Python Lists

  • a kind of container along with tuples, and dictionaries

general format and indexes

  • format is list_name = [element_0, element_1, element2]
  • can mix types on a single list
  • can have empty list list = []
  • reference an element by list_name[element_index]
  • to get length of list len(list_name)
  • 0 indexed
  • when using negative indexes -1 is the last item on the list
    • think length of 'list - negative_index = positive_index'; ex: if you have a list whose length is 5 and you want index -3, then 'len(list) - 3 = 2', +2 is the same index as -3
  • trying to access things outside index range gives index error
  • list.index(element) - returns the index of the first occurrence of the element
    • error if element isn't on list, so check first

adding elements

  • list.insert(index, element) - -inserts element at that index and shifts all the following elements down
    • note: if you assign an element with list[index] = element to an index that already has an element in it the existing element is overwritten rather than inserted
  • add elements to list with list_name.append(new_element) and the new element will be added to the end of the list
  • list.extend(other_list) - all the elements on other_list are now at the end of list
  • note: .append adds single element, .extend merges a list onto another
    • example:
      >>> list1 = [1, 2, 3]
      >>> list = [4, 5]
      >>> list1.append(list2)
      >>> list1
      [1, 2, 3, [4, 5]]
      >>> list1 = [1, 2, 3]
      >>> list = [4, 5]
      >>> list1.extend(list2)
      >>> list1
      [1, 2, 3, 4, 5]
  • list1 + list2 - works like extend but creates a new list and leaves the original lists unchanged
  • list1 += list2 - works just like extend

removing elements

  • remove elements from list with list_name.pop() removes the last element
    • use list.pop(index) to get an element that isn't the last one on the list
    • it returns the popped element
  • list.remove(element) - removes the element
    • if the element occurs multiple times in the list it removes only the first occurrence
    • get error if try to remove an element that isn't on the list, so good to check that it is in the list first
  • note: .pop takes an index, .remove takes an element

checking if element in list

  • to see if a list contains an element use in and it will evaluate to a Boolean, so it can be used anywhere you can use a Boolean test --- assignment, loops, conditionals
    • example:
      x = 1
      if x in list_name:
          # do something


  • list.copy() - returns copy of the list
  • max(list), min(list), sum(list) - returns the min value, max value, or sum of the elements in the list

iterating through list

  • standard for loop:
    for i in range(len(list)):
      # do something
  • for each loop:
    for elem in collection:
      # do something with element
  • both for loops iterate over all elements of the list in order


  • for i in range(len(list)):
      list[i] += 1    # modifying list in place
    • use 'for loop with range' when modifying primitive type elements in a list
    • you can always use this one
  • for elem in list  # modifying local variable elem; if elem is primitive type, not changing the list!!!
      elem += 1
    • use 'for-each loop' when not modifying elements or when the elements are not primitive types

passing as parameters

  • lists are passed by reference
  • in the function, value changes in list persist after the function ends
  • creating a new list in a function means you're not dealing with the list passed in as parameter
    • it's like the URL you are using is pointing to a new page and you are no longer changing the parameter list
    • example:
      def create_new_list(num_list):
          num_list = [1, 2, 3]
      def main():
          values = [5, 6, 7, 8]
      Output is [5, 6, 7, 8, 9]

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