%autocall: Python 3.9 and PEP 585

[EhsanKia]

PEP 585 in Python 3.9 introduces __class_getitem__, which allows for some types such as concurrent.futures.Future to have a type alias, such as:

IntFuture = concurrent.futures.Future[int]

The above works as expected in ipython 7.24.0, but if you don't assign it to a name, it throws an exception:

In [28]: IntFuture = concurrent.futures.Future[int]

In [29]: concurrent.futures.Future[int]
-------> concurrent.futures.Future([int])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-29-8b8b5de9d20e> in <module>
----> 1 concurrent.futures.Future([int])

TypeError: __init__() takes 1 positional argument but 2 were given

This seems to be a bug with ipython, as it works fine in normal python terminal.

From my understanding, this is due to a feature in ipython that lets you just write len array without parenthesis, and it'll automatically add parentheses for you. In this case, it tries to call Future([int]) as if it was an array.

EDIT: I believe the feature is called "autocall"

[Carreau]

Are you sure yo don't have a config option that set autocall to 2 ?

By default IPython behaves properly:

$ ipython
Python 3.9.0 | packaged by conda-forge | (default, Oct 10 2020, 20:36:05)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.25.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: import concurrent

In [2]: concurrent.futures.Future[int]
Out[2]: concurrent.futures._base.Future[int]

What does get_ipython().config give you ?

You can try autocall 0, autocall 1, autocall 2 or no arguments but the default for autocall should be 1.

With autocall set to 2 even other stuff get improperly called:

In [8]: class A:
   ...:
   ...:     def __call__(self):
   ...:         print('a called')
   ...:

In [9]: a = A(); b = A()

In [10]: a.b = b

In [11]: a.b
-------> a.b()
a called

which is why it's not the default.

[EhsanKia]

My autocall is set to 1 (default), I have not changed it.

Maybe it's fixed on 7.25 dev already?

In [5]: get_ipython().config
Out[5]: {'TerminalInteractiveShell': {'autocall': 1, 'confirm_exit': False}}

[Carreau]

My bad, the default auf autocall is 0, not 1, with 1 I can indeed reproduce.

[Carreau]

narrowed it down to

    handler_name = Unicode('auto')
    esc_strings = List([ESC_PAREN, ESC_QUOTE, ESC_QUOTE2])

    def handle(self, line_info):

Which get called and decide that () should be added.

[EhsanKia]

Awesome, thank you for looking into it. Note that this is only an issue with classes that have __class_getitem__ defined, so maybe that can be used to decide when autocall=smart?

[Carreau]

Autocall is decided purely on the syntax, it does not use reflexion or inspection. So that might be hard.

[EhsanKia]

Hmm, if it's working purely on the syntax tree, how does any getattr work? How can it tell that this is safe?

Future = {int: 1}
Future[int]

[MrMino]

Reading the conversation it seems to me like this issue is specific to %autocall; correct me if I'm wrong.

[EhsanKia]

It is. With autocall = 1, When doing a __getattr__ on a class object, it tries to call the class. In may have made sense in the past, but now that we have __class_getitem__, that no longer holds. Not sure what logic smart autocall uses to decide whether to call or not, but the logic needs to be extended to classes too.