[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Guido van Rossum]

Change by Guido van Rossum :


--
resolution:  -> wont fix
stage: patch review -> resolved
status: open -> closed

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Jarry Shaw]

Jarry Shaw  added the comment:

Apparently static checkers like mypy doesn't rely on the 
`typing.get_type_hints` function to implement its type checking functions (as I 
had browsed through the code repo).

$ cat test.py
def foo(arg) -> ' str': ...
$ mypy test.py
Success: no issues found in 1 source file

If type checkers *think* this is acceptable, but the standard library doesn't, 
this can be some sort of inconsistency in my perspective.

As Saiyang Gou had suggested, I would rather prefer to change the behaviour of 
`compile` in `eval` mode to preserve the internal consistency of builtin 
functions and therefore to eliminate this *buggy* behaviour of 
`typing.get_type_hints`.

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Guido van Rossum]

Guido van Rossum  added the comment:

The more I think about this, the less I like it. AFAIK static checkers like 
mypy don’t strip leading white space from forward references either. (If I’m 
wrong, please show evidence.)

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Guido van Rossum]

Guido van Rossum  added the comment:

Also at this point it's too late to get this into 3.10 (given that we're still 
debating this it's clearly not a simple bugfix).

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Saiyang Gou]

Saiyang Gou  added the comment:

I think it might be a good idea to just strip leading spaces and tabs for 
`compile(x, ..., 'eval')` if we want consistent behavior. `compile` might be 
used in more locations in the whole Python source tree apart from 
`typing.get_type_hints`. Technically the only behavior difference between 
`eval(x)` and `compile(x, ..., 'eval')` (when `x` is a string) should be that 
the latter one does not execute the generated byte code.

--
nosy: +gousaiyang

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Jarry Shaw]

Jarry Shaw  added the comment:

To me, I discovered this issue because of a typo in my code. And apparently, I 
do not suggest people will write their type annotations with leading 
whitespaces purposely.

Here’s another thing though: aligning the behaviour of the two builtin 
functions is good for us users, since logically, ``compile`` is just one 
*execution* step away from ``eval`` to my understanding.

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Guido van Rossum]

Guido van Rossum  added the comment:

So the question is, whether anyone actually writes `x: ' str'`. Does the fix 
satisfy a real need?

If it does, why don't we change compile(x, ..., 'eval') as you suggested in 
your second comment?

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Jarry Shaw]

Jarry Shaw  added the comment:

> as discussed in https://bugs.python.org/issue41887

After some additional thoughts, I am thinking that changing the behaviour of 
``compile`` in ``'eval'`` mode directly might be a better idea, for consistency 
all over the builtin functions. 

Especially that it is a bit complex to decide if ``compile`` is called in 
``'eval'`` mode all over the code base which may need to patch with ``lstrip`` 
sanitisation.

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Ken Jin]

Change by Ken Jin :


--
nosy: +gvanrossum, kj, levkivskyi

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Jarry Shaw]

Change by Jarry Shaw :


--
keywords: +patch
pull_requests: +24209
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/25484

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[issue43893] typing.get_type_hints does not accept type annotations with leading whitespaces

[Jarry Shaw]

New submission from Jarry Shaw :

`typing.get_type_hints` does not accept type annotations (in string capsulated 
form) with leading whitespaces.

```
>>> def foo(arg) -> ' str': ...
>>> typing.get_type_hints(foo)
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/typing.py", line 519, in __init__
code = compile(arg, '', 'eval')
  File "", line 1
str
IndentationError: unexpected indent

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "", line 1, in 
  File "/usr/local/lib/python3.9/typing.py", line 1448, in get_type_hints
value = ForwardRef(value)
  File "/usr/local/lib/python3.9/typing.py", line 521, in __init__
raise SyntaxError(f"Forward reference must be an expression -- got {arg!r}")
SyntaxError: Forward reference must be an expression -- got ' str'
```

When elaborating on this issue, an inconsistency of hevaiour between ``eval`` 
function call and ``compile`` in ``'eval'`` mode was also noticed, where the 
former accepts leading whitespaces and the latter does not.

However, as discussed in https://bugs.python.org/issue41887 , I would then 
propose manually ``lstrip`` whitespaces from the type annotations:

```
519c519
< code = compile(arg.lstrip(' \t'), '', 'eval')
---
> code = compile(arg, '', 'eval')
```

--
components: Library (Lib)
messages: 391434
nosy: jarryshaw
priority: normal
severity: normal
status: open
title: typing.get_type_hints does not accept type annotations with leading 
whitespaces
type: behavior
versions: Python 3.10

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