[issue35001] ImportFrom level cannot be optional

[Brett Cannon]

Brett Cannon  added the comment:

There aren't any type hints in the 'ast' module in Python itself, so this is an 
issue with typeshed. Closing as "third party".

--
nosy: +brett.cannon
resolution:  -> third party
stage:  -> resolved
status: open -> closed

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[issue35001] ImportFrom level cannot be optional

[thautwarm]

thautwarm  added the comment:

Firstly, allowing to construct ImportFrom without `level` specified could be 
the result of referring Python-asdl.

Secondly, in C level, `level` is always an integer.

Last but not the least, when you can access `level` from an ImportFrom AST, it 
must be an integer, not None.

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[issue35001] ImportFrom level cannot be optional

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The level argument of ast.ImportFrom is optional. If it is not specified or 
None is passes, the corresponding attribute is set to the default value 0.

Type hints are not used in the stdlib. If this is a bug in mypy, this is not 
the proper tracker for reporting it.

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[issue35001] ImportFrom level cannot be optional

[thautwarm]

thautwarm  added the comment:

@Anthony

> I don't think mypy has an annotation for "sometimes has an attribute"

Yes.. The annotation for an attribute that **maybe exist** cannot be expressed 
by mypy..

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[issue35001] ImportFrom level cannot be optional

[thautwarm]

thautwarm  added the comment:

Hi, Serhiy, for Python-asdl has made `level` able to optional, certainly we 
could construct an ImportFrom AST without giving `level`.

Moreover, this might evidence that `level` cannot be optional in fact:

https://github.com/python/cpython/blob/8e73ad38ab7d218b9ef8976032865928dfad00f1/Python/compile.c#L2918

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[issue35001] ImportFrom level cannot be optional

[Anthony Sottile]

Anthony Sottile  added the comment:

Hmmm, I don't think mypy has an annotation for "sometimes has an attribute" -- 
`Optional[T]` is `Union[T, None]` (why I tried `None`).

But you're right, `FromImport` is constructable without a `level` -- it seems 
to behave as `level=0` (I guess as expected!)

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[issue35001] ImportFrom level cannot be optional

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

None is not integer. This field is optional:

>>> from ast import *
>>> x = Module(body=[ImportFrom(names=[alias(name='path', asname=None)], 
>>> lineno=1, col_offset=0)])
>>> compile(x, '', 'exec')
 at 0x7f73b754da00, file "", line 1>

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nosy: +serhiy.storchaka

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[issue35001] ImportFrom level cannot be optional

[Anthony Sottile]

Anthony Sottile  added the comment:

In fact, trying to use an `ImportFrom` without an integer `level` results in a 
`ValueError`:

>>> x = ast.parse('from os import path')
>>> x.body[0].level = None
>>> compile(x, '', 'exec')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: invalid integer value: None

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[issue35001] ImportFrom level cannot be optional

[Anthony Sottile]

Anthony Sottile  added the comment:

It appears it has always had this bug since introduction of absolute/relative 
imports in 
https://github.com/python/cpython/commit/f7f438ba3b05eb4356e7511401686b07d9dfb6d8

Agree with changing this to `# type: int` and correcting the documentation

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[issue35001] ImportFrom level cannot be optional

[Sebastian Rittau]

Change by Sebastian Rittau :


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nosy: +srittau

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[issue35001] ImportFrom level cannot be optional

[thautwarm]

New submission from thautwarm :

This issue is found from a type hinting problem:

```
class ImportFrom(stmt): class ImportFrom(stmt):
module = ...  # type: Optional[_identifier] module = ...  # type: 
Optional[_identifier]
names = ...  # type: typing.List[alias] names = ...  # type: 
typing.List[alias]
level = ...  # type: Optional[int]
```

As we can see that `level` here is optional, and it's strange.

I tried `ast.parse` on both Python 3.5/3.6, and found that None of `from a 
import *`, `from .a import *`, `from ..a import *` and other regular cases 
result into an ImportFrom AST whose `level` is None.


Then I went to Python-asdl: 
https://github.com/python/cpython/blob/137b0632dccb992ca11e9445142fb33a29c33a51/Parser/Python.asdl#L44

and got 
   
   ImportFrom(identifier? module, alias* names, int? level)


It seems like a bug. To validate it, I went to 
https://github.com/python/cpython/blob/97cf0828727ac2a269c89c5aa09570a69a22c83c/Python/ast.c#L3311

and got 

static stmt_ty
ast_for_import_stmt(struct compiling *c, const node *n){
   int idx, ndots = 0;
   ...
   return ImportFrom(modname, aliases, ndots, lineno, col_offset, 
c->c_arena);
   ...
}

It seems that no reason for `level` being None.

If it's really a bug, IMO it could be also an example that type hinting helps 
to error detection :-)

--
assignee: docs@python
components: Documentation
messages: 327840
nosy: docs@python, thautwarm
priority: normal
severity: normal
status: open
title: ImportFrom level cannot be optional
type: enhancement
versions: Python 3.6, Python 3.7, Python 3.8

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