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()
 

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

my_dictionary.get('letter')

A) Return the key for the value ‘letter’ from my_dictionary
B) Add the key ‘letter’ to my_dictionary
C) Add the value ‘letter’ to my_dictionary
D) Return the value for the key ‘letter’ from my_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, because .keys() is not an in-place method
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 that uses methods:

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`#