Black reports code that is not equivalent when formatting a doc-string in a file that contains the exec statement

[deymundson]

Description
If a file contains a doc-string that requires formatting, and the exec statement, Black will complain that the change to the doc-string is not equivalent.

Note that this only applies to the exec statement. The exec() function works as expected. Therefore this issue only applies if the target Python version is 2.7.

To Reproduce

1. Using a file with the following contents

def Foo():
    """
    """


exec ""

2. Run Black on it with these arguments --target-version=py27
3. See error

Expected behavior
Black would format the file, including the doc-string, without an error stating that the code is not equivalent.

Environment

• Version: 20.8b1
• OS and Python version: Windows/Python 3.9

Does this bug also happen on master?
Yes

Additional context
The same issue exists when Black reformats a doc-string with an Exception handler that uses the "comma" syntax to get the instance of the exception. i.e. except Exception, e:
See here for a snippet in the Black playground.

[ichard26]

Closing as a duplicate of #1635 (er actually this is a duplicate of #1663 apparently)..

(black) ichard26@acer-ubuntu:~/programming/oss/black$ python --version ; git log -n 1 --oneline
Python 3.8.5
4070527 (HEAD -> main, upstream/master, caching) Switch to pytest and tox (#1763)
(black) ichard26@acer-ubuntu:~/programming/oss/black$ cat temp.py 
def Foo():
    """
    """


exec ""
(black) ichard26@acer-ubuntu:~/programming/oss/black$ black temp.py -t py27 --fast --diff --color
--- temp.py	2020-10-26 22:16:15.861441 +0000
+++ temp.py	2020-10-26 22:16:42.500120 +0000
@@ -1,6 +1,5 @@
 def Foo():
-    """
-    """
+    """"""
 
 
 exec ""
would reformat temp.py
All done! ✨ 🍰 ✨
1 file would be reformatted.