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Chapter 3

Functions and Packages

You'll learn how to use functions, methods, and packages to efficiently leverage the code that brilliant Python developers have written. The goal is to reduce the amount of code you need to solve challenging problems!

slides_link
https://projector-video-pdf-converter.datacamp.com/735/chapter3.pdf

nb_of_exercises	title
4	Functions

nb_of_exercises	title
4	Methods

nb_of_exercises	title
4	Packages

Functions

type: VideoExercise
key: 5c5f365930
xp: 50

@projector_key 1204d914b0e53100529827e07441ee6c

Familiar functions

type: NormalExercise
key: c422ee929b
lang: python
xp: 100
skills:
  - 2

Out of the box, Python offers a bunch of built-in functions to make your life as a data scientist easier. You already know two such functions: print() and type(). There are also functions like str(), int(), bool() and float() to switch between data types. You can find out about them here. These are built-in functions as well.

Calling a function is easy. To get the type of 3.0 and store the output as a new variable, result, you can use the following:

result = type(3.0)

@instructions

Use print() in combination with type() to print out the type of var1.
Use len() to get the length of the list var1. Wrap it in a print() call to directly print it out.
Use int() to convert var2 to an integer. Store the output as out2.

@hint

Call the type() function like this: type(var1).
Call print() like you did so many times before. Simply put the variable you want to print in parentheses.
int(x) will convert x to an integer.

@pre_exercise_code

@sample_code

# Create variables var1 and var2
var1 = [1, 2, 3, 4]
var2 = True

# Print out type of var1
____

# Print out length of var1
____

# Convert var2 to an integer: out2
out2 = ____

@solution

# Create variables var1 and var2
var1 = [1, 2, 3, 4]
var2 = True

# Print out type of var1
print(type(var1))

# Print out length of var1
print(len(var1))

# Convert var2 to an integer: out2
out2 = int(var2)

@sct

msg = "You don't have to change or remove the predefined variables."
Ex().check_object("var1", missing_msg=msg).has_equal_value(incorrect_msg=msg)
Ex().check_object("var2", missing_msg=msg).has_equal_value(incorrect_msg=msg)

patt = "__JINJA__:Make sure to print out the %s of `var1` with `{{sol_call}}`."
Ex().has_printout(0, not_printed_msg = patt % 'type')
Ex().has_printout(1, not_printed_msg = patt % 'length')

int_miss_msg = "Have you used `int()` to make an integer of `var2`?"
int_incorr_msg = "Have you passed `var2` to `int()`?"
Ex().check_correct(
  check_object("out2").has_equal_value(incorrect_msg="You called `int()` correctly; now make sure to assign the result of this call to `out2`."),
  check_function("int", missing_msg=int_miss_msg).has_equal_value(incorrect_msg=int_incorr_msg)
)
success_msg("Great job! The `len()` function is extremely useful; it also works on strings to count the number of characters!")

Help!

type: MultipleChoiceExercise
key: 679b852978
lang: python
xp: 50
skills:
  - 2

Maybe you already know the name of a Python function, but you still have to figure out how to use it. Ironically, you have to ask for information about a function with another function: help(). In IPython specifically, you can also use ? before the function name.

To get help on the max() function, for example, you can use one of these calls:

help(max)
?max

Use the IPython Shell to open up the documentation on pow(). Do this by typing ?pow or help(pow) and hitting Enter.

Which of the following statements is true?

@possible_answers

pow() takes three arguments: base, exp, and mod. Without mod, the function will return an error.
pow() takes three required arguments: base, exp, and None.
pow() requires base and exp arguments; mod is optional.
pow() takes two arguments: exp and mod. Missing exp results in an error.

@hint

Optional arguments are set = to a default value, which the function will use if that argument is not specified.

@pre_exercise_code

@sct

msg1 = "Not quite. `mod` has a default value that will be used if you don't specify a value."
msg2 = "Incorrect. `None` is the default value for the `mod` argument."
msg3 = "Perfect! Using `help()` can help you understand how functions work, unleashing their full potential!"
msg4 = "Incorrect. `pow()` takes three arguments, one of which has a default value."
Ex().has_chosen(3, [msg1, msg2, msg3, msg4])

Multiple arguments

type: NormalExercise
key: e30486d7c1
lang: python
xp: 100
skills:
  - 2

In the previous exercise, you identified optional arguments by viewing the documentation with help(). You'll now apply this to change the behavior of the sorted() function.

