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title_meta title description attachments lessons
Chapter 2
Python Lists
Learn to store, access, and manipulate data in lists: the first step toward efficiently working with huge amounts of data.
nb_of_exercises title
5
Python Lists
nb_of_exercises title
6
Subsetting Lists
nb_of_exercises title
5
Manipulating Lists

Python Lists

type: VideoExercise
key: a5886d213f
xp: 50

@projector_key a0530c4542f10988847b2dbb91f717c3

Create a list

type: NormalExercise
key: e6c527bf41
lang: python
xp: 100
skills:
  - 2

As opposed to int, bool etc., a list is a compound data type; you can group values together:

a = "is"
b = "nice"
my_list = ["my", "list", a, b]

After measuring the height of your family, you decide to collect some information on the house you're living in. The areas of the different parts of your house are stored in separate variables for now, as shown in the script.

@instructions

  • Create a list, areas, that contains the area of the hallway (hall), kitchen (kit), living room (liv), bedroom (bed) and bathroom (bath), in this order. Use the predefined variables.
  • Print areas with the print() function.

@hint

  • You can use the variables that have already been created to build the list: areas = [hall, kit, ...].
  • Put print(areas) in your script to print out the list when submitting.

@pre_exercise_code

@sample_code

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Create list areas


# Print areas

@solution

# Area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Create list areas
areas = [hall, kit, liv, bed, bath]

# Print areas
print(areas)

@sct

predef_msg = "Don't remove or edit the predefined variables!"
areas_msg = "Define `areas` as the list containing all the area variables, in the correct order: `[hall, kit, liv, bed, bath]`. Watch out for typos. The list shouldn't contain anything else!"

Ex().check_correct(
    has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the `areas` list at the end of your script?"),
    check_correct(
        check_object("areas").has_equal_value(incorrect_msg = areas_msg),
        multi(
            check_object('hall', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('kit', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('liv', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('bed', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('bath', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg)
        )
    )
)

success_msg("Nice! A list is way better here, isn't it?")

Create list with different types

type: NormalExercise
key: 1702a8bcdc
lang: python
xp: 100
skills:
  - 2

A list can contain any Python type. Although it's not really common, a list can also contain a mix of Python types including strings, floats, booleans, etc.

The printout of the previous exercise wasn't really satisfying. It's just a list of numbers representing the areas, but you can't tell which area corresponds to which part of your house.

The code in the editor is the start of a solution. For some of the areas, the name of the corresponding room is already placed in front. Pay attention here! "bathroom" is a string, while bath is a variable that represents the float 9.50 you specified earlier.

@instructions

  • Finish the code that creates the areas list. Build the list so that the list first contains the name of each room as a string and then its area. In other words, add the strings "hallway", "kitchen" and "bedroom" at the appropriate locations.
  • Print areas again; is the printout more informative this time?

@hint The first three 4 elements of the list areas are coded as ["hallway", hall, "kitchen", kit, ....

@pre_exercise_code

@sample_code

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Adapt list areas
areas = [hall, kit, "living room", liv, bed, "bathroom", bath]

# Print areas

@solution

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Adapt list areas
areas = ["hallway", hall, "kitchen", kit, "living room", liv, "bedroom", bed, "bathroom", bath]

# Print areas
print(areas)

@sct

objs = ["hall", "kit", "liv", "bed", "bath"]
predef_msg = "Don't remove or edit the predefined variables!"
areas_msg = "You didn't assign the correct value to `areas`. Have another look at the instructions. Make sure to place the room name before the variable containing the area each time. The order matters here! Watch out for typos."

Ex().check_correct(
  check_object("areas").has_equal_value(incorrect_msg = areas_msg),
  multi([ check_object(obj, missing_msg = predef_msg).has_equal_value(incorrect_msg = predef_msg) for obj in objs])
)

Ex().has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the `areas` list at the end of your script?")

success_msg("Nice! This list contains both strings and floats, but that's not a problem for Python!")

