Section 3: Python Object and Data Structure Basics Flashcards
What does “Formatted String Literals (f-strings)” look like:
name = ‘Fred’
print(f”He said his name is {name}.”)
What do Lists look like?
# Assign a list to an variable named my_list my_list = [1,2,3]
How to find the number of objects in a List?
len(my_list)
How do you slice a list?
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]
How do you add an item to a List?
# Append
list1.append('append me!')How do you remove the last item in a list and store it in a variable?
# Assign the popped element, remember default popped index is -1 popped_item = list1.pop()
How do you reverse the items in a List?
# Use reverse to reverse order (this is permanent!) new_list.reverse()
How to sort a List (so that the sort is permanent, done in place) ?
# Use sort to sort the list (in this case alphabetical order, but for numbers it will go ascending) new_list.sort()
How can you nest Lists?
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]]
What does List Comprehensions look like?
# 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]
What does a dictionary look like”
# Make a dictionary with {} and : to signify a key and a value
my_dict = {'key1':'value1','key2':'value2'}How do call a value with a dictionary?
# Call values by their key my_dict['key2']
produces:
value2
How would you subtract from a value in a dictionary?
# Subtract 123 from the value my_dict['key1'] = my_dict['key1'] - 123
How do you add a new key/value pair to a dictionary?
# Create a new key through assignment d['animal'] = 'Dog'
# Can do this with any object d['answer'] = 42
How do you create a tuple?
# Create a tuple t = (1,2,3)
# Can also mix object types
t = ('one',2)
For Tuples, how do you find the index of a value?
# Use .index to enter a value and return the index
t.index('one')For a Tuple, how can you count the number times a value appears?
# Use .count to count the number of times a value appears
t.count('one')Are Tuples Immutable or Mutable
Immutable
Because of this immutability, tuples can’t grow. Once a tuple is made we can not add to it.
What is a Set?
Sets are an unordered collection of unique elements. We can construct them by using the set() function.
How do you create an empty set?
x = set()
How can you add to a Set?
# We add to sets with the add() method x.add(1)
What do Sets and Dictionaries have in common?
curly brackets { }
What happens if you convert a list with repeats into a set?
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}
How can you quickly write a file in Jupyter Notebook?
This function is specific to jupyter notebooks!
%%writefile test.txt
Hello, this is a quick test file.
How do you open a file in python?
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:
How to read a file”
First:
# Open the text.txt we made earlier
my_file = open('test.txt')
# We can now read the file my_file.read()
What happens if you try to reread a file?
# 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:
How can we reset a file that has already been read?
# Seek to the start of file (index 0) my_file.seek(0)
How do you read a file line by line?
Readlines returns a list of the lines in the file
my_file.seek(0)
my_file.readlines()
How do you close a file once you are finished with it?
my_file.close()
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.
How can you write to a file?
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+’)
Opening a file with ‘w’ or ‘w+’ will ….
truncates the original, meaning that anything that was in the original file is deleted!
To append to a file with Jupyter:
%%writefile -a test.txt
This is text being appended to test.txt
And another line here.
How do you append to a file in Python?
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.’)
How can you iternate through a file:
for line in open(‘test.txt’):
print(line)
