performance
• Good data structure choices can make your code easier to follow

Lists & Tuples vs Dictionaries

• Does the data have a natural order?
  • Yes → consider a list, dictionaries can be sorted by their keys, but order is not inherent
  • All access to list data is either by looping through it in order, or by referring to an entry by it’s ordinal place in the list
  • All access to dictionary data is either by looping through it, perhaps in some arbitrary order, or by referring to an entry by it’s semantic meaning
• Does the addition of keys add semantic value?
  • Yes → probably benefit from a dictionary
  • No → fabricating keys that don’t have semantic value can be counter-productive
    • If the order of entries IS the semantic value, then use a list

Lists vs Tuples

• Do you want to clearly indicate that certain data has an immutable relationship?
  • Yes → choose a tuple; immutability provides a weak form of semantics since order is fixed

Comparing Different Data Structures

Key Points

• Iterable data types are collections of objects.

• Ordered data types perserve the order of creation.

• Unordered data types return data in a random order.

• Mutable data types can be changed in place.

• Immutable data types cannot be changed in place. To modify them, you must create a new object.

• All iterable types have a length, the number of items they hold

• Choose an iterable type that makes your code safer and easier to understand.