Select the valid list

type: MultipleChoiceExercise
key: 416b80a405
lang: python
xp: 50
skills:
  - 2

A list can contain any Python type. But a list itself is also a Python type. That means that a list can also contain a list! Python is getting funkier by the minute, but fear not, just remember the list syntax:

my_list = [el1, el2, el3]

Can you tell which ones of the following lines of Python code are valid ways to build a list?

A. [1, 3, 4, 2] B. [[1, 2, 3], [4, 5, 7]] C. [1 + 2, "a" * 5, 3]

@possible_answers

  • A, B and C
  • B
  • B and C
  • C

@hint Try out all the different lines in the Python shell and see which ones generate an error. Maybe none of them go wrong?

@pre_exercise_code

@sct

msg1 = "Correct! As funny as they may look, all these commands are valid ways to build a Python list."
msg2 = "Command B is valid, but it's not the only one!"
msg3 = "Both command B and C are valid; what about command A? Try it out in the console."
msg4 = "Command C is valid, but it's not the only one!"
Ex().has_chosen(1,[msg1,msg2,msg3,msg4])

List of lists

type: NormalExercise
key: 9158c577b0
lang: python
xp: 100
skills:
  - 2

As a data scientist, you'll often be dealing with a lot of data, and it will make sense to group some of this data.

Instead of creating a flat list containing strings and floats, representing the names and areas of the rooms in your house, you can create a list of lists. The script in the editor can already give you an idea.

Don't get confused here: "hallway" is a string, while hall is a variable that represents the float 11.25 you specified earlier.

@instructions

  • Finish the list of lists so that it also contains the bedroom and bathroom data. Make sure you enter these in order!
  • Print out house; does this way of structuring your data make more sense?
  • Print out the type of house. Are you still dealing with a list?

@hint

  • Add sublists to the house list by adding ["bedroom", bed] and ["bathroom", bath] inside the square brackets.
  • To print a variable x, write print(x) on a new line in the Python script.
  • To print out the type of a variable x, you can use print(type(x)).

@pre_exercise_code

@sample_code

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# house information as list of lists
house = [["hallway", hall],
         ["kitchen", kit],
         ["living room", liv]]

# Print out house


# Print out the type of house

@solution

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# house information as list of lists
house = [["hallway", hall],
         ["kitchen", kit],
         ["living room", liv],
         ["bedroom", bed],
         ["bathroom", bath]]

# Print out house
print(house)

# Print out the type of house
print(type(house))

@sct

predef_msg = "Don't remove or edit the predefined variables!"
house_msg = "You didn't assign the correct value to `house`. Have another look at the instructions. Extend the list of lists so it incorporates a list for each pair of room name and room area. Mind the order and typos!"

Ex().check_correct(
    check_object("house").has_equal_value(incorrect_msg = house_msg),
    multi(
        check_object('hall', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('kit', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('liv', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('bed', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('bath', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg)
    )
)

Ex().has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the contents of `house`?")
Ex().has_printout(1, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the type of the `house` variable?")

success_msg("Great! Get ready to learn about list subsetting!")

Subsetting Lists

type: VideoExercise
key: c076b5a69c
xp: 50

@projector_key fc15ba5cb9485456df8589130b519ea3

Subset and conquer

type: NormalExercise
key: c3ce582e32
lang: python
xp: 100
skills:
  - 2

Subsetting Python lists is a piece of cake. Take the code sample below, which creates a list x and then selects "b" from it. Remember that this is the second element, so it has index 1. You can also use negative indexing.

x = ["a", "b", "c", "d"]
x[1]
x[-3] # same result!

Remember the areas list from before, containing both strings and floats? Its definition is already in the script. Can you add the correct code to do some Python subsetting?

@instructions

  • Print out the second element from the areas list (it has the value 11.25).
  • Subset and print out the last element of areas, being 9.50. Using a negative index makes sense here!
  • Select the number representing the area of the living room (20.0) and print it out.

