X-Git-Url: https://git.madduck.net/etc/vim.git/blobdiff_plain/ffaaf4838228c922b586a87f717ed402031fcc0a..d97b7898b34b67eb3c6839998920e17ac8c77908:/src/black/__init__.py

diff --git a/src/black/__init__.py b/src/black/__init__.py
index 2786861..7d7ddbe 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -917,7 +917,7 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """
-    if not src_contents.strip():
+    if not mode.preview and not src_contents.strip():
         raise NothingChanged
 
     if mode.is_ipynb:
@@ -1014,6 +1014,9 @@ def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileCon
     Operate cell-by-cell, only on code cells, only for Python notebooks.
     If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
     """
+    if mode.preview and not src_contents:
+        raise NothingChanged
+
     trailing_newline = src_contents[-1] == "\n"
     modified = False
     nb = json.loads(src_contents)
@@ -1106,6 +1109,13 @@ def _format_str_once(src_contents: str, *, mode: Mode) -> str:
     dst_contents = []
     for block in dst_blocks:
         dst_contents.extend(block.all_lines())
+    if mode.preview and not dst_contents:
+        # Use decode_bytes to retrieve the correct source newline (CRLF or LF),
+        # and check if normalized_content has more than one line
+        normalized_content, _, newline = decode_bytes(src_contents.encode("utf-8"))
+        if "\n" in normalized_content:
+            return newline
+        return ""
     return "".join(dst_contents)