Methods

Methods#

string, list, & dictionary
in place vs not in place
relationship to functions

Class Question #1#

What will the following code snippet print?

def my_func(my_dictionary):
    
    output = []

    for item in my_dictionary:
        value = my_dictionary[item]
        output.append(value) #append method adds an item to end of a list
    
    return output

# create dictionary and execute function
dictionary = {'first' : 1, 'second' : 2, 'third' : 3}
out = my_func(dictionary)

print(out)

A) [‘first’, ‘second’, ‘third’]
B) {1, 2, 3}
C) [‘first’, 1, ‘second’, 2, ‘third’, 3]
D) [1, 2, 3]
E) None

Methods#

Methods are functions that are defined and called directly on an object.

For our purposes, objects are any data variable.

Method Examples#

A method is a function applied directly to the object you call it on.

General form of a method:

object.method()

In other words: methods “belong to” an object.

# The `append` method, defined on lists
my_list = [1, 2, 3]
my_list.append(4)
print(my_list) 

The method append() is called directly on the list my_list

# append is a method for lists 
# this will error with a string
my_string = 'cogs18'
my_string.append('!')

# The `is_integer()` method, defined on floats
12.0
12.3.is_integer()

# The `is_integer()` method, attempted on an integer
# this code will produce an error
my_int = 12
my_int.is_integer()

String Methods#

There are a whole bunch of string methods, all described here. We’ll review a few of the most commonly used here.

# Make a string all lower case
'aBc'.lower()

# Make a string all upper case
'aBc'.upper()

# Capitalize a string
'python is great'.capitalize()

# Find the index of where a string starts 
'Hello, my name is'.find('name')

Class Question #2#

What will the following code snippet print out?

inputs = ['fIx', 'tYpiNg', 'lIkE', 'tHiS']
output = ''

for element in inputs:
    output = output + element.lower() + ' '

output.capitalize()

A) ‘fix typing like this ‘
B) [‘fix’, ‘typing’, ‘like’, ‘this’]
C) ‘Fix typing like this ‘
D) ‘Fix typing like this’
E) ‘Fixtypinglikethis’

List Methods#

There are also a bunch of list methods, all described here. You’ve seen some of these before, but we’ll review a few of the most commonly used here.

# sort sorts integers in numerical orders
ints = [16, 88, 33, 40, 40, 40, 3]
ints.sort()
ints

ints.sort(reverse=True)
ints

# append adds to the end of a list
ints.append(2) 
ints

# remove value from list
ints.remove(40)
ints

list.remove?

# reverse order of list
ints.reverse()
ints

Class Question #3#

What will the following code snippet print out?

list_string = ['a', 'c', 'd', 'b']
list_string.sort()
list_string.reverse()
list_string

A) [‘a’, ‘c’, ‘d’, ‘b’]
B) [‘a’, ‘b’, ‘c’, ‘d’]
C) [‘d’, ‘c’, ‘b’, ‘a’]
D) [‘d’, ‘b’, ‘a’, ‘c’]
E) [‘d’, ‘a’, ‘b’, ‘c’]

Dictionary Methods#

As with string and list methods, there are many described here that are helpful when working with dictionaries.

car = {
  "brand": "BMW",
  "model": "M5",
  "year": 2019
}

# keys() returns the keys of a dictionary
car.keys()
 
#for key in cars

# get returns the value of a specified key
mod = car.get('model')

# equivalent 
mod2 = car['model']

print(mod)
print(mod2)

# previously done this by indexing
print(car['model'])

# update adds a key-value pair
car.update({"color": "Black"})

print(car) 

Class Question #4#

Assuming dictionary is a dictionary that exists, what would the following accomplish:

dictionary.get('letter')

A) Return the key for the value ‘letter’ from dictionary
B) Add the key ‘letter’ to dictionary
C) Add the value ‘letter’ to dictionary
D) Return the value for the key ‘letter’ from dictionary

Class Question #5#

Which method would you use to add a new key-value pair to a dictionary?

A) .append()
B) .get()
C) .keys()
D) .update()

Methods: In Place vs Not In Place#

Some methods update the object directly (in place), whereas others return an updated version of the input.

