For instance this file:

foo = f"""{}
foo"""

$ python3.8 --version --version
Python 3.8.0b1 (default, Jun  6 2019, 03:44:52) 
[GCC 7.4.0]

$ python3.8 t.py | wc -c
  File "t.py", line 1


<<snipped, but very very many spaces>>

                                   ^
SyntaxError: f-string: empty expression not allowed


$ python3.8 t.py |& wc -c
49134

I can also trigger this with:

```
1+ """\q
"""
```

And `python -Werror`

so it seems not necessarily related to fstrings

Debugging further, there's a token with a `multi_line_start` pointing at uninitialized memory -- let's see if I can't track down where that gets set and fix it :)

This looks like the same issue I mentioned here https://bugs.python.org/msg344764

On Windows 10 with current 3.8 I see correct output, so the bug seems OS specific, even though the simple fix (see PR) is not.

C:\Users\Terry>py f:/python/a/tem4.py
  File "f:/python/a/tem4.py", line 1
    f'''{}
    ^
SyntaxError: f-string: empty expression not allowed

just an f-string doesn't trigger this, there needs to be tokens before it

OK, very obnoxious.  Fix restores behavior above.

Thanks Anthony Sottile for the fix!

I suggest adding more test cases that do not involve f-strings, what do you think? Can you create another PR for those?

sure! the only other case I could come up with involves multi-line strings and invalid escape sequences + Werror

let me see if I can fold that in as well

This seems related. It's also possible I'm misunderstanding what is supposed to happen here.

If you create test.py with just the 2 lines:

"""
a

and run python test.py from CPython master, you get

$./python.exe test.py
  File "/Users/aaronmeurer/Documents/cpython/test.py", line 4
    a
    ^
SyntaxError: EOF while scanning triple-quoted string literal

Notice that it reports line 4 even though the file only has 2 lines.

The offset in the syntax error is 6 columns (line numbers and column offsets
in SyntaxErrors count from 1)

>>> try:
...     compile('"""\na', '<none>', 'exec')
... except SyntaxError as e:
...     print(repr(e))
...
SyntaxError('EOF while scanning triple-quoted string literal', ('<none>', 2, 6, '"""\na\n'))

seems unrelated and does the same on older versions of python:

```
$ python3.5 t.py
  File "t.py", line 4
    
    ^
SyntaxError: EOF while scanning triple-quoted string literal
```

I'd suggest a new issue or finding the dupe

The initial issue has been fixed in master by the commit 5b94f3578c662d5f1ee90c0e6b81481d9ec82d89:

commit 5b94f3578c662d5f1ee90c0e6b81481d9ec82d89
Author: Anthony Sottile <asottile@umich.edu>
Date:   Mon Jul 29 06:59:13 2019 -0700

    Fix `SyntaxError` indicator printing too many spaces for multi-line strings (GH-14433)

Backport to 3.8: commit cf52bd0b9b1c1b7ecdd91e1ebebd15ae5c213a2c.

Thanks Anthony Sottile! I close the issue.

--

If there is a different issue in Python 3.7 and you consider that it must be fixed, please file a new issue. Python 3.5 and 3.6 don't accept bugfixes anymore:
https://devguide.python.org/#status-of-python-branches

nosy: + vstinner
messages: + msg350074

resolution: fixed
stage: patch review -> resolved

messages: + msg346872
title: syntax error in f-string in multiline string produces ~40k spaces of output -> syntax error in multiline f-string produces ~40k spaces output