Possibly spurious SyntaxError: annotated name can't be global

[rohanpadhye]

BPO	34939
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi, @pablogsal, @tirkarthi, @rohanpadhye
PRs	bpo-34939: Allow annotated global names in module namespace #9844

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-10-14.17:07:08.454>
created_at = <Date 2018-10-09.01:58:38.509>
labels = ['interpreter-core', 'type-bug', '3.7']
title = "Possibly spurious SyntaxError: annotated name can't be global"
updated_at = <Date 2018-10-14.17:07:08.453>
user = 'https://github.com/rohanpadhye'

bugs.python.org fields:

activity = <Date 2018-10-14.17:07:08.453>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-14.17:07:08.454>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2018-10-09.01:58:38.509>
creator = 'rohanpadhye'
dependencies = []
files = []
hgrepos = []
issue_num = 34939
keywords = ['patch']
message_count = 8.0
messages = ['327378', '327384', '327386', '327420', '327421', '327424', '327554', '327716']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi', 'pablogsal', 'xtreak', 'rohanpadhye']
pr_nums = ['9844']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34939'
versions = ['Python 3.7']

[rohanpadhye]

The following code when run as a script file gives syntax error:

def set_x():
    global x
    x = 1

x:int = 0   # SyntaxError: annotated name 'x' can't be global

PEP-526 does not seem to forbid this. The error message "annotated name [...] can't be global" is usually seen when using the global x declaration in the same scope as an annotated assignment. In the above case, the annotated assignment is outside the function scope, yet Python 3.7 gives a syntax error.

Is this a bug in CPython? Or should the PEP-526 document say something about forward references?

Interestingly, if the above program is run in interactive mode, there is no syntax error.

In interactive mode:

>>> def set_x():
...     global x
...     x = 1
... 
>>> x:int = 0
>>> set_x()
>>> print(x)
1

Further, forward references work fine with nonlocal. For example, the following works fine both as a script file and in interactive mode:

def outer():
    def inner():
        nonlocal y
        y = 1
    y:int = 0

I don't see why a forward reference in global is a problem.

[tirkarthi]

The error message was added with 6cff874 and bpo-27999. The docs were also changed as below with the commit :

https://docs.python.org/3.8/reference/simple_stmts.html#the-global-statement

Names listed in a global statement must not be defined as formal parameters or in a for loop control target, class definition, function definition, import statement, or variable annotation.

Thanks

[ilevkivskyi]

Hm, I think this should be allowed. The formulation in the docs is not very clear, but the wording in the PEP clarifies the intention. Indeed, only annotations at the same scope with global declarations should be prohibited.

So I think this is a bug.

[gvanrossum]

I agree with Ivan.

[serhiy-storchaka]

This looks similar to say

    x = 1
    global x

It is an error in a file, but is not an error in interactive mode.

[rohanpadhye]

Another point I'd like to make is that there is no error in either file or interactive if the annotated assignment appears before the global declaration like so:

x:int = 0

def set_x():
    global x
    x = 1

# Works fine!

The syntax error specifically occurs when the annotated assignment occurs after a global declaration in the program, even if the assignment is in a different scope.

Neither the docs nor the PEP say anything about such an ordering constraint.

[ilevkivskyi]

I think we can just go ahead and allow this. If there is a volunteer, please go ahead, otherwise I will try to find time for this myself.

[pablogsal]

New changeset de2aea0 by Pablo Galindo in branch 'master':
bpo-34939: Allow annotated global names in module namespace (GH-9844)
de2aea0