Lists

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I store multiple values?

Objectives

• Explain why programs need collections of values.

• Write programs that create flat lists, index them, slice them, and modify them through assignment and method calls.

A list stores many values in a single structure.

• Doing calculations with a hundred variables called pressure_001, pressure_002, etc., would be at least as slow as doing them by hand.
• Use a list to store many values together.
  • Contained within square brackets [...].
  • Values separated by commas ,.
• Use len to find out how many values are in a list.

subjects = ['women', 'history', 'social conditions']
print('subjects:', subjects)
print('length:', len(subjects))

subjects: ['women', 'history', 'social conditions']
length: 3

Use an item’s index to fetch it from a list.

• Just like strings.

print('first item of subjects:', subjects[0])
print('third item of subjects:', subjects[2])

first item of subjects: women
third item of subjects: social conditions

Lists’ values can be replaced by assigning to them.

• Use an index expression on the left of assignment to replace a value.

subjects[0] = 'youth'
print('subjects is now:', subjects)

subjects is now: ['youth', 'history', 'social conditions']

Appending items to a list lengthens it.

• Use list_name.append to add items to the end of a list.

formats = ["image", "pdf", "web page"]
print('formats is initially', formats)
formats.append("archived web site")
formats.append("drawing")
print('formats has become:', formats)

formats is initially ['image', 'pdf', 'web page']
formats has become: ['image', 'pdf', 'web page', 'archived web site', 'drawing']

• append is a method of lists.
  • Like a function, but tied to a particular object.
• Use object_name.method_name to call methods.
  • Deliberately resembles the way we refer to things in a library.
• We will meet other methods of lists as we go along.
  • Use help(list) for a preview.
• extend is similar to append, but it allows you to combine two lists. For example:

mime_types = ["image/gif", "image/jpg"]
other_mime_types = ["image/tif", "image/jpeg"]
print('mime_types is currently:', mime_types)
mime_types.extend(other_mime_types)
print('mime_types has now become:', mime_types)
formats.append(mime_types)
print('formats has finally become:', formats)

mime_types is currently: ['image/gif', 'image/jpg']
mime_types has now become: ['image/gif', 'image/jpg', 'image/tif', 'image/jpeg']
formats has finally become: ['image', 'pdf', 'web page', 'archived web site', 'drawing', ['image/gif', 'image/jpg', 'image/tif', 'image/jpeg']]

Note that while extend maintains the “flat” structure of the list, appending a list to a list makes the result two-dimensional.

Use del to remove items from a list entirely.

• del list_name[index] removes an item from a list and shortens the list.
• Not a function or a method, but a statement in the language.

pages = [2, 3, 12, 17, 95]
print('pages before removing last item:', pages)
del pages[4]
print('pages after removing last item:', pages)

pages before removing last item: [2, 3, 12, 17, 95]
pages after removing last item: [2, 3, 12, 17]

The empty list contains no values.

• Use [] on its own to represent a list that doesn’t contain any values.
  • “The zero of lists.”
• Helpful as a starting point for collecting values (which we will see in the next episode).

Lists may contain values of different types.

• A single list may contain numbers, strings, and anything else.

tags = [1865, "Tennessee, "railroads"]

Character strings can be indexed like lists.

• Get single characters from a character string using indexes in square brackets.

medium = 'photograph'
print('zeroth character:', medium[0])
print('third character:', medium[3])

zeroth character: p
third character: t

Character strings are immutable.

• Cannot change the characters in a string after it has been created.
  • Immutable: cannot be changed after creation.
  • In contrast, lists are mutable: they can be modified in place.
• Python considers the string to be a single value with parts, not a collection of values.

medium[0] = 'C'

TypeError: 'str' object does not support item assignment

Indexing beyond the end of the collection is an error.

• Python reports an IndexError if we attempt to access a value that doesn’t exist.
  • This is a kind of runtime error.
  • Cannot be detected as the code is parsed because the index might be calculated based on data.

print('99th letter of mediume is:', element[99])

IndexError: string index out of range

String methods

You can use the method split() on a string to split it up on a separator character. Default is space.

title = "Insurance maps of the city of New York"
title_words = title.split()
print(title_words)

['Insurance', 'maps', 'of', 'the', 'city', 'of', 'New', 'York']

Split can take a string to specify another character to split on.

year_info = "2019/2020"
years_list = year_info.split("/")
print(years_list)

['2019', '2020']

How would get just the first year in that list and assign it to a new variable?

start_year = years_list[0]
print(start_year)

2019

Exercise: Split on something else

Given the subject heading below, split apart the subdivisions and assign the results to a list called “subheadings”.

subject = "Railroads--United States--Maps"

subheadings = subject.split("--")
form = subheadings[-1]

Bonus to join them together again

new_subject = '--'.join(subheadings)

Railroads--United States--Maps

Key Points

• A list stores many values in a single structure.

• Use an item’s index to fetch it from a list.

• Lists’ values can be replaced by assigning to them.

• Appending items to a list lengthens it.

• Use del to remove items from a list entirely.

• The empty list contains no values.

• Lists may contain values of different types.

• Character strings can be indexed like lists.

• Character strings are immutable.

• Indexing beyond the end of the collection is an error.