@hint

  • Use x[1] to select the second element of a list x. Make sure to wrap your subsetting operation in a print() call.
  • Use x[-1] to select the last element of a list x. Make sure to wrap your subsetting operation in a print() call.
  • The number representing the area of the living room is the 6th element in the list, so you'll need [5] here.

@pre_exercise_code

@sample_code

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Print out second element from areas
print(areas[_])

# Print out last element from areas
print(areas[__])

# Print out the area of the living room
print(areas[_])

@solution

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Print out second element from areas
print(areas[1])

# Print out last element from areas
print(areas[-1])

# Print out the area of the living room
print(areas[5])

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)
Ex().has_printout(0, not_printed_msg = "Have another look at your code to print out the second element in `areas`, which is at index `1`.")
Ex().has_printout(1, not_printed_msg = "Have another look at your code to print out the last element in `areas`, which is at index `-1`.")
Ex().has_printout(2, not_printed_msg = "Have another look at your code to print out the area of the living room. It's at index `5`.")
success_msg("Good job!")

Subset and calculate

type: NormalExercise
key: 58c969f11f
lang: python
xp: 100
skills:
  - 2

After you've extracted values from a list, you can use them to perform additional calculations. Take this example, where the second and fourth element of a list x are extracted. The strings that result are pasted together using the + operator:

x = ["a", "b", "c", "d"]
print(x[1] + x[3])

@instructions

  • Using a combination of list subsetting and variable assignment, create a new variable, eat_sleep_area, that contains the sum of the area of the kitchen and the area of the bedroom.
  • Print the new variable eat_sleep_area.

@hint

  • Add areas[3] to areas[-3] to calculate eat_sleep_area.
  • Print out eat_sleep_area: print(eat_sleep_area).

@pre_exercise_code

@sample_code

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Sum of kitchen and bedroom area: eat_sleep_area


# Print the variable eat_sleep_area

@solution

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Sum of kitchen and bedroom area: eat_sleep_area
eat_sleep_area = areas[3] + areas[-3]

# Print the variable eat_sleep_area
print(eat_sleep_area)

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)
Ex().check_object("eat_sleep_area").has_equal_value(incorrect_msg = "Be sure to assign the correct value to `eat_sleep_area`. You'll need to sum `areas[3]` and `areas[-3]`.")
Ex().has_printout(0, not_printed_msg = "Don't forget to print out `eat_sleap_area` after calculating it!")
success_msg("Bellissimo!")

Slicing and dicing

type: NormalExercise
key: 7f08642d18
lang: python
xp: 100
skills:
  - 2

Selecting single values from a list is just one part of the story. It's also possible to slice your list, which means selecting multiple elements from your list. Use the following syntax:

my_list[start:end]

The start index will be included, while the end index is not.

The code sample below shows an example. A list with "b" and "c", corresponding to indexes 1 and 2, are selected from a list x:

x = ["a", "b", "c", "d"]
x[1:3]

The elements with index 1 and 2 are included, while the element with index 3 is not.

@instructions

  • Use slicing to create a list, downstairs, that contains the first 6 elements of areas.
  • Do a similar thing to create a new variable, upstairs, that contains the last 4 elements of areas.
  • Print both downstairs and upstairs using print().

@hint

  • Use the brackets [0:6] to build downstairs.
  • Use the brackets [6:10] to build upstairs.
  • Simply add two print() calls to the script to print out downstairs and upstairs.

@pre_exercise_code

@sample_code

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Use slicing to create downstairs


# Use slicing to create upstairs


# Print out downstairs and upstairs

@solution

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Use slicing to create downstairs
downstairs = areas[0:6]

# Use slicing to create upstairs
upstairs = areas[6:10]

# Print out downstairs and upstairs
print(downstairs)
print(upstairs)

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)

patt = "`%s` is incorrect. Use `areas[%s]` and slicing to select the elements you want, or something equivalent."
Ex().check_object("downstairs").has_equal_value(incorrect_msg = patt % ('downstairs', '0:6'))
Ex().check_object("upstairs").has_equal_value(incorrect_msg = patt % ('upstairs', '6:10'))

Ex().has_printout(0, not_printed_msg="Have you printed out `downstairs` after calculating it?")
Ex().has_printout(1, not_printed_msg="Have you printed out `upstairs` after calculating it?")

success_msg("Great!")

