len is a built-in function: https://docs.python.org/3/library/functions.html#len

When you do len("something") you are passing the string something to 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"})
3

If you were to try to do "something".len() it would try to call the function "len" that exists on str. 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.

This is a great question. len(string) is the only one that works in Python but the choice is fairly arbitrary. Many other languages - probably most actually use the second form.

There are also languages that support "uniform function call syntax" where both work and are equivalent (e.g. D).

IMO the biggest advantage of the string.len() form is that it works with autocomplete. That may also explain why Python doesn't use it, because it didn't have static typing (and therefore proper auto-complete) until very recently. Many people still don't use it.

Note that even in Python the free function form len(string) is kind of a lie - it gets translated to string.__len__() so really they've just done both but made one of them really ugly.

Why? God knows. Python is full of insane design decisions. This is one of the milder ones.

What is the difference between these types of statements?

Technically, len() is a Python built-in function, while "some string".len() would be an instance method of a string object, if such an instance method existed.

How do I think about this to know which one I should expect to work?

As a very general rule of thumb, I would recommend to keep the list of built-in functions close and memorize the "popular ones" over time. These are special. Anything else you encounter usually is an instance or class method, or a plain function without any object-oriented shenanigans, depending on how structured the code is you are looking at.

Learn to navigate the Python reference docs. Use the search function liberally if you don't come from a search engine result, anyway, or jump directly to the docs for the module you're interested in to see what functionality it offers, and how to use it.

Python is annoyingly flexible, and does not strictly enforce a single way to do anything, so learning what to expect always ends up as building an intuition, and actively looking up documentation for the modules, classes, or functions you intend to use until you have encountered enough (good) Python code to have reasonable expectations in the first place.

TL;DR: grep the relevant module's docs, or Python built-ins. It's typically one or the other where you find detailed help.

len(string) passes the object 'string' to a function 'len' that returns its length.

sting.len() (edit: hypothetically if it existed) calls the method 'len' which is an inherent part of any object of string type.

In practice the result is the same, but the latter is the more object-oriented approach.

You can't use string.len() on List Datatype iirc