INTERNAL ERROR: unstable assert formatting for certain length strings

[jonppe]

Black is not able to reliably format code with assert and certain length string with formatting.
This happens with both 19.10b0 and the current master 19.10b1.dev154+gcd3a93a.d20200822.
19.3b0 does not have the issue.

--- source
+++ first pass
@@ -1,4 +1,6 @@
-assert some_type_of_boolean_expression, (
-    "format-string-and-arguments with certain total length %s %s xxxxx"
-    % (xxxxxxxxx, xxx)
+assert (
+    some_type_of_boolean_expression
+), "format-string-and-arguments with certain total length %s %s xxxxx" % (
+    xxxxxxxxx,
+    xxx,
 )
--- first pass
+++ second pass
@@ -1,6 +1,4 @@
-assert (
-    some_type_of_boolean_expression
-), "format-string-and-arguments with certain total length %s %s xxxxx" % (
-    xxxxxxxxx,
-    xxx,
+assert some_type_of_boolean_expression, (
+    "format-string-and-arguments with certain total length %s %s xxxxx"
+    % (xxxxxxxxx, xxx,)
 )

Some additional notes:

If I remove one x, black reliably gives me output:

assert (
    some_type_of_boolean_expression
), "format-string-and-arguments with certain total length %s %s xxxx" % (xxxxxxxxx, xxx)

Anyway, here the longest line is 88 chars so this format can not be used with the original code.

On the other hand, if I add one X to the original code, black again works and gives me format:

assert some_type_of_boolean_expression, (
    "format-string-and-arguments with certain total length %s %s xxxxxX"
    % (xxxxxxxxx, xxx)

This is a format that could be used with the original code.
And this seems to be same format than output of the first pass.
Finally, black uses this same format if I add a trailing comma to the string format arguments:

assert some_type_of_boolean_expression, (
    "format-string-and-arguments with certain total length %s %s xxxxx"
    % (xxxxxxxxx, xxx,)
)

But I wonder is this against the "magical trailing comma" rule?

[ichard26]

Fixed by #1621.

(black-rHKUX7ap) R:\Programming\black>type temp.py
assert some_type_of_boolean_expression, (
    "format-string-and-arguments with certain total length %s %s xxxxx"
    % (xxxxxxxxx, xxx)
)
(black-rHKUX7ap) R:\Programming\black>git checkout cd3a93a
HEAD is now at cd3a93a Property-based fuzz test

(black-rHKUX7ap) R:\Programming\black>black temp.py --check
error: cannot format temp.py: INTERNAL ERROR: Black produced different code on the second pass of the formatter.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: C:\Users\User\AppData\Local\Temp\blk_7487p0vh.log
Oh no! 💥 💔 💥
1 file would fail to reformat.

(black-rHKUX7ap) R:\Programming\black>git checkout 586d24236e6b57bc3b5da85fdbe2563835021076
Previous HEAD position was cd3a93a Property-based fuzz test
HEAD is now at 586d242 Address pre-existing trailing commas when not in the rightmost bracket pair

(black-rHKUX7ap) R:\Programming\black>black temp.py --check
would reformat temp.py
Oh no! 💥 💔 💥
1 file would be reformatted.