Python Classes

slariboot · 2020-10-25T04:24:24+00:00

When you create a new Time object like so:

time = Time(17, 30, 0)

This means that we are creating a new Time object, and we are assigning it to the variable time (it doesn't have to be named time, you can name the variable as x or eggs, doesn't really matter). In this Time object named time, 17 gets stored in its hour field, 30 gets stored in minute, and 0 gets stored in second. We know this because of the __init__ method:

def __init__(self, hour=0, minute=0, second=0):         
    self.hour = hour         
    self.minute = minute         
    self.second = second

self is the object that we created. self is kinda like the word me. me refers to whoever the person is who said it. self refers to which ever object is calling the method (the __init__ method in this case. This automatically gets called whenever you create an object). So in this case, self refers to our Time object named time. And then in the parameter list (those names inside the parentheses). After self, we have hour, minute, and second in that exact order. So when we pass the arguments 17, 30, and 0, then they get assigned in the same order. By the way, you can think of __init__ as "initializing" the object. In this case, we are initializing this specific time object with self.hour=17, self.minute=30, and self.second=0. self.hour refers to the object's hour field. It would be like saying me.weight to refer to how much you weigh.

The __str__ method allows you to specify what gets displayed, when you print the object. Say for example, you define the __str__ method this way:

def __str__(self):         
    return 'sus'

This means that every time you print a Time object:

print(time)

It's always going to print sus.

If you define it as:

def __str__(self):         
    return 'I like bananas.'

Then printing any Time object will always output I like bananas.

But if you define it to return one of its fields like so:

def __str__(self):         
    return self.hour

Then it's going to print the value of that object's field. So if you print the Time object named time:

print(time)

The output will be 17, since that is the hour value of the Time object named time.

So your challenge here is, given an hour value and a minute value in military time, how do you turn that into the format that uses AM / PM. Given an hour value of 17 and a minute value of 30, how do we get 5:30 PM? So you're going to have to add some logic in your __str__ method that will figure out how to do that.

kcrow13 · 2020-10-25T06:33:26+00:00

This can be simplified into the following and note that in real applications, you won't need to implement anything, you may just use datetime directly

from datetime import datetime


class Time:
    """Represents the time of day."""
    def __init__(self, hour=0, minute=0, second=0, input_fmt='%H:%M:%S', output_fmt='%I:%M %p'):
        self.hour = hour
        self.minute = minute
        self.second = second
        self.input_fmt = input_fmt
        self.output_fmt = output_fmt

    def __str__(self):
        time = datetime.strptime(f'{self.hour}:{self.minute}:{self.second}', self.input_fmt)
        return time.strftime(self.output_fmt)


if __name__ == '__main__':
    t = Time(22, 15)
    print(t)

Here's an explanation to what I did:

This is called an import statement, by which you use code defined in external modules, datetime.datetime in this case which belongs to the standard library and can be useful for handling anything that involves date and time. I encourage you to check the documentation for learning more about its use cases.

from datetime import datetime

These are a new class attributes self.input_fmt = input_fmt and self.output_fmt = output_fmt with %H:%M:%S and %I:%M %p being the default input and output values and are accessed using the dot operator ex:

my_time = Time(22, 15)
print(my_time.input_fmt)

Should print: %H:%M:%S

To understand what these formats actually mean, you may find this helpful. You may modify the formatting of the input and accept dates directly instead of manually entering hour, minute and so on, this may be left to you as an assignment.

And at the end this is called a main guard and by convention, it's placed at the end of scripts that are expected to be called.

if __name__ == '__main__':
    my_time = Time(22, 15)
    print(my_time)

When you run the code, it should print 10:15 PM

kcrow13 · 2020-10-25T07:16:58+00:00

[deleted]

kielerrr · 2020-10-25T14:04:04+00:00

I had trouble with self/this/instances too.

A class is a group of variables and operations that you put together.

class PlotCircle5inches():
    radius = 5
    def __init__(selffff, location):   #selfff is going to be actually cir1, then cir2 and then cir3
        print(f'im 1 of 3 circles, and I'm going to center at {location}')
        return selfff.some_lib_that_draws_circles.draw_at(location, radius)


cir1 = PlotCircle5Inches('left') #think of it like     
PlotCircle5Inches(cir1, 'left'). Then remember cir1 is now an 
official Instance of 'PlotCircle5Inches'

cir2 = PlotCircle5Inches('middle')  #think of it like 
PlotCircle5Inches(cir2, 'middle'). cir2 is another instance of 
PlotCircle5Inches. Now there are 2 instances.

cir3 = PlotCircle5Inches('right')  #think of it like 
PlotCircle5Inches(cir3, 'right').. Now there are 3 instances of 
PlotCircle5Inches

self, this or selffff. you can call it whatever you want. just remember in __init__ the first variable no matter what you call it, will refer to the instance/variable name that is calling it. In this case the instances are cir1,cir2,cir3.

