NameError: name 'start_time_of_the_script' is not defined

Question

import time
import datetime

class TimeCounter():

    def startTime():

        start_time_of_the_script = time.time()

    def endTime():

        end_time_of_the_script = time.time()
        process_time_in_seconds = end_time_of_the_script - 
start_time_of_the_script
        print(str(datetime.timedelta(seconds=process_time_in_seconds)))

def main():

    TimeCounter.startTime()
    TimeCounter.endTime()

main()

I know that I am doing some fundamental mistake using python functions. Would you mind sharing a solution. Thanks.

Dodge · Answer 1 · 2018-09-05T18:49:14.920

1

You can assign values you want to retrieve later to self. like the following:

import time
import datetime

class TimeCounter:

   def startTime(self):

        self.start_time_of_the_script = time.time()

    def endTime(self):

        self.end_time_of_the_script = time.time()
        self.process_time_in_seconds = self.end_time_of_the_script - self.start_time_of_the_script
        print(str(datetime.timedelta(seconds=self.process_time_in_seconds)))

The self variable explained

edited Sep 05 '18 at 18:49

answered Sep 05 '18 at 00:58

Dodge

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In the above example we have not written object inside of **class TimeCounter():** while creating the TimeCounter class and the code did work. I wonder in which situations do we have to write **object** inside of class paranthesis? – questioneer1 Sep 05 '18 at 18:37
@questioneer1 Yes parentheses are used when a class is subclassing another class. It is not necessary here it seems. I referenced this post: https://stackoverflow.com/questions/4109552/python-class-definition-syntax. It looks like we have both learned something here. I will remove that from my answer. – Dodge Sep 05 '18 at 18:48
And here is one more link that explains that further: https://stackoverflow.com/questions/4015417/python-class-inherits-object – Dodge Sep 05 '18 at 18:51
Yes very important thing I have learned too. What if I wanted to call say for example the : **process_time_in_seconds** from python shell or from another function later on, what should I do? because it does not simply work like writing **process_time_in_seconds** in the shell as I am using functions in the classes.. – questioneer1 Sep 05 '18 at 19:17
You can retrieve class attributes from instances of the class after instantiation. So if your class instance is `tc` you can retrieve said value with `tc.process_time_in_seconds` . – Dodge Sep 05 '18 at 19:23
Hmm. for the code most above it says : **NameError: name 'tc' is not defined** when I give **tc.process_time_in_seconds** to the shell – questioneer1 Sep 05 '18 at 19:30
yes you would need to instantiate the class with `tc = TimeCounter()` – Dodge Sep 05 '18 at 19:45
yes i do it under the **def main():** function already. like @vthorsteinsson given example .. should i do something else? – questioneer1 Sep 05 '18 at 19:57

vthorsteinsson · Accepted Answer · 2018-09-05T12:35:00.233

You need to distinguish between classes and instances. Once you have defined TimeCounter as a class, you can create one or many instances of it. This is done below in the assignment tc = TimeCounter(), which creates a fresh instance of TimeCounter and assigns it to the variable tc.

When methods (functions) are called on instances of a class, the instance is passed to the method as a parameter, traditionally called self. So when the code below calls tc.startTime(), the self parameter in startTime will refer to the tc instance. Also, when startTime sets self.start_time_of_the_script, it is creating a new property of the tc instance - which is then read again in endTime.

import time
import datetime

class TimeCounter:  # Parentheses not necessary

    def startTime(self):  # Note self parameter

        self.start_time_of_the_script = time.time()  # Set a property of the instance

    def endTime(self):  # Note self parameter

        end_time_of_the_script = time.time()  # This is a local variable
        process_time_in_seconds = end_time_of_the_script - self.start_time_of_the_script
        print(datetime.timedelta(seconds=process_time_in_seconds))

def main():

    tc = TimeCounter()  # Create a fresh instance of TimeCounter and assign to tc
    tc.startTime()  # Call method startTime on the tc instance
    tc.endTime()

main()

Thanks. It did work! However, another question came up as; why we also don't write self.end_time_of_the_script for: end_time_of_the_script and self.process_time_in_seconds for : process_time_in_seconds. why it does't give any error when we don't.? – — questioneer1, Sep 05 '18 at 18:29

NameError: name 'start_time_of_the_script' is not defined

2 Answers2