"INTERNAL ERROR" on minimal python file

[DavidHowlett]

Describe the bug

I created a file with "# fmt: off" and no matching "# fmt: on" and black suffered an internal error.

To Reproduce

Run black on this file:

def foo():
    # fmt: off
    pass

def bar():
    pass

Example run on my computer:

(base) ➜  ~ cat demo.py
def foo():
    # fmt: off
    pass

def bar():
    pass
(base) ➜  ~ black demo.py
error: cannot format demo.py: INTERNAL ERROR: Black produced code that is not equivalent to the source on pass 1.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /var/folders/6f/1jnzmq1d0_v9qyk4pbr96cmc0000gn/T/blk_r87c8kh7.log
Oh no! 💥 💔 💥
1 file failed to reformat.

Expected behavior

I believe the correct behaviour is for black to not format the rest of the file after the marker.

Environment

• Black's version: black, 21.12b0 (compiled: no)
• OS and Python version: MacOS 12.1 Python 3.9.7

Additional context

I discovered this accidentally while applying black to the codebase I am working on. Thanks for including a link to where to report the bug.

[felix-hilden]

Thanks for reporting. We do require a fmt: on as well, but your rationale is fair. And at the very least we should not output an internal error, but rather "fmt: on not found" or something akin to that. We're tracking these in #569 and #1817, so I'll close this for housekeeping.

[DavidHowlett]

Thanks for the fast reply. Sorry about the duplicate.

[DavidHowlett]

Interestingly I can use "fmt: on" without a "fmt:off". Most of the time it works as expected and does not crash.