Correct usage of self in python instance

Question

I have a class that I defined in the following way:

class Time:
    def print_time(self, am_pm):
        if am_pm == 'AM':
            tot_seconds = self.hour * 3600 + self.minute * 60 + self.second
        else: 
...         tot_seconds = self.hour * 3600 + self.minute * 60 + self.second + 12*3600.0
        print('%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second))
        print('Seconds passed since midnight: ', tot_seconds)

I create an instance of Time with start = Time and then I specify the instance attributes as start.hour = 10, start.minute = 5, start.second = 2.

When I call the method print_time, if I do Time.print_time(start, 'AM') then I get the correct output. But if I try to pass start as self it does not work:

start.print_time('AM')
Traceback (most recent call last):

  File "<ipython-input-68-082a987f3007>", line 1, in <module>
    start.print_time('PM')

TypeError: print_time() missing 1 required positional argument: 'am_pm'

But why it is so? I thought that start would be the subject of the method invocation and so it would count as the first parameter and so I would need to specify only the am_pm parameter in print_time(). Why is this wrong?

You have to change start = Time to start = Time(). Time.print_time(start, 'AM') works coincidentally, start is not actually an instance of Time here. — Mario Ishac
– Mario Ishac, Commented May 11, 2020 at 21:58

Nate Norris · Accepted Answer · 2020-05-11 22:03:07Z

3

start = Time does not actually construct an instance of your class. Rather, you assign the class definition Time to the variable start. You need open/close parentheses after a class name in order to construct an instance.

In other words, start.print_time() in your case is attempting to treat print_time like a static function -- as if you were simply calling Time.print_time() without an instance.

Hope that helps!

answered May 11, 2020 at 22:03

Nate Norris

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2 Comments

user12394113 Over a year ago

Oh gosh what a silly mistake. I was banging my head for nothing, thanks a lot for your quick reply! :)

Nate Norris Over a year ago

Haha! No prob, I've done it myself more times than I care to admit.

3 revs · Accepted Answer · 2020-05-11 23:21:45Z

0

class Time:

def __init__(self, am_pm, ):

    self.am_pm=am_pm

    self.hour=10

    self.minute=5

    self.second=2

def print_time(self):

    if self.am_pm=="AM":

        tot_seconds=self.hour * 3600 + self.minute * 60 + self.second

        print(tot_seconds)

    else:

        tot_seconds = self.hour * 3600 + self.minute * 60 + self.second + 12*3600.0

        print('%.2d:%.2d:%.2d' %(self.hour, self.minute, self.second))

        print( 'Seconds passed since midnight: ',tot_seconds)

start=Time("AM")

start.print_time()

except you change the object of the class ( start ) as self. ie

      self=Time("AM")
      self.print_time()

You can specify any object but not keywords

edited May 11, 2020 at 23:21

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