Have a look at the documentation of sorted() by typing help(sorted) in the IPython Shell.

You'll see that sorted() takes three arguments: iterable, key, and reverse. In this exercise, you'll only have to specify iterable and reverse, not key.

Two lists have been created for you.

Can you paste them together and sort them in descending order?

@instructions

Use + to merge the contents of first and second into a new list: full.
Call sorted() and on full and specify the reverse argument to be True. Save the sorted list as full_sorted.
Finish off by printing out full_sorted.

@hint

Sum first and second as if they are two numbers and assign the result to full.
Use sorted() with two inputs: full and reverse=True.
To print out a variable, use print().

@pre_exercise_code

@sample_code

# Create lists first and second
first = [11.25, 18.0, 20.0]
second = [10.75, 9.50]

# Paste together first and second: full
full = ____ + ____

# Sort full in descending order: full_sorted
full_sorted = ____

# Print out full_sorted
____

@solution

# Create lists first and second
first = [11.25, 18.0, 20.0]
second = [10.75, 9.50]

# Paste together first and second: full
full = first + second

# Sort full in descending order: full_sorted
full_sorted = sorted(full, reverse=True)

# Print out full_sorted
print(full_sorted)

@sct

msg = "You don't have to change or remove the already variables `first` and `second`."
Ex().multi(
  check_object("first", missing_msg=msg).has_equal_value(incorrect_msg=msg),
  check_object("second", missing_msg=msg).has_equal_value(incorrect_msg=msg)
)
Ex().check_correct(
  check_object("full_sorted").has_equal_value(incorrect_msg="Make sure you assign the result of calling `sorted()` to `full_sorted`."),
  check_function("sorted").multi(
    check_args(0).has_equal_value(),
    check_args('reverse').has_equal_value()
  )
)

success_msg("Cool! Head over to the video on Python methods.")

Methods

type: VideoExercise
key: 2b66cb66b1
xp: 50

@projector_key 8e387776f3a264a745128b68aa8d8f83

String Methods

type: NormalExercise
key: 4039302ee0
lang: python
xp: 100
skills:
  - 2

Strings come with a bunch of methods. Follow the instructions closely to discover some of them. If you want to discover them in more detail, you can always type help(str) in the IPython Shell.

A string place has already been created for you to experiment with.

@instructions

Use the .upper() method on place and store the result in place_up. Use the syntax for calling methods that you learned in the previous video.
Print out place and place_up. Did both change?
Print out the number of o's on the variable place by calling .count() on place and passing the letter 'o' as an input to the method. We're talking about the variable place, not the word "place"!

@hint

You can call the .upper() method on place without any additional inputs.
To print out a variable x, you can write print(x).
Make sure to wrap your place.count(____) call in a print() function so that you print it out.

@pre_exercise_code

@sample_code

# string to experiment with: place
place = "poolhouse"

# Use upper() on place
place_up = 

# Print out place and place_up



# Print out the number of o's in place

@solution

# string to experiment with: place
place = "poolhouse"

# Use upper() on place
place_up = place.upper()

# Print out place and place_up
print(place)
print(place_up)

# Print out the number of o's in place
print(place.count('o'))

@sct

msg = "You don't have to change or remove the predefined variables."
Ex().check_object("place", missing_msg=msg).has_equal_value(incorrect_msg=msg)

patt = "Don't forget to print out `%s`."
Ex().has_printout(0, not_printed_msg=patt % "place")
Ex().check_correct(
    has_printout(1, not_printed_msg=patt % "place_up"),
    check_correct(
        check_object("place_up").has_equal_value(incorrect_msg="Assign the result of your `place.upper()` call to `place_up`."),
        check_function("place.upper", signature=False)
    )
)    

# check count of place
Ex().check_correct(
  has_printout(2, not_printed_msg = "You have calculated the number of o's in `place` fine; now make sure to wrap `place.count('o')` call in a `print()` function to print out the result."),
  check_function("place.count", signature=False).check_args(0).has_equal_value()
)

success_msg("Nice! Notice from the printouts that the `upper()` method does not change the object it is called on. This will be different for lists in the next exercise!")