List methods that are in place#

# Reverse a list
my_list = ['a', 'b', 'c']
my_list.reverse()

#print(my_list)

# Sort a list
my_numbers = [13, 3, -1]
my_numbers.sort()

print(my_numbers)

Dictionary methods that are not in place#

# let's remember what is stored in the car dictionary:
car

# Return the keys in the dictionary
out = car.keys() 
print(out)

# print keys
print(type(out))
print(out)

# car has not changed
print(type(car))
print(car)

# Return the values in the dicionary
car.values()

Finding Methods#

Typing the object/variable name you want to find methods for followed by a ‘.’ and then pressing tab will display all the methods available for that type of object.

# Define a test string
my_string = 'Python'

# See all the available methods on an object with tab complete
my_uppercase_string = my_string.
print(my_string)
print(my_uppercase_string)

Using the function dir() returns all methods available

# For our purposes now, you can ignore any leading underscores (these are special methods)
dir(my_string)

Correspondance Between Functions & Methods#

All methods are functions. Methods are special functions attached to a variable type. All functions are NOT methods.

Note that:

my_variable.function_call()

acts like:

function_call(my_variable)

A function that we can call directly on a variable (a method) acts like a shortcut for passing that variable into a function.

Functions: Reminders#

Additional notes included here to remind you of points we’ve already discussed.

def defines a function
function_name() - parentheses are required to execute a function
function_name(input1) - input parameters are specified within the function parentheses
function_name(input1, input2) - functions can take multiple parameters as inputs
- input1 and input2 can then be used within your function when it executes
To store the output from a function, you’ll need a return statement

For example….#

If you write a function called is_odd() which takes an input value,

def is_odd(value):
    if value % 2 != 0:
        answer = True
    else:
        answer = False
    
    return answer

to use that function, you would execute is_odd(value) ….something like is_odd(value = 6)

out = is_odd(6)
out

Later on, if you wanted to use that function within another function you still have to pass an input to the function.

def new_function(my_list):
    output = []
    for val in my_list:
        if is_odd(val):
            output.append('yay!')
    return output
            

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

More Functions & Methods#

`sort_array`#

A version of a selection sort:

loop through current list
find lowest item
put that at the front of your sorted list
remove lowest item from current list
wash, rinse, repeat

def sort_array(array_to_sort):
    """A function to sort an array."""

    is_sorted = False    # Keeps track of when we are done sorting
    sorted_array = []    # A new list that we will use to 
     
    while not is_sorted:

        lowest = None
        for item in array_to_sort:
            if lowest == None:         # If not defined (first element) set the current element as lowest
                lowest = item
            if item < lowest:
                lowest = item
        
        # these next two lines uses some new (to us) list methods
        # these methods save us from having to loop through the list again
        sorted_array.append(lowest)    # Add the lowest value to our sorted array output
        array_to_sort.remove(lowest)   # Drop the now sorted value from the original array

        if len(array_to_sort) == 0:    # When `array_to_sort` is empty, we are done sorting
            is_sorted = True 
    
    return sorted_array

Using `sort_array`#

# Sort an array of integers
unsorted_array = [12, 7, 19, 12, 25]
sorted_array = sort_array(unsorted_array)
print(sorted_array)

# Sort an array of floats
unsorted_array = [21.3, 56.7, 2.3, 2.9, 99.9]
sorted_array = sort_array(unsorted_array)
print(sorted_array)

SideNote: `sorted`#

# Sort a list, with `sorted`
data = [7.8, 4.2, 6.0]
sorted(data)

# what about a list of strings?
sorted(['asdf','abcd'])

# Sort different data types
print(sorted(['a', 'c', 'b']))
print(sorted([True, False, True]))
print(sorted([[1, 4], [1, 2]]))

Methods

Contents

Methods#

Class Question #1#

Methods#

Method Examples#

String Methods#

Class Question #2#

List Methods#

Class Question #3#

Dictionary Methods#

Class Question #4#

Class Question #5#

Methods: In Place vs Not In Place#

List methods that are in place#

Dictionary methods that are not in place#

Finding Methods#

Correspondance Between Functions & Methods#

Functions: Reminders#

For example….#

More Functions & Methods#

sort_array#

Using sort_array#

SideNote: sorted#

`sort_array`#

Using `sort_array`#

SideNote: `sorted`#