CODEWITHSUNDEEP
pythonpython-internals
yrrah2
yrrah2

Reputation: 61

How are lists created without the class name?

It is possible to go about creating lists like this:

l = list()

l.append(1)

however the most common method is to create a list like this:

l = [1]

My question is simple in the concept but probably extremely complicated in the answer but how is it that the list is created with a notation outside the usual class notation?

I have looked through the source code at https://github.com/python/cpython/blob/master/Objects/listobject.c and cant find where it defines the list notation [].

I am not asking about the difference between [] and list() - I am asking why you can use [] at all. How is it possible that this object is created differently from other objects?

Upvotes: 1

Views: 169

Answers (2)

chepner
chepner

Reputation: 531683

The list literal [...] is defined by the grammar, the same as keywords like if and while:

atom: ... | '[' [testlist_comp] '] | ...

When the code generator encounters a list literal in the AST, it generates a specific byte code to create a list:

>>> import dis
>>> dis.dis('[]')
  1           0 BUILD_LIST               0
              2 RETURN_VALUE

whereas a call to list is handled like any other callable:

>>> dis.dis('list()')
  1           0 LOAD_NAME                0 (list)
              2 CALL_FUNCTION            0
              4 RETURN_VALUE

For non-empty lists, the expressions in the literal are added to the stack for BUILD_LIST to use:

>>> dis.dis('list([1,2])')
  1           0 LOAD_NAME                0 (list)
              2 LOAD_CONST               0 (1)
              4 LOAD_CONST               1 (2)
              6 BUILD_LIST               2
              8 CALL_FUNCTION            1
             10 RETURN_VALUE

A call like list([1,2]) necessarily uses BUILD_LIST first to create a list before passing that to list as an argument.

Upvotes: 3

rdas
rdas

Reputation: 21285

[1] is a list-literal - it implicitly creates the list() with 1 in it (stores it in the heap). I'm sure you can find this if you dig around in the code-base a bit more.

Upvotes: 1

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