List Methods

type: NormalExercise
key: 0dbe8ed695
lang: python
xp: 100
skills:
  - 2

Strings are not the only Python types that have methods associated with them. Lists, floats, integers and booleans are also types that come packaged with a bunch of useful methods. In this exercise, you'll be experimenting with:

.index(), to get the index of the first element of a list that matches its input and
.count(), to get the number of times an element appears in a list.

You'll be working on the list with the area of different parts of a house: areas.

@instructions

Use the .index() method to get the index of the element in areas that is equal to 20.0. Print out this index.
Call .count() on areas to find out how many times 9.50 appears in the list. Again, simply print out this number.

@hint

To print out the index, wrap the areas.index(___) call in a print() function.
To print out the number of times an element x occurs in the list, wrap the areas.count(___) call in a print() function.

@pre_exercise_code

@sample_code

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

# Print out the index of the element 20.0


# Print out how often 9.50 appears in areas

@solution

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

# Print out the index of the element 20.0
print(areas.index(20.0))

# Print out how often 9.50 appears in areas
print(areas.count(9.50))

@sct

predef_msg = "You don't have to change or remove the predefined list `areas`."

Ex().check_object("areas", missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg)

Ex().check_function("print", index=0).check_args(0).check_function('areas.index', signature=False).check_args(0).has_equal_value()


Ex().check_function("print", index=1).check_args(0).check_function('areas.count', signature=False).has_equal_value()

success_msg("Nice! These were examples of `list` methods that did not change the list they were called on.")

List Methods (2)

type: NormalExercise
key: 1fbeab82d0
lang: python
xp: 100
skills:
  - 2

Most list methods will change the list they're called on. Examples are:

.append(), that adds an element to the list it is called on,
.remove(), that removes the first element of a list that matches the input, and
.reverse(), that reverses the order of the elements in the list it is called on.

You'll be working on the list with the area of different parts of the house: areas.

@instructions

Use .append() twice to add the size of the poolhouse and the garage again: 24.5 and 15.45, respectively. Make sure to add them in this order.
Print out areas
Use the .reverse() method to reverse the order of the elements in areas.
Print out areas once more.

@hint

For the first instruction, use the areas.append(___) call twice.
To print out a variable x, simply write print(x).
The .reverse() method does not require additional inputs; just use the dot notation and empty parentheses: .reverse().
To print out a variable x, simply write print(x).

@pre_exercise_code

@sample_code

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

# Use append twice to add poolhouse and garage size



# Print out areas


# Reverse the orders of the elements in areas


# Print out areas

@solution

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

# Use append twice to add poolhouse and garage size
areas.append(24.5)
areas.append(15.45)

# Print out areas
print(areas)

# Reverse the orders of the elements in areas
areas.reverse()

# Print out areas
print(areas)

@sct

Ex().multi(
  check_function("areas.append", index=0, signature=False).check_args(0).has_equal_value(),
  check_function("areas.append", index=1, signature=False).check_args(0).has_equal_value(),
  check_function("print", index=0).check_args(0).has_equal_ast(),
  check_function("areas.reverse", index=0, signature=False),
  check_function("print", index=1).check_args(0).has_equal_ast()
)

success_msg("Great!")

Packages

type: VideoExercise
key: ab96a17c5e
xp: 50

@projector_key cedcfb34350be8545599768f96695cdd

Import package

type: NormalExercise
key: 7432a6376f
lang: python
xp: 100
skills:
  - 2

Let's say you wanted to calculate the circumference and area of a circle. Here's what those formulas look like:

$$C = 2 \pi r$$ $$A = \pi r^2 $$

Rather than typing the number for pi, you can use the math package that contains the number

For reference, ** is the symbol for exponentiation. For example 3**4 is 3 to the power of 4 and will give 81.

@instructions

Import the math package.
Use math.pi to calculate the circumference of the circle and store it in C.
Use math.pi to calculate the area of the circle and store it in A.

@hint

You can simply use import math, and then refer to pi with math.pi.
Use the equation in the assignment text to find C. Use *
Use the equation in the assignment text to find A. Use * and **.

