How to use [] with a custom builtin list?

Question

I'm trying to understand how __builtin__ works in Python. My code is as follows:

import __builtin__

class MyList(list):
    pass

__builtin__.list = MyList

a = [1,2,3,4,5]
b = list([1,2,3,4,5])

print 'A: ', type(a)
print 'B: ', type(b)

When I print the types of both of the lists, I get:

A:  <type 'list'>
B:  <class '__main__.MyList'>

Why aren't both lists of type MyList and how can I achieve that [] syntax would also be of type MyList?

@martineau I read the question too quickly and misunderstood. The OP is asking about overriding literal syntax. — juanpa.arrivillaga, Jan 25 '17 at 23:59
So, you could hack the syntax tree to achieve this, maybe, but it sounds like a lot of effort for very little reward. — juanpa.arrivillaga, Jan 26 '17 at 00:00
So, see for example here: http://stackoverflow.com/questions/7878933/override-the-notation-so-i-get-an-ordereddict-instead-of-a-dict/37259917 — juanpa.arrivillaga, Jan 26 '17 at 00:01
Assigning to `__builtin__` variables will never affect the Python language internals the way you're hoping. The `__builtin__` module is simply the standard fallback for global variable lookup, and assignments to `__builtin__` variables can only affect global variable lookup. Evaluating literals like `[1, 2, 3, 4, 5]` involves no global variable lookup. — user2357112, Jan 26 '17 at 00:30

score 2 · Accepted Answer · answered Jan 26 '17 at 00:14

If you check how [] is handled by CPython interprerter with dis module, you would see that it spawns BUILD_LIST instruction:

>>> def x():
...     x = [1,2]
... 
>>> dis.dis(x)
  2           0 LOAD_CONST               1 (1)
              3 LOAD_CONST               2 (2)
              6 BUILD_LIST               2
              9 STORE_FAST               0 (x)
             12 LOAD_CONST               0 (None)
             15 RETURN_VALUE

The corresponding instruction triggers direct call to C function PyList_New (it is handled in ceval.c):

2202        case BUILD_LIST:
2203            x =  PyList_New(oparg);

I think that if CPython would search list constructor in __builtin__ pseudo-module each time [] are used in source, it would be much slower, so this behavior can be considered an optimization.

So what you asking is not achievable in vanilla CPython.

How to use [] with a custom __builtin__ list?

1 Answers1

How to use [] with a custom builtin list?