Slicing and dicing (2)

type: NormalExercise
key: dfc9a168a3
lang: python
xp: 100
skills:
  - 2

In the video, Hugo first discussed the syntax where you specify both where to begin and end the slice of your list:

my_list[begin:end]

However, it's also possible not to specify these indexes. If you don't specify the begin index, Python figures out that you want to start your slice at the beginning of your list. If you don't specify the end index, the slice will go all the way to the last element of your list. To experiment with this, try the following commands in the IPython Shell:

x = ["a", "b", "c", "d"]
x[:2]
x[2:]
x[:]

@instructions

  • Create downstairs again, as the first 6 elements of areas. This time, simplify the slicing by omitting the begin index.
  • Create upstairs again, as the last 4 elements of areas. This time, simplify the slicing by omitting the end index.

@hint

  • To get the first 5 elements of a list, l, you would use l[:5].
  • To get everything except the first 5 elements, you would use l[5:].

@pre_exercise_code

@sample_code

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Alternative slicing to create downstairs


# Alternative slicing to create upstairs

@solution

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Alternative slicing to create downstairs
downstairs = areas[:6]

# Alternative slicing to create upstairs
upstairs = areas[6:]

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)

patt = "Your definition of `%s` is incorrect. Use the `areas[...]` and slicing to select the elements you want. You could use `%s` where the dots are, for example."
Ex().check_object("downstairs").has_equal_value(incorrect_msg = patt % ("downstairs",":6"))
Ex().check_object("upstairs").has_equal_value(incorrect_msg = patt % ("upstairs",":6"))

success_msg("Wonderful!")

Subsetting lists of lists

type: MultipleChoiceExercise
key: dbbbd306cf
lang: python
xp: 50
skills:
  - 2

You saw before that a Python list can contain practically anything; even other lists! To subset lists of lists, you can use the same technique as before: square brackets. Try out the commands in the following code sample in the IPython Shell:

x = [["a", "b", "c"],
     ["d", "e", "f"],
     ["g", "h", "i"]]
x[2][0]
x[2][:2]

x[2] results in a list, that you can subset again by adding additional square brackets.

What will house[-1][1] return? house, the list of lists that you created before, is already defined for you in the workspace. You can experiment with it in the IPython Shell.

@possible_answers

  • A float: the kitchen area
  • A string: "kitchen"
  • A float: the bathroom area
  • A string: "bathroom"

@hint house[-1] selects the last element of house, which is the list ["bathroom", 9.50]. What's the result if you then subset this sublist with [1]? You can always try out the command in the IPython Shell!

@pre_exercise_code

house = [["hallway", 11.25],
         ["kitchen", 18.0],
         ["living room", 20.0],
         ["bedroom", 10.75],
         ["bathroom", 9.50]]

@sct

msg1 = msg2 = "Wrong. `house[-1]` selects the last element of `house`, which is the list `[\"bathroom\", 9.50]`."
msg3 = "Correctomundo! The last piece of the list puzzle is manipulation."
msg4 = "Incorrect. `house[-1]` indeed selects the list that represents the bathroom information, but `[1]` selects the second element of the sublist, not the first. Python uses zero-based indexing!"
Ex().has_chosen(3, [msg1, msg2, msg3, msg4])

Manipulating Lists

type: VideoExercise
key: d7fe818b3a
xp: 50

@projector_key 355ed52d2fb0d67508c6a311b7cbc6d3

Replace list elements

type: NormalExercise
key: 4e1bba1b55
lang: python
xp: 100
skills:
  - 2

Replacing list elements is pretty easy. Simply subset the list and assign new values to the subset. You can select single elements or you can change entire list slices at once.

