What is the guidance around list()?

[cjohnson318]

In Python, I usually define empty lists using list(), since it screams "list" to me louder than []. Will Mojo support this keyword in future? Is this an anti-pattern in Mojo?

[soraros]

Mojo currently lacks a data type equivalent to Python's list, which is likely due to ongoing work on traits. An alternative, such as DynamicVector, exists, but it is not yet integrated with the ListLiteral mechanism. Therefore, relying on constructors (akin to Python's list()) remains necessary.

That said, I think list() is an anti-pattern in Python. Because list is really a dynamic variable that usually binds to the buildins.list, Python will generate an extra LOAD_GLOBAL so the resultant byte code is slightly worse. I don't think it will greatly affect the actual performance of your Python code, it's just a good to know. dict has some slight overhead for the same reason.

from dis import dis

def f():
  l = []

def g():
  l = list()

print('f:')
dis(f)
print('g:')
dis(g)

Result:

f:
  3           0 RESUME                   0

  4           2 BUILD_LIST               0
              4 STORE_FAST               0 (l)
              6 LOAD_CONST               0 (None)
              8 RETURN_VALUE
g:
  6           0 RESUME                   0

  7           2 LOAD_GLOBAL              1 (NULL + list)
             14 PRECALL                  0
             18 CALL                     0
             28 STORE_FAST               0 (l)
             30 LOAD_CONST               0 (None)
             32 RETURN_VALUE

Similar overhead won't be observed in Mojo (at least at runtime), as it's compiled and names are resolved statically.

[cjohnson318]

Thanks, I think DynamicVector is probably my best bet then.