Section 3: Python Object and Data Structure Basics

Question 1

What does “Formatted String Literals (f-strings)” look like:

Answer 1

name = ‘Fred’

print(f”He said his name is {name}.”)

Question 2

What do Lists look like?

Answer 2

# Assign a list to an variable named my_list
my_list = [1,2,3]

Question 3

How to find the number of objects in a List?

Answer 3

len(my_list)

Question 4

How do you slice a list?

Answer 4

my_list = [‘one’, ‘two’, ‘three’, 4, 5]

# Grab index 1 and everything past it
my_list[1:]

# Grab everything UP TO index 3
my_list[:3]

Question 5

How do you add an item to a List?

Answer 5

# Append
list1.append('append me!')

Question 6

How do you remove the last item in a list and store it in a variable?

Answer 6

# Assign the popped element, remember default popped index is -1
popped_item = list1.pop()

Question 7

How do you reverse the items in a List?

Answer 7

# Use reverse to reverse order (this is permanent!)
new_list.reverse()

Question 8

How to sort a List (so that the sort is permanent, done in place) ?

Answer 8

# Use sort to sort the list (in this case alphabetical order, but for numbers it will go ascending)
new_list.sort()

Question 9

How can you nest Lists?

Answer 9

Let’s make three lists
lst_1=[1,2,3]
lst_2=[4,5,6]
lst_3=[7,8,9]

# Make a list of lists to form a matrix
matrix = [lst_1,lst_2,lst_3]

Show
matrix
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Question 10

What does List Comprehensions look like?

Answer 10

# Show matrix
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# Build a list comprehension by deconstructing a for loop within a []
first_col = [row[0] for row in matrix]

first_col
[1, 4, 7]

Question 11

What does a dictionary look like”

Answer 11

# Make a dictionary with {} and : to signify a key and a value
my_dict = {'key1':'value1','key2':'value2'}

Question 12

How do call a value with a dictionary?

Answer 12

# Call values by their key
my_dict['key2']

produces:
value2

Question 13

How would you subtract from a value in a dictionary?

Answer 13

# Subtract 123 from the value
my_dict['key1'] = my_dict['key1'] - 123

Question 14

How do you add a new key/value pair to a dictionary?

Answer 14

# Create a new key through assignment
d['animal'] = 'Dog'

# Can do this with any object
d['answer'] = 42

Question 15

How do you create a tuple?

Answer 15

# Create a tuple
t = (1,2,3)

# Can also mix object types
t = ('one',2)

Question 16

For Tuples, how do you find the index of a value?

Answer 16

# Use .index to enter a value and return the index
t.index('one')

Question 17

For a Tuple, how can you count the number times a value appears?

Answer 17

# Use .count to count the number of times a value appears
t.count('one')

Question 18

Are Tuples Immutable or Mutable

Answer 18

Immutable

Because of this immutability, tuples can’t grow. Once a tuple is made we can not add to it.

Question 19

What is a Set?

Answer 19

Sets are an unordered collection of unique elements. We can construct them by using the set() function.

Question 20

How do you create an empty set?

Answer 20

x = set()

Question 21

How can you add to a Set?

Answer 21

# We add to sets with the add() method
x.add(1)

Question 22

What do Sets and Dictionaries have in common?

Answer 22

curly brackets { }

Question 23

What happens if you convert a list with repeats into a set?

Answer 23

there will be no repeats in a set:

# Create a list with repeats
list1 = [1,1,2,2,3,4,5,6,1,1]

# Cast as set to get unique values
set(list1)

{1, 2, 3, 4, 5, 6}

Question 24

How can you quickly write a file in Jupyter Notebook?

Answer 24

This function is specific to jupyter notebooks!

%%writefile test.txt
Hello, this is a quick test file.

Question 25

How do you open a file in python?

Answer 25

myfile = open(‘whoops.txt’)

It is very easy to get an error on this step:
To avoid this error,make sure your .txt file is saved in the same location as your notebook, to check your notebook location, use pwd:

Question 26

How to read a file”

Answer 26

First:
# Open the text.txt we made earlier
my_file = open('test.txt')

# We can now read the file
my_file.read()

Question 27

What happens if you try to reread a file?

Answer 27

# But what happens if we try to read it again?
my_file.read()

You get => “” (an empty string)

This happens because you can imagine the reading “cursor” is at the end of the file after having read it. So there is nothing left to read. We can reset the “cursor” like this:

Question 28

How can we reset a file that has already been read?

Answer 28

# Seek to the start of file (index 0)
my_file.seek(0)

Question 29

How do you read a file line by line?

Answer 29

Readlines returns a list of the lines in the file
my_file.seek(0)
my_file.readlines()

Question 30

How do you close a file once you are finished with it?

Answer 30

my_file.close()

Question 31

By default, the open() function will only ….

Answer 31

allow us to read the file. We need to pass the argument ‘w’ to write over the file.

Question 32

How can you write to a file?

Answer 32

By default, the open() function will only allow us to read the file. We need to pass the argument 'w' to write over the file. For example:
   # Add a second argument to the function, 'w' which stands for write.
   # Passing 'w+' lets us read and write to the file

my_file = open(‘test.txt’, ‘w+’)

Question 33

Opening a file with ‘w’ or ‘w+’ will ….

Answer 33

truncates the original, meaning that anything that was in the original file is deleted!

Question 34

To append to a file with Jupyter:

Answer 34

%%writefile -a test.txt

This is text being appended to test.txt
And another line here.

Question 35

How do you append to a file in Python?

Answer 35

Passing the argument ‘a’ opens the file and puts the pointer at the end, so anything written is appended. Like ‘w+’, ‘a+’ lets us read and write to a file. If the file does not exist, one will be created.

my_file = open(‘test.txt’,’a+’)
my_file.write(‘\nThis is text being appended to test.txt’)
my_file.write(‘\nAnd another line here.’)

Question 36

How can you iternate through a file:

Answer 36

for line in open(‘test.txt’):

print(line)