Use the IPython Shell to experiment with the commands below. Can you tell what's happening and why?

x = ["a", "b", "c", "d"]
x[1] = "r"
x[2:] = ["s", "t"]

For this and the following exercises, you'll continue working on the areas list that contains the names and areas of different rooms in a house.

@instructions

  • Update the area of the bathroom area to be 10.50 square meters instead of 9.50.
  • Make the areas list more trendy! Change "living room" to "chill zone".

@hint

  • Use areas[-1] = ___ to update the bathroom area. Can you fill in the blanks?
  • Similarly, you can use areas[4] = ___ to update the element "living room".

@pre_exercise_code

@sample_code

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Correct the bathroom area


# Change "living room" to "chill zone"

@solution

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Correct the bathroom area
areas[-1] = 10.50

# Change "living room" to "chill zone"
areas[4] = "chill zone"

@sct

bathroom_msg = 'You can use `areas[-1] = 10.50` to update the bathroom area.'
chillzone_msg = 'You can use `areas[4] = "chill zone"` to update the living room name.'
Ex().check_correct(
  check_object('areas').has_equal_value(incorrect_msg = 'Your changes to `areas` did not result in the correct list. Are you sure you used the correct subset operations? When in doubt, you can use a hint!'),
  multi(
    has_equal_value(expr_code='areas[-1]', override=10.50, incorrect_msg = bathroom_msg),
    has_equal_value(expr_code='areas[4]', override='chill zone', incorrect_msg = chillzone_msg),
  )
)
success_msg('Sweet! As the code sample showed, you can also slice a list and replace it with another list to update multiple elements in a single command.')

Extend a list

type: NormalExercise
key: ff0fe8d967
lang: python
xp: 100
skills:
  - 2

If you can change elements in a list, you sure want to be able to add elements to it, right? You can use the + operator:

x = ["a", "b", "c", "d"]
y = x + ["e", "f"]

You just won the lottery, awesome! You decide to build a poolhouse and a garage. Can you add the information to the areas list?

@instructions

  • Use the + operator to paste the list ["poolhouse", 24.5] to the end of the areas list. Store the resulting list as areas_1.
  • Further extend areas_1 by adding data on your garage. Add the string "garage" and float 15.45. Name the resulting list areas_2.

@hint

  • Follow the code sample in the assignment. x is areas here, and ["e", "f"] is ["poolhouse", 24.5].
  • To add more elements to areas_1, use areas_1 + ["element", 123].

@pre_exercise_code

@sample_code

# Create the areas list and make some changes
areas = ["hallway", 11.25, "kitchen", 18.0, "chill zone", 20.0,
         "bedroom", 10.75, "bathroom", 10.50]

# Add poolhouse data to areas, new list is areas_1


# Add garage data to areas_1, new list is areas_2

@solution

# Create the areas list (updated version)
areas = ["hallway", 11.25, "kitchen", 18.0, "chill zone", 20.0,
         "bedroom", 10.75, "bathroom", 10.50]

# Add poolhouse data to areas, new list is areas_1
areas_1 = areas + ["poolhouse", 24.5]

# Add garage data to areas_1, new list is areas_2
areas_2 = areas_1 + ["garage", 15.45]

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)
Ex().check_object("areas_1").has_equal_value(incorrect_msg = "Use `areas + [\"poolhouse\", 24.5]` to create `areas_1`. Watch out for typos!")
Ex().check_object("areas_2").has_equal_value(incorrect_msg = "Use `areas_1 + [\"garage\", 15.45]` to create `areas_2`. Watch out for typos!")
success_msg("Cool! The list is shaping up nicely!")

Delete list elements

type: MultipleChoiceExercise
key: 85f792356e
lang: python
xp: 50
skills:
  - 2

Finally, you can also remove elements from your list. You can do this with the del statement:

x = ["a", "b", "c", "d"]
del(x[1])

Pay attention here: as soon as you remove an element from a list, the indexes of the elements that come after the deleted element all change!

