title_meta | title | description | attachments |
---|---|---|---|
Chapter 3 |
Functions and Packages |
To leverage the code that brilliant Python developers have written, you'll learn about using functions, methods and packages. This will help you to reduce the amount of code you need to solve challenging problems! |
type: VideoExercise
lang: python
xp: 50
skills: 2
key: 2dde2f90b8
@video_link
//player.vimeo.com/video/154563189
@video_hls
//videos.datacamp.com/transcoded/735_intro_to_python/v3/hls-ch3_1.master.m3u8
*** =projector_key abe8835964fe3385a3f0283b7a605f5f
type: NormalExercise
lang: python
xp: 100
skills: 2
key: c422ee929b
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()
. You've also used the functions str()
, int()
, bool()
and float()
to switch between data types. 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)
The general recipe for calling functions and saving the result to a variable is thus:
output = function_name(input)
@instructions
- Use
print()
in combination withtype()
to print out the type ofvar1
. - Use
len()
to get the length of the listvar1
. Wrap it in aprint()
call to directly print it out. - Use
int()
to convertvar2
to an integer. Store the output asout2
.
@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 convertx
to an integer.
@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
@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()`](https://docs.python.org/3/library/functions.html#int) to make an integer of `var2`?"
int_incorr_msg = "Have you passed `var2` to [`int()`](https://docs.python.org/3/library/functions.html#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()`](https://docs.python.org/3/library/functions.html#len) function is extremely useful; it also works on strings to count the number of characters!")
type: MultipleChoiceExercise
lang: python
xp: 50
skills: 2
key: 679b852978
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 Shell on the right to open up the documentation on complex()
. Which of the following statements is true?
@instructions
complex()
takes exactly two arguments:real
and[, imag]
.complex()
takes two arguments:real
andimag
. Both these arguments are required.complex()
takes two arguments:real
andimag
.real
is a required argument,imag
is an optional argument.complex()
takes two arguments:real
andimag
. If you don't specifyimag
, it is set to 1 by Python.
@hint
The help file shows complex(real[, imag])
. Do you remember what Filip told you about these square brackets?
@sct
msg1 = "Incorrect. `[, imag]` shows that `imag` is an optional argument."
msg2 = "This statement is false. `imag` is not a required argument."
msg3 = "Perfect!"
msg4 = "This is almost true, but not entirely. If you don't specify `imag`, it is set to 0."
Ex().has_chosen(3, [msg1, msg2, msg3, msg4])
type: NormalExercise
lang: python
xp: 100
skills: 2
key: e30486d7c1
In the previous exercise, the square brackets around imag
in the documentation showed us that the imag
argument is optional. But Python also uses a different way to tell users about arguments being optional.
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
.
key=None
means that if you don't specify the key
argument, it will be None
. reverse=False
means that if you don't specify the reverse
argument, it will be False
.
In this exercise, you'll only have to specify iterable
and reverse
, not key
. The first input you pass to sorted()
will be matched to the iterable
argument, but what about the second input? To tell Python you want to specify reverse
without changing anything about key
, you can use =
:
sorted(___, reverse = ___)
Two lists have been created for you on the right. Can you paste them together and sort them in descending order?
Note: For now, we can understand an iterable as being any collection of objects, e.g. a List.
@instructions
- Use
+
to merge the contents offirst
andsecond
into a new list:full
. - Call
sorted()
onfull
and specify thereverse
argument to beTrue
. Save the sorted list asfull_sorted
. - Finish off by printing out
full_sorted
.
@hint
- Simply sum
first
andsecond
as if they are two numbers and assign the result tofull
. - Use
sorted()
with two inputs:full
andreverse = True
. - To print out a variable, use
print()
.
@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
# Sort full in descending order: 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.")
type: VideoExercise
lang: python
xp: 50
skills: 2
key: e1aaeb300b
@video_link
//player.vimeo.com/video/154563307
@video_hls
//videos.datacamp.com/transcoded/735_intro_to_python/v1/hls-ch3_2.master.m3u8
*** =projector_key cf2471efdf2df82d911fb0cfcf0466f6
type: NormalExercise
lang: python
xp: 100
skills: 2
key: 4039302ee0
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 onplace
and store the result inplace_up
. Use the syntax for calling methods that you learned in the previous video. - Print out
place
andplace_up
. Did both change? - Print out the number of o's on the variable
place
by callingcount()
onplace
and passing the letter'o'
as an input to the method. We're talking about the variableplace
, not the word"place"
!
@hint
- You can call the
upper()
method onplace
without any additional inputs. - To print out a variable
x
, you can writeprint(x)
. - Make sure to wrap your
place.count(___)
call in aprint()
function so that you print it out.
@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_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()`](https://docs.python.org/3/library/stdtypes.html#str.upper) method does not change the object it is called on. This will be different for lists in the next exercise!")
type: NormalExercise
lang: python
xp: 100
skills: 2
key: 0dbe8ed695
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 andcount()
, 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 inareas
that is equal to20.0
. Print out this index. - Call
count()
onareas
to find out how many times9.50
appears in the list. Again, simply print out this number.
