Methods

Methods#

Q&A

Q: Does range just make a list of numbers?
A: For simplest answer, yes. For a more nuanced answer, it’s not technically a list but a range object. This is NOT a nuance I will test you on. You can think of it as a list of numbers.

Q: When is a good time to use loops and do you add break just to stop the loop?
A: Any time you want code to run multiple times, a loop is the perfect solution! And, yup, break stops the loop. It can be helpful when debugging or when you want to stop a loop after you’ve found what you’re looking for and don’t need to look any longer.

Course Announcements

Due this week:

VQ10 due Wed
CL5 due Fri
Mid-course survey “due” (for extra credit) Friday - link also on Canvas assignment

Notes:

Oral Exam weeks 7-10 slots released on Thursday; will send Canvas announcement
A4 now available - we’re just starting this material; due next Sun

Exam Summary

Well done, overall!
Median: 86%
Perfect scores: 17 students (3%)!
You will be able to view your exams in PL (end-of-week)…however, due to the wifi issues, students are still taking the exam
- if you want to see them sooner/discuss struggles, ask a staff member in office hours :)

Methods#

First: review recent uncertainties & debugging

string, list, & dictionary
in place vs not in place
relationship to functions
objects
- attributes
- methods

…and try/except

Oral Exam: Collections#

Given the code below, take a minute to determine how the code is working and then describe what the function accomplishes overall and the logic within the code…You can assume encoder is a function that has been previously defined that takes in a single character and returns a different character

def custom_encoder(char, key=200):

    custom_encodings = {
        'a' : 'r', 
        'e' : 'p', 
        'i' : 'm',
        'o' : 'n',
        'u' : 's'
    }
    
    if char in custom_encodings:
        output_char = custom_encodings[char]
    else:
        output_char = encoder(char=char, key=key)
        
    return output_char

If you struggled on the back-up oral exam question last week, a very similar question was discussed in Q1 of E1-Review (Thurs of week 4; 4/24)

Debugging#

Let’s say you were working on this lab question last week and you were confused…how do you go about doing this and debugging?

First: the goal#

First, you need to understand what the goal of the function is…

Take a string as input
Convert all the characters in the string to some other character using ord() and this key
Return that new string from the function

Second: expectations#

Second, I would want to figure out some possible inputs and expected outputs…given my understanding.

Let’s consider the string ‘hi’, with the default key of 200:

ord('h') + 200

chr(304)

'İ'

ord('i') + 200

chr(305)

'ı'

so…with an input and the default key, ‘hi’ I’d expect the output ‘İı’

…but what about a nondefault key of say 300…

chr(ord('h') + 300)

'Ɣ'

chr(ord('i') + 300)

'ƕ'

so with an input of ‘hi’ and a key of 300, I’d expect an output ‘Ɣƕ’

Third: write some code#

Two approaches:

parts -> whole
all at once -> debug

Option A: Get parts of it working at a time…using print() statements to check understanding

# loop through a string
def encoder(input_string):
    for ltr in input_string:
        print(ltr)

encoder('hi')

h
i

# for each letter, get the "new" character
def encoder(input_string):
    for ltr in input_string:
        print(chr(ord(ltr) + 200))

encoder('hi')

İ
ı

# add each new letter to a growing string - return that string
def encoder(input_string, key=200):
    output_string = ''
    for ltr in input_string:
        output_string = output_string + chr(ord(ltr) + key)
    return output_string

encoder('hi')

'İı'

# work for other keys?
encoder('hi', 300)

'Ɣƕ'

# using this to test
encoder('hi')

Option B: Try to write it all at once…and then debug

# THIS HAS ERRORS WE'RE GOING TO DEBUG
def encoder(input_string, key=200):
    
    for char in input_string:
        output_string = chr(ord(char) + 200)
    
    return output_string

encoder('hi')

'ı'

# we'll debug here so you can see the "original"
def encoder(input_string, key=200):

    output_string = ''
    
    for char in input_string:
        output_string = output_string + chr(ord(char) + key)
    
    return output_string

# using this to test
encoder('hi', 300)

'Ɣƕ'

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 method append() is called directly on the list my_list

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

[1, 2, 3, 4]

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

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[45], line 4
      1 # append is a method for lists
      2 # this will error with a string
      3 my_string = 'cogs18'
----> 4 my_string.append('!')

AttributeError: 'str' object has no attribute 'append'

Available methods:#

string methods
- .lower(), .upper(), .capitalize(), etc.
list methods
- .append(), sort(), reverse(), etc.
dictionary methods
- .keys(), .values(), .items(), etc.