The updated and extended version of areas that you've built in the previous exercises is coded below. You can copy and paste this into the IPython Shell to play around with the result.

areas = ["hallway", 11.25, "kitchen", 18.0,
        "chill zone", 20.0, "bedroom", 10.75,
         "bathroom", 10.50, "poolhouse", 24.5,
         "garage", 15.45]

There was a mistake! The amount you won with the lottery is not that big after all and it looks like the poolhouse isn't going to happen. You decide to remove the corresponding string and float from the areas list.

The ; sign is used to place commands on the same line. The following two code chunks are equivalent:

# Same line
command1; command2

# Separate lines
command1
command2

Which of the code chunks will do the job for us?

@possible_answers

  • del(areas[10]); del(areas[11])
  • del(areas[10:11])
  • del(areas[-4:-2])
  • del(areas[-3]); del(areas[-4])

@hint You can simply try all the different options to see if they work. Just make sure to re-initialize the areas list again before you try a new option.

@pre_exercise_code

areas = ["hallway", 11.25, "kitchen", 18.0,
        "chill zone", 20.0, "bedroom", 10.75,
         "bathroom", 10.50, "poolhouse", 24.5,
         "garage", 15.45]

@sct

msg1 = "If you first remove `areas[10]`, all elements after index 10 move up a spot. If you then do `del(areas[11])`, you are deleting the element that was originally at index `12`."
msg2 = "`areas[10:11])` will only select the element at index `10`."
msg3 = "Correct! You'll learn about easier ways to remove specific elements from Python lists later on."
msg4 = "This code chunk will not correctly remove the poolhouse-related information. Try again."
Ex().has_chosen(3, [msg1, msg2, msg3, msg4])

Inner workings of lists

type: NormalExercise
key: af72db9915
lang: python
xp: 100
skills:
  - 2

At the end of the video, Hugo explained how Python lists work behind the scenes. In this exercise you'll get some hands-on experience with this.

The Python code in the script already creates a list with the name areas and a copy named areas_copy. Next, the first element in the areas_copy list is changed and the areas list is printed out. If you hit Run Code you'll see that, although you've changed areas_copy, the change also takes effect in the areas list. That's because areas and areas_copy point to the same list.

If you want to prevent changes in areas_copy from also taking effect in areas, you'll have to do a more explicit copy of the areas list. You can do this with list() or by using [:].

@instructions Change the second command, that creates the variable areas_copy, such that areas_copy is an explicit copy of areas. After your edit, changes made to areas_copy shouldn't affect areas. Submit the answer to check this.

@hint Change the areas_copy = areas call. Instead of assigning areas, you can assign list(areas) or areas[:].

@pre_exercise_code

@sample_code

# Create list areas
areas = [11.25, 18.0, 20.0, 10.75, 9.50]

# Create areas_copy
areas_copy = areas

# Change areas_copy
areas_copy[0] = 5.0

# Print areas
print(areas)

@solution

# Create list areas
areas = [11.25, 18.0, 20.0, 10.75, 9.50]

# Create areas_copy
areas_copy = list(areas)

# Change areas_copy
areas_copy[0] = 5.0

# Print areas
print(areas)

@sct

Ex().check_correct(
  check_object("areas_copy").has_equal_value(incorrect_msg = "It seems that `areas_copy` has not been updated correctly."),
  check_function("list", missing_msg = "Make sure to use `list(areas)` to create an `areas_copy`.")
)

mmsg = "Don't remove the predefined `areas` list."
imsg = "Be sure to edit ONLY the copy, not the original `areas` list. Have another look at the exercise description if you're unsure how to create a copy."
Ex().check_correct(
  check_object("areas", missing_msg = mmsg).has_equal_value(incorrect_msg = imsg),
  check_function("list", missing_msg = "Make sure to use `list(areas)` to create an `areas_copy`.")
)

success_msg("Nice! The difference between explicit and reference-based copies is subtle, but can be really important. Try to keep in mind how a list is stored in the computer's memory.")