Just started as in, I’m about an hour into a 4 hour intro video. Seeing two basic ways of manipulating things and don’t understand the difference.
If I want to know the length of a string and I just guess at how to do it I would try one of these two things,
- Len(string)
- string.len()
What is the difference between these types of statements? How do I think about this to know which one I should expect to work?
lenis a built-in function: https://docs.python.org/3/library/functions.html#lenWhen you do
len("something")you are passing the stringsomethingto it, and it returns how long it is. You can pass it other things like lists or sets, and it will tell you how many things are in them, too.>>> len <built-in function len> >>> len("something") 9 >>> len([1,2,3,4]) 4 >>> len({"a", "b", "c"}) 3If you were to try to do
"something".len()it would try to call the function “len” that exists onstr. There isn’t one.>>> "something".len() Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'str' object has no attribute 'len'https://docs.python.org/3/library/stdtypes.html#textseq
Scroll down a little to “String Methods” and you can see what methods are available on strings.
This is kind of language specific. Now you know that when you want to know how long something is in Python, you generally use the built-in
len. If you’re dealing with some other type of object, you’d check what methods it provides and what it inherits from. There’s a lot of documentation reading in software development. A good IDE also helps.At the end of the day,
len(ob)just defers toob.__len__()so both are correct, just one’s more functional and one’s more object oriented.Things prefixed with two underscores are considered private, and typically should not be accessed directly.
https://docs.python.org/3/tutorial/classes.html#private-variables
Keyword “typically”. If I’m overriding dunder methods, then I’ll typically need to call the super method as well. It’s not like it’s forbidden.
Consider the following:
class MyStr(str): def len(self): return len(self) # OR return self.__len__()Both of the above return values are perfectly valid Python.