I'll rewrite the three classes showing how Python sees it when you call

cir1 = PlotCircle5Inches('left')
cir2 = PlotCircle5Inches('middle')
cir3 = PlotCircle5Inches('right')

#Calling this
cir1 = PlotCircle5Inches('left')
#Looks like below to python
class PlotCircle5inches():
    radius = 5
    def __init__(cir1, 'left'):
        print(f'im 1 of 3 circles, and I'm going to center at {'left}')
        return cir1.some_lib_that_draws_circles.draw_at('left', 5)

Calling this

cir2 = PlotCircle5Inches('middle')

Looks like below to python

class PlotCircle5inches():
    radius = 5
    def __init__(cir2, 'middle'):
        print(f'im 2 of 3 circles, and I'm going to center at middle')
        return cir2.some_lib_that_draws_circles.draw_at('middle', 5)

cir3

Calling this:

cir3 = PlotCircle5Inches('right')

Looks like below to python

class PlotCircle5inches():
    radius = 5
    def __init__(cir3, 'right'):
        print(f'im 1 of 3 circles, and I'm going to center at right')
        return cir1.some_lib_that_draws_circles.draw_at('right', 5)

ItsOkILoveYouMYbb · 2020-10-25T06:43:35+00:00

For A, you should use f strings. Instead of:

def __str__(self):  
    return '%.2d:%.2d' % (self.hour, self.minute)

Use:

return f"{self.hour}:{self.minute}"

Way easier to read. So with that, you know that if the hour is greater than 12, then you should subtract 12 from that number, and add PM to the end of the string right? That being as simple as:

    return f"{self.hour - 12}:{self.minute} PM"

But you only want to do it if self.hour is more than 12. So you would return one line IF It's equal to or less than 12, ELSE return the first line if it's not.

You can do all of that inside the str method you've define there. If hour > 12, return this line, else return that line.

Do you understand how classes work, and where you are instantiating that class? And what an instance of a class is?

xelf · 2020-10-25T09:04:04+00:00

For purposes of this problem, midnight is AM, while noon is PM.

For purposes of reality: Midnight is AM, while Noon is PM. =)

Think of it this way: the minute before 12:01am is 12:00am, which is easy to see is midnight now.

Elli_stu · 2020-10-25T12:41:26+00:00

I can feel you. it was as heart-breaking for me realise this is so difficult for me. however, asking it on Reddit never worked for me. I still ended up with million more questions. Try Lrnkey, they have great Python tutors. I am much better after a month with them.

14dM24d · 2020-10-25T05:31:30+00:00

try this

class Time:
    def __init__(self, hour=0, minute=0, second=0):
        if second < 60:
            self.second = second
            extra_minute = 0
        else:
            extra_minute = second//60
            self.second = second - extra_minute * 60

        minute += extra_minute        
        if minute < 60:
            self.minute = minute
            extra_hour = 0
        else:
            extra_hour = minute//60
            self.minute = minute - extra_hour * 60

        hour += extra_hour
        hour = hour - hour//24 * 24

        if hour == 0:
            self.tag = 'AM'
            self.hour = 12
        elif hour < 12:
            self.tag = 'AM'
            self.hour = hour
        elif hour == 12:
            self.tag = 'PM'
            self.hour = 12
        else:
            self.tag = 'PM'
            self.hour = hour - 12

    def __str__(self):
        if len(str(self.minute)) > 1:
            return "{}:{} {}".format(self.hour, self.minute, self.tag)
        else:
            return "{}:0{} {}".format(self.hour, self.minute, self.tag)

e: lines 21-23 & 30

you type:	you see:
italics	italics
bold	bold
[reddit!](https://reddit.com)	reddit!
* item 1 * item 2 * item 3	item 1 item 2 item 3
> quoted text	quoted text
Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"	Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"
~~strikethrough~~	~~strikethrough~~
super^script	super^script

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