With --incompatible_string_join_requires_strings=false, this code was previously accepted by Bazel:

>> ",".join(["a", 1, None, (2, 3)])
.. 
"a,1,None,(2, 3)"

However the argument of string.join should an iterable of strings. Bazel used to implicitly convert each item to a string (https://github.com/bazelbuild/starlark/blob/master/spec.md#stringjoin).

Since Bazel 0.27, this is an error and the flag --incompatible_string_join_requires_strings is set to true. If your code relies on the old behavior, do instead an explicit conversion to string with str:

- ", ".join(my_list)
+ ", ".join([str(e) for e in my_list])

A temporary workaround is to use --incompatible_string_join_requires_strings=false. This flag will be removed in the future.

Error message

If you see an error message similar to this (with a call to string.join in the stack trace), your code is likely affected by this change:

		" ".join(([python_bin, ctx.path(ctx.attr....))
sequence element must be a string (got 'path'). See https://github.com/bazelbuild/bazel/issues/7802 for information about --incompatible_string_join_requires_strings.