@hint
- To print out the index, wrap the
areas.index(___)
call in aprint()
function. - To print out the number of times an element
x
occurs in the list, wrap theareas.count(___)
call in aprint()
function.
@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_correct(
has_printout(0, not_printed_msg="You have calculated the index of element 20.0 correctly; now make sure to print it!"),
multi(
check_object("areas", missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
check_function('areas.index', signature=False).check_args(0).has_equal_value()
)
)
Ex().check_correct(
has_printout(1, not_printed_msg="You have calculated the number of times `9.50` appears correctly; now make sure to print it!"),
multi(
check_object("areas", missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
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.")
type: NormalExercise
lang: python
xp: 100
skills: 2
key: 1fbeab82d0
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, andreverse()
, 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
and15.45
, respectively. Make sure to add them in this order. - Print out
areas
- Use the
reverse()
method to reverse the order of the elements inareas
. - Print out
areas
once more.
@hint
- For the first instruction, use the
areas.append(___)
call twice. - To print out a variable
x
, simply writeprint(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 writeprint(x)
.
@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!")
type: VideoExercise
lang: python
xp: 50
skills: 2
key: 2b89c5a9d8
@video_link
//player.vimeo.com/video/146994272
@video_hls
//videos.datacamp.com/transcoded/735_intro_to_python/v1/hls-ch3_3.master.m3u8
*** =projector_key c550d2f388d2718361d55e101c6c3887
type: NormalExercise
lang: python
xp: 100
skills: 2
key: 7432a6376f
As a data scientist, some notions of geometry never hurt. Let's refresh some of the basics.
For a fancy clustering algorithm, you want to find the circumference, r
, you can calculate
To use the constant pi
, you'll need the math
package. A variable r
is already coded in the script. Fill in the code to calculate C
and A
and see how the print()
functions create some nice printouts.
@instructions
- Import the
math
package. Now you can access the constantpi
withmath.pi
. - Calculate the circumference of the circle and store it in
C
. - Calculate the area of the circle and store it in
A
.
@hint
- You can simply use
import math
, and then refer topi
withmath.pi
. - Use the equation in the assignment text to find
C
. Use*
- Use the equation in the assignment text to find
A
. Use*
and**
.
@sample_code
# Definition of radius
r = 0.43
# Import the math package
# Calculate C
C = 0
# Calculate A
A = 0
# Build printout
print("Circumference: " + str(C))
print("Area: " + str(A))
@solution
# Definition of radius
r = 0.43
# Import the math package
import math
# Calculate C
C = 2 * r * math.pi
# Calculate A
A = math.pi * r ** 2
# Build printout
print("Circumference: " + str(C))
print("Area: " + str(A))
@sct
msg = "You don't have to change or remove the predefined variables."
patt = "Your calculation of `%s` is not quite correct. Make sure to use `math.pi`."
Ex().multi(
check_object('r', missing_msg=msg).has_equal_value(incorrect_msg=msg),
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!")
type: NormalExercise
lang: python
xp: 100
skills: 2
key: fe65eff50a
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
Let's say the Moon's orbit around planet Earth is a perfect circle, with a radius r
(in km) that is defined in the script.
@instructions
- Perform a selective import from the
math
package where you only import theradians
function. - Calculate the distance travelled by the Moon over 12 degrees of its orbit. Assign the result to
dist
. You can calculate this asr * phi
, wherer
is the radius andphi
is the angle in radians. To convert an angle in degrees to an angle in radians, use theradians()
function, which you just imported. - Print out
dist
.
@hint
- Use
from math import radians
to do the selective import. - You can simply use the
radians()
function now. Pass the function the number 12 to get the angle in radians. - To print out a variable
x
, simply typeprint(x)
.
@sample_code
# Definition of radius
r = 192500
# Import radians function of math package
# Travel distance of Moon over 12 degrees. Store in dist.
# Print out dist
@solution
# Definition of radius
r = 192500
# Import radians function of math package
from math import radians
# Travel distance of Moon over 12 degrees. Store in dist.
dist = r * radians(12)
# Print out dist
print(dist)
@sct
msg = "You don't have to change or remove the predefined variables."
Ex().check_object("r", missing_msg=msg).has_equal_value(incorrect_msg=msg)
Ex().has_import("math.radians", not_imported_msg = "Be sure to import [`radians()`](https://docs.python.org/3/library/math.html#math.radians) from the `math` package. You should use the `from ___ import ___` notation.", incorrect_as_msg = "Don't set any alias for [`radians()`](https://docs.python.org/3/library/math.html#math.radians). Just type `from math import radians`.")
Ex().check_correct(
check_object("dist").has_equal_value(),
check_function("math.radians", signature=False).check_args(0).has_equal_value()
)
Ex().has_printout(0)
success_msg("Nice! Head over to the next exercise.")
type: MultipleChoiceExercise
lang: python
xp: 50
skills: 2
key: f1b2675a2a
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?
@instructions
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.
@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()`](https://docs.python.org/3/library/functions.html#inv) is now available as `my_inv()`."
Ex().has_chosen(4, [msg1, msg2, msg3, msg4])