Python '!=' Is Not 'is not': Comparing Objects in Python (Summary)

I learned about interning! I had encountered this concept before but wasn’t aware of the lower and upper bounds (-5 to 256), nor was I aware of using the intern() method itself. Pretty cool!

Great to hear, Kevin! Glad you got a chance to learn something new :)

I was following along in pycharm and the variables, “a” and “b” when set to, “This is a string”, are equal when using the is comparison operator. I am guessing that pycharm intern’s string automagically for us or something.

I thought these lessons were very helpful, thank you.

Interesting @bday29. I suppose that could be possible – pycharm does a lot of cool optimizations under the hood. Glad you found the lessons helpful otherwise!

Thanks for the course, Liam! Very helpful in explaining the differences. Had never heard of “intern” before so I did a little research. Based on this excellent Medium article, string interning follows a few simple rules:

1. The string must be a compile-time constant
2. The string must be not be subject to constant folding or no longer than 20 characters
3. The string consists exclusively of ASCII letters, digits, or underscores

When I was following along with experimenting with the interning example #3 is the one that tricked me. I was using short strings with no spaces and it seemed like they were getting automatically interned. This was true! Spaces in the string disqualify it from being automatically interned. is statements worked like a charm on the short, space-less strings.

Hope this helps someone else who’s learning and is too lazy to type longer strings. Thanks again.

Thanks for that super informative comment @d0ctajones! I wasn’t even aware of all of the specifics of the third rule, so you’ve taught me something today :)

Really liked the way you explained this stuff, i liked also the approach of going over a few times some concepts and at good pace.....i like the colour of the screen where the commands popped out nicely....good work....i am pretty new to python and i got to say that i really like your way to cover and teach stuff…thanks!

Really glad I could be of help @paolotagliente! Stay tuned for some new tutorials I’ll have out in the coming weeks :)

Good course. Looking forward to new tutorials from you!

Ditto the comments above, Liam. As you’ve implied, and just to mention to other newbies like me: “it’s complicated”. E.g. the overwrite of __eq__ has implications on the hash-a-bility of an object such as the example user-defined SillyString. Other RP tutorials point to stackoverflow.com/questions/14535730/what-does-hashable-mean-in-python and eng.lyft.com/hashing-and-equality-in-python-2ea8c738fb9d. The first states “Objects which are instances of user-defined classes are hashable by default; they all compare unequal, and their hash value is their id().” Actually, it’s id()>>4. However, hash(SillyString('a')) returns for me TypeError: unhashable type: 'SillyString'. The simple overwrite of __eq__ in SillyString breaks an __eq__, __hash__ contract honored in the default. There be dragons here!

Very good course; thanks!

I have created a subclass from str class as below:

class SillyString(str):    # SillyString subclass of str class
    def __eq__(self, other):
        print(f"Comparing {self} to {other}")
        return len(self) == len(other)  #silly comparison

Now, I am creating the objects and printing output as below:

s1 = "hello"
s3 = SillyString("world")
print('{0} == [SillyString] {1}'.format(s1, s3), s1 == s3)

My output looks as below:

Comparing world to hello
hello == [SillyString] world True

Please help me understand why Comparing world to hello is printing before the hello == [SillyString] world.

Also why hello == [SillyString] world is printed out before True?

@hemant-technerd The comparison operator in s1 == s3 makes your SillyString.__eq__() method to run first. Since this method contains a call to print() in its body, it prints out a message “Comparing (…)” prior to returning a Boolean result. That’s why you’re seeing it before your other print statement.

On the other hand, the order of values printed out later is determined by order of arguments to print().