int('123') in python - is it a function call or constructor call of 'int' class?

Question

I am learning Python, and I am a little confused about the data types of Python. I am reading this line again and again.:

'Everything is an object in Python'

This includes integer, floats, string, sets, lists, etc. and when we write like this: [1, 2, 3], so we are actually creating an object of list class (or not)? So, when we write an integer like 123, does it mean it's an object of int class? But when we read in books it says that to convert a string into integer we need to use the int method?

But what I am thinking is that int is a class that also accepts the string object in constructor and then we get integer object, right?

So, is int a method or class in Python?

Answer 1

Yes int is a class (and it's also called a type; see Python : terminology 'class' VS 'type'), and doing int('123') returns an instance of an int object.

However, (in standard Python aka CPython) for small integers (in the range -5 to 256, inclusive) the int constructor doesn't actually build a new integer object. For efficiency reasons the interpreter has a cache of small integers and the constructor simply returns a reference to the existing int object. This topic is discussed in the answers to “is” operator behaves unexpectedly with integers.

Your book that calls int() "the int method" is being a tiny bit sloppy, IMHO. Pedantically speaking, int itself is a class, which is a callable object, and when you call a class that call gets automatically converted into a call to the class's constructor method (that is, its __new__ method). But informally it's common to refer to int() as a function call or method call.

I almost forgot about the question in your first paragraph. When we write

[1, 2, 3]

the interpreter creates the 3 int objects and puts them inside a fresh list instance. (To be more precise, it puts references to the int objects into the list).

Using the standard dis module you can disassemble the bytecode for this operation:

from dis import dis
dis('a=[1,2,3]')    

output

  1           0 LOAD_CONST               0 (1)
              3 LOAD_CONST               1 (2)
              6 LOAD_CONST               2 (3)
              9 BUILD_LIST               3
             12 STORE_NAME               0 (a)
             15 LOAD_CONST               3 (None)
             18 RETURN_VALUE

So even though we're "just" creating a literal list it's still a fully-fledged list instance object. Unlike some OOP languages, Python doesn't have any "primitive" datatypes that aren't objects, so literal integers and literal strings are also objects. Thus a literal string comes equipped with all the standard string methods. Eg,

print('hello'.lower)

output

built-in method lower of str object at 0xb72e7880>

shows us that the literal string 'hello' has the standard lower() method.