Add support for collections.namedtuple

[akaihola]

namedtupletest.py:

from collections import namedtuple

Test:

$ mypy namedtupletest.py
/tmp/namedtupletest.py, line 1: Module has no attribute 'namedtuple'

[rockneurotiko]

It's not implemented yet.
Code

[akaihola]

I now noticed that some work-arounds are mentioned in the "Common issues" section of the "Creating stubs for Python modules" wiki page.

[JukkaL]

The most primitive support would allow basic type checking with implicit Any item types:

X = namedtuple('X', ['a', 'b'])
x = X(1, 'x')  # Ok
x.c  # Type error
x.a + [1]  # No error, since a has implicit type Any

We also need to support namedtuple as a base class:

class X(namedtuple('X', ['a', 'b'])):
    ...

The above features would allow running mypy on code that uses named tuples and perform rudimentary type checking.

The next step (potentially as a separate issue) would be to allow specifying item types. Here is a potential syntax:

X = namedtuple('X', ['a', 'b'])  # type: (int, str)

Coming up with a syntax for the base class case is more difficult. Suggestions are welcome, or we can require the code to be rewritten like this:

X = namedtuple('X', ['a', 'b'])  # type: (int, str)
class X(X):
    ...

[JukkaL]

I'm working on this.

[JukkaL]

I just pushed an experimental, partial implementation of namedtuple support.

This is supported:

from collections import namedtuple
X = namedtuple('X', ['a', 'b'])
x = X(1, 'x')  # Ok
x = X(1, c='x')  # Error
x.c  # Error
x.a + [1]  # No error, since a has implicit type Any
class Y(X): 
    """you can use a namedtuple class as a base class"""

Also, you can define the types of named tuple items using typing.NamedTuple:

from typing import NamedTuple
X = NamedTuple('X', [('a', int), ('b', str)])
x = X(1, 'x')  # Ok
x = X(1, 2)  # Error

A bunch of stuff is still broken or unsupported, but this should be good enough for (most) library stubs that need namedtuple. I'm going to continue working on this.

[JukkaL]

This is now implemented. It's not quite perfect yet, but it should be usable and I'll file issues for the remaining limitations (or just fix them).

Now you can also use named tuples in the base class list:

from typing import NamedTuple
class Foo(NamedTuple('Bar', [('item1', int), ('item2', str)])):    # OK!
    ...

[gvanrossum]

I don't see NamedTuple defined in the https://github.com/JukkaL/typing project yet. mypy's own typing.py seems to have diverged (e.g. it has Callable, mypy still has Function, but mypy has NamedTuple).

[ghost]

Mypy disallows usage of _replace() method from namedtuple (and likewise for NamedTuple).

from collections import namedtuple
Person = namedtuple("Person", 'name')
p = Person("bob")
p._replace(name="sam")

==> "Person" has no attribute "_replace"

[JukkaL]

Yeah, the namedtuple implementation is missing a few things. Added a new issue: #661