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Python Basic Object-Oriented Python Welcome to OOP Methods

Posted by Kasey Knudsen
Kasey Knudsen
5,888 Points

self.is_moving = False

I'm not quite clear on why we are setting self.is_moving to False in the stop method if it's already set to False in the def init method?

3 Answers

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Good question. While the __init__ method establishes the initial value for the is_moving attribute, at some point the value may be set to True by another means. The stop() method could check to see if the is_moving attribute needs to be reset before reassigning. Sometimes simply resetting the value regardless of the current value works as well and makes the code intent obvious. Instead of saying "reset if set" it says "reset regardless".

Post back if you want more help. Good luck!!

Laura Mora Freeman
Laura Mora Freeman
8,192 Points
Laura Mora Freeman
Laura Mora Freeman
8,192 Points

Can you comment a little bit more about it, i had the same question, also wasn't it easy to set it to true at the begging?. And why the first time when the stop function is called, it prints the else statement instead of the if? The if is true and the else is false?, so when we reasure self.is_moving = False it goes to the else statement? An also in the go statement i didn't understand the if not self.is_moving and then set to true again. i don't know if i made myself clear. Thanx in advance.

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Sure Laura Mora Freeman,

wasn't it easy to set [self.is_moving] to true at the beginning?

Yes, in the __init__ method, self.is_moving is set to False when an instance is created.

why the first time when the stop function is called, it prints the else statement instead of the if?

Initially, the car not moving (self.is_moving is False). So, the statement is if False… this triggers the else block.

The if is true and the else is false?, so when we reasure self.is_moving = False it goes to the else statement?

Not exactly. The self.is_moving is only set to True if the go method is called.

An also in the go statement i didn't understand the if not self.is_moving and then set to true again.

If the car is not moving (is_moving is False), then if not False… will execute. Expanding my answer above, if the only action after checking if something is False is to set it to True, then the if isn’t strictly necessary since regardless of its current value, self.is_moving becomes True. However, since there is a print statement that should only execute on the first call to go the if is needed to block the print on subsequent calls to go if the car is already moving.

The methods run are:

  • __init__ – starts False
  • stop – still False
  • go – becomes True
  • go – still True
  • stop – becomes False
  • stop – still False

In summary,

  • __init__ initializes self.is_moving to False,
  • only the go method sets self.is_moving to True, and
  • only the stop method sets it to False.

Hope I’ve covered your question. Post back if you need more help. Good luck!!!

Laura Mora Freeman
Laura Mora Freeman
8,192 Points
Laura Mora Freeman
Laura Mora Freeman
8,192 Points

Thank you so much Chris for taking the time, it really clarifies it.

Ali Colak
Ali Colak
2,192 Points
Ali Colak
Ali Colak
2,192 Points

Hey Chris, Im a little confused on this bit.

"why the first time when the stop function is called, it prints the else statement instead of the if?

Initially, the car not moving (self.is_moving is False). So, the statement is if False… this triggers the else block."

  def stop(self):
        if self.isMoving:
            print("The car is stopped!")
            self.isMoving = False
        else:
            print("The car been stopped")

So if self.isMoving -> if False would be the interpretation I'm assuming because it was set to false in:

    def __init__(self, model, year, make = "Ford!"):
        self.make = make
        self.model = model
        self.year = year
        self.isMoving = False

Wouldn't if False = True because the self.isMoving is False, prompting the first "The Car is Stopped"? Im a little lost as to why the if statement exists if it'll never run

[MOD: added ```python formatting -cf]

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Hey Ali Colak, be careful when reading truthy statements:

# this relies on the actual value of the attribute
# as the entire conditional statement
If could_be_false:
    print(“could_be_false was True”
else:
    print(“could_be_false was False”

# the above behaves exactly the same as
if could_be_false == True:

So, self.isMoving being initiated to False means the first block of code will not be executed. 

# initially 
if self.isMoving: # evaluate as False -> execute else block
# same as
if self.isMoving == True: # evaluates as False -> execute else block

And, as you have stated, the first block of code may never be executed if another method doesn’t set self.isMoving = True.

The go method will set this attribute to True this will enable the first block to execute.

Post back if you need more help. Good luck!!!