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)

['c', 'b', 'a']

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

print(my_numbers)

[-1, 3, 13]

Dictionary methods that are not in place#

car = {'make': 'Hyundai',
       'model': 'Santa Fe',
       'year': 2009}

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

dict_keys(['make', 'model', 'year'])

# car has not changed
car

{'make': 'Hyundai', 'model': 'Santa Fe', 'year': 2009}

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

dict_values(['Hyundai', 'Santa Fe', 2009])

# string methods operate NOT in place
my_string = 'hello'
new_string = my_string.upper()

new_string

'HELLO'

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.

Activity: Methods#

Please complete the two questions here: https://forms.gle/oeH6B6fSpvj6jNHk7. For these, best to read the code and think before trying the code out and getting the answer.

You are encouraged to:

talk to one another!
ask each other questions
ask us questions

`.items()`: Using dictionary methods…to loop#

car

{'make': 'Hyundai', 'model': 'Santa Fe', 'year': 2009}

for key, val in car.items():
    print('Loop Iteration:')
    print(key)
    print(val)

Loop Iteration:
make
Hyundai
Loop Iteration:
model
Santa Fe
Loop Iteration:
year
2009

Activity: `.items()`#

Include your code for this question in this form: https://forms.gle/EmZwrUBVD6KkVHQd9

Can you re-write this function (accomplishing the same task!) using .items():

def passing_students(scores_dict, passing_score):
    passed_students = []
    
    for student in scores_dict:
        if scores_dict[student] >= passing_score:
            passed_students.append(student)

    return passed_students

Notes:

We initially drafted some of this code in the Loops notes.
Including students below so you don’t have to type out a dictionary with students’ names and scores

students = {
    'Alondra': 85,
    'Holly': 58,
    'Brooke': 92,
    'Pauline': 47,
    'Minqi': 76
}

# CODE
def passing_students(scores_dict, passing_score):
    passed_students = []
    
    for key, val in scores_dict.items():
        if val >= passing_score:
            passed_students.append(key)

    return passed_students

# TRY IT OUT
passing_students(students, 70)

['Alondra', 'Brooke', 'Minqi']

`try`/`except`#

What if you want Python to try to run some code…but do something else if it encounters an exception?

Note: input() allows you to get input from a user.

def get_an_input_integer():
    in_string = input('Enter your favorite whole number:\n')

    try:
        in_integer = int(in_string)
    except:
        in_integer = None
        
    return in_integer

get_an_input_integer()

What would this look like without a try/except?

def get_an_input_integer_wo():
    in_string = input('Enter your favorite whole number:\n')
    in_integer = int(in_string)
    return in_integer

get_an_input_integer_wo()

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[72], line 1
----> 1 get_an_input_integer_wo()

Cell In[70], line 3, in get_an_input_integer_wo()
      1 def get_an_input_integer_wo():
      2     in_string = input('Enter your favorite whole number:\n')
----> 3     in_integer = int(in_string)
      4     return in_integer

ValueError: invalid literal for int() with base 10: 'Shannon'

Where are we going?#

class - creating our own object types!

attributes
methods <- we know what these are now!

So let’s talk about attributes…

Objects#

Objects are an organization of data (called attributes), with associated code to operate on that data (functions defined on the objects, called methods).

from datetime import date

my_date = date(year=1988, month=9, day=29)
print(my_date)

1988-09-29

type(my_date)

datetime.date

Attributes#

Attributes look up & return information about the object.

attributes maintain the object’s state, simply returning information about the object to you

my_date.day

Methods#

These are functions that belong to and operate on the object directly.

methods modify the object’s state

# Method to return what day of the week the date is
my_date.weekday()

my_date.weekday?

Docstring:
Return the day of the week represented by the date.
Monday == 0 ... Sunday == 6
Type:      builtin_function_or_method

Activity: Methods#

Complete the Google Form questions here: https://forms.gle/JLzC4cgFmhs2Z8h27

Feel free to chat with and ask questions of your neighbors!

Objects Summary#

Objects allow for data (attributes) and functions (methods) to be organized together
- methods operate on the object type (modify state)
- attributes store and return information (data) about the object (maintain state)
dir() returns methods & attributes for an object
Syntax:
- obj.method()
- obj.attribute
date and datetime are two types of objects in Python

Methods

Contents

Methods#

Methods#

Oral Exam: Collections#

Debugging#

First: the goal#

Second: expectations#

Third: write some code#

Methods#

Method Examples#

Available methods:#

Methods: In Place vs Not In Place#

List methods that are in place#

Dictionary methods that are not in place#

Correspondance Between Functions & Methods#

Activity: Methods#

.items(): Using dictionary methods…to loop#

Activity: .items()#

try/except#

Think-Pair-Share#

Where are we going?#

Objects#

Attributes#

Methods#

Activity: Methods#

Objects Summary#

`.items()`: Using dictionary methods…to loop#

Activity: `.items()`#

`try`/`except`#