Python for Programmers: List Basics

• Python's lists are what other languages call "arrays": they're mutable sequences of values, with integer indexes starting at zero.

• >

  letters = ["a", "b", "c"]
  letters

  Result:

  Run Code
• Note: this code example reuses elements (variables, etc.) defined in earlier examples.

  >

  letters[0]

  Result:

  'a'

• Lists can contain mixed types. For example, we can mix numbers and strings.

• >

  user_ids = [18, "72fa", 99]
  user_ids

  Result:

  Run Code

• In a previous lesson, we saw that len(some_string) gives us the length of a string. That same len function also works on lists, as well as most other types that have lengths.

• >

  len("Hello")

  Result:

  5

• >

  len(["a", "b", "c", "d"])

  Result:

  4

• Calling len on values without a length raises a TypeError exception. (You can type error when a code example will cause an error.)

• >

  len(100)

  Result:

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

• Lists are mutable, which means that we can change them after they're created.

• >

  user_ids = [18, 37, 99]
  user_ids[1] = "72fa"
  user_ids

  Result:

  [18, '72fa', 99]

• However, we can only assign to indexes that already exist. If we assign to an index that's not in the list, that's an IndexError.

• >

  visitor_log = ["Amir", "Betty", "Cindy"]
  visitor_log[3] = "Dalili"
  visitor_log

  Result:

  IndexError: list assignment index out of range

• Some languages, including JavaScript, let us write to non-existent indexes. They allow that even though it usually indicates a mistake on our part. This is another place where Python is more strict or conservative than other dynamic languages.

• Python's indexing strictness also applies to accessing elements: when we access an index that doesn't exist, Python raises an exception. By contrast, JavaScript returns undefined, which can lead to bugs that are difficult to track down.

• >

  ["gasoline", "kerosene", "coal"][3]

  Result:

  IndexError: list index out of range

• We can join two lists together with +, like we did with strings in an earlier lesson.

• >

  my_list = ["A", "B"]
  my_list + [1, 2]

  Result:

  ['A', 'B', 1, 2]

• We can only use + to build lists from other lists. If we try to add a non-list to a list, Python raises a TypeError exception. For example, we can't use + to append a new integer to a list.

• >

  my_list = [1, 2, 3]
  my_list = my_list + 4
  my_list

  Result:

  TypeError: can only concatenate list (not "int") to list

• When writing larger list literals, we often write one element per line for readability.

• >

  alphabet_mnemonic = [
    "apple",
    "ball",
    "cat",
    "dog",
    "elephant"
  ]
  
  len(alphabet_mnemonic)

  Result:

  5

• Like most languages, Python allows a trailing comma after the last element of a list.

• >

  alphabet_mnemonic = [
    "apple",
    "ball",
    "cat",
    "dog",
    "elephant",
  ]
  
  len(alphabet_mnemonic)

  Result:

  5

• The trailing comma makes it slightly easier to edit the list later. Trailing commas are allowed in any list, but are normally used only for multi-line lists.

• Here's a code problem:

  Finish this first_three_elements function. It takes a list, then returns a list with only the first three elements. It doesn't need to handle lists with fewer than three elements.

  9

  1

  2

  def first_three_elements(the_list):

  return [the_list[0], the_list[1], the_list[2]]

  11

  3

  4

  5

  6

  assert first_three_elements([1, 2, 3]) == [1, 2, 3]

  assert first_three_elements([0, 2, 4, 6, 8]) == [0, 2, 4]

  assert first_three_elements([5, 3, 2, 1, 6]) == [5, 3, 2]

  assert first_three_elements([1, 4, 9, 16, 25]) == [1, 4, 9]

  Goal:

  None

  Yours:

  None

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