@pre_exercise_code

@sample_code

# Import the math package
import ____

# Calculate C
C = 2 * 0.43 * ____

# Calculate A
A = ____ * 0.43 ** 2

print("Circumference: " + str(C))
print("Area: " + str(A))

@solution

# Import the math package
import math

# Calculate C
C = 2 * 0.43 * math.pi

# Calculate A
A = math.pi * 0.43 ** 2

print("Circumference: " + str(C))
print("Area: " + str(A))

@sct

patt = "Your calculation of `%s` is not quite correct. Make sure to use `math.pi`."
Ex().multi(
  has_import('math', same_as=False),
  check_object('C').has_equal_value(incorrect_msg=patt%'C'),
  check_object('A').has_equal_value(incorrect_msg=patt%'A')
)

Ex().multi(
  has_printout(0, not_printed_msg = "__JINJA__:Keep `{{sol_call}}` in there to print out the circumference."),
  has_printout(1, not_printed_msg = "__JINJA__:Keep `{{sol_call}}` in there to print out the area.")
)

success_msg("Nice! If you know how to deal with functions from packages, the power of a lot of Python programmers is at your fingertips!")

Selective import

type: NormalExercise
key: fe65eff50a
lang: python
xp: 100
skills:
  - 2

General imports, like import math, make all functionality from the math package available to you. However, if you decide to only use a specific part of a package, you can always make your import more selective:

from math import pi

Try the same thing again, but this time only use pi.

@instructions

Perform a selective import from the math package where you only import the pi function.
Use math.pi to calculate the circumference of the circle and store it in C.
Use math.pi to calculate the area of the circle and store it in A.

@hint

Use from math import pi to do the selective import.
Now, you can use pi on it's own!

@pre_exercise_code

@sample_code

# Import pi function of math package
from math import ____

# Calculate C
C = 2 * 0.43 * ____

# Calculate A
A = ____ * 0.43 ** 2

print("Circumference: " + str(C))
print("Area: " + str(A))

@solution

# Import pi function of math package
from math import pi

# Calculate C
C = 2 * 0.43 * pi

# Calculate A
A = pi * 0.43 ** 2

print("Circumference: " + str(C))
print("Area: " + str(A))

@sct

patt = "Your calculation of `%s` is not quite correct. Make sure to use only `pi`."

Ex().has_import("math.pi", not_imported_msg = "Be sure to import `pi` from the `math` package. You should use the `from ___ import ___` notation.",)

Ex().multi(
  check_object('C').has_equal_value(incorrect_msg=patt%'C'),
  check_object('A').has_equal_value(incorrect_msg=patt%'A')
)

Ex().multi(
  has_printout(0, not_printed_msg = "__JINJA__:Keep `{{sol_call}}` in there to print out the circumference."),
  has_printout(1, not_printed_msg = "__JINJA__:Keep `{{sol_call}}` in there to print out the area.")
)

success_msg("Nice! Head over to the next exercise.")

Different ways of importing

type: MultipleChoiceExercise
key: f1b2675a2a
lang: python
xp: 50
skills:
  - 2

There are several ways to import packages and modules into Python. Depending on the import call, you'll have to use different Python code.

Suppose you want to use the function inv(), which is in the linalg subpackage of the scipy package. You want to be able to use this function as follows:

my_inv([[1,2], [3,4]])

Which import statement will you need in order to run the above code without an error?

@possible_answers

import scipy
import scipy.linalg
from scipy.linalg import my_inv
from scipy.linalg import inv as my_inv

@hint

Try the different import statements in the IPython shell and see which one causes the line my_inv([[1, 2], [3, 4]]) to run without errors. Hit enter to run the code you have typed.

@pre_exercise_code

@sct

msg1 = msg2 = msg3 = "Incorrect, try again. Try the different import statements in the IPython shell and see which one causes the line `my_inv([[1, 2], [3, 4]])` to run without errors."
msg4 = "Correct! The `as` word allows you to create a local name for the function you're importing: `inv()` is now available as `my_inv()`."
Ex().has_chosen(4, [msg1, msg2, msg3, msg4])

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chapter3.md

chapter3.md

Functions

Familiar functions

Help!

Multiple arguments

Methods

String Methods

List Methods

List Methods (2)

Packages

Import package

Selective import

Different ways of importing

Files

chapter3.md

Latest commit

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chapter3.md

File metadata and controls

Functions

Familiar functions

Help!

Multiple arguments

Methods

String Methods

List Methods

List Methods (2)

Packages

Import package

Selective import

Different ways of importing