REPL incorrectly accepts errorneous `global` statement

[matthewyu0311]

Bug report

Bug description:

According to the docs on global statement,

The global statement applies to the entire scope of a function or class body. A SyntaxError is raised if a variable
is used or assigned to prior to its global declaration in the scope.

this is clearly a syntax error (global after assignment), yet REPL accepts it.

>>> x = 0; global x; x = 1; print(x)
1
>>> x = 0
... global x
... x = 1
... print(x)
... 
1

But if put into a module file or command line python -c "x=0; global x; x=1", it is correctly rejected.

exec also rejects it:

>>> exec("x = 0; global x; x = 1; print(x)")
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    exec("x = 0; global x; x = 1; print(x)")
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1
SyntaxError: name 'x' is assigned to before global declaration

In any of the above cases the global is redundant, yet only the REPL ignores it silently. If this is intended behavior of the REPL, I think the REPL should give a SyntaxWarning pointing out this syntax would be invalid in other cases.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

• gh-137376: ensure that SyntaxError are properly detected in REPL #137378

[picnixz]

That's likely because the AST parsing doesn't fail:

>>> import ast
>>> ast.parse("x = 1; global x; x = 0")
Module(body=[Assign(targets=[Name(id='x', ctx=Store(...))], value=Constant(value=1, kind=None), type_comment=None), ..., Assign(targets=[Name(id='x', ctx=Store(...))], value=Constant(value=0, kind=None), type_comment=None)], type_ignores=[])

I found out that we're using ast.PyCF_ONLY_AST in InteractiveColoredConsole.runsource:

    def runsource(self, source, filename="<input>", symbol="single"):
        try:
            tree = self.compile.compiler(
                source,
                filename,
                "exec",
                ast.PyCF_ONLY_AST,
                incomplete_input=False,
            )
        except SyntaxError as e:
            ...

I think it's also a combination with how flags are actually passed to compile. I'll try to see what I can do.

EDIT 1: it appears that the statement is split after ; and does not run in one go. So, global x as a standalone statement is fine.
EDIT 2: actually, the global statements seems to be entirely ignored when we're writing lines in the REPL.
EDIT 3: this should be detected before processing the lines. Namely this should be detected when parsing the AST itself.

[picnixz]

Ok, the issue can be fixed as follows:

• first we validate the syntax using compile to get the AST only.
• then, we can validate other stuff by calling symtable.symtable. The reason is that there are some checks that are performed at the symtable level and not during AST parsing.

We can't get rid of ast.PyCF_ONLY_AST because otherwise we end up with a non-AST object. But we still need the symtable checks.

[picnixz]

Arf, it actually won't work perfectly because of future statements apparently. But I have an idea!

[picnixz]

So I made a dirty fix because I need to leave now. I can only rework on this in 2 or 3 days.