Fixed issue #1189: '--diff' broken when reading from standard input.

CHANGELOG.md

@@ -4,7 +4,8 @@ Changelog
 NOTE: isort follows the [semver](https://semver.org/) versioning standard.
 
 ### 5.1.1 July 15, 2020
-  - Fix issue #1322: Occasionally two extra newlines before comment with `-n` & `--fss`.
+  - Fixed issue #1322: Occasionally two extra newlines before comment with `-n` & `--fss`.
+  - Fixed issue #1189: `--diff` broken when reading from standard input.
 
 ### 5.1.0 July 14, 2020
   - isort now throws an exception if an invalid settings path is given (issue #1174).

isort/api.py

@@ -100,6 +100,7 @@ def sort_stream(
     config: Config = DEFAULT_CONFIG,
     file_path: Optional[Path] = None,
     disregard_skip: bool = False,
+    show_diff: bool = False,
     **config_kwargs,
 ):
     """Sorts any imports within the provided code stream, outputs to the provided output stream.
@@ -113,6 +114,28 @@ def sort_stream(
     - **disregard_skip**: set to `True` if you want to ignore a skip set in config for this file.
     - ****config_kwargs**: Any config modifications.
     """
+    if show_diff:
+        _output_stream = StringIO()
+        _input_stream = StringIO(input_stream.read())
+        changed = sort_stream(
+            input_stream=_input_stream,
+            output_stream=_output_stream,
+            extension=extension,
+            config=config,
+            file_path=file_path,
+            disregard_skip=disregard_skip,
+            **config_kwargs,
+        )
+        _output_stream.seek(0)
+        _input_stream.seek(0)
+        show_unified_diff(
+            file_input=_input_stream.read(),
+            file_output=_output_stream.read(),
+            file_path=file_path,
+            output=output_stream,
+        )
+        return changed
+
     config = _config(path=file_path, config=config, **config_kwargs)
     content_source = str(file_path or "Passed in content")
     if not disregard_skip:

isort/format.py

@@ -28,7 +28,9 @@ def format_natural(import_line: str) -> str:
     return import_line
 
 
-def show_unified_diff(*, file_input: str, file_output: str, file_path: Optional[Path]):
+def show_unified_diff(
+    *, file_input: str, file_output: str, file_path: Optional[Path], output=sys.stdout
+):
     file_name = "" if file_path is None else str(file_path)
     file_mtime = str(
         datetime.now() if file_path is None else datetime.fromtimestamp(file_path.stat().st_mtime)
@@ -43,7 +45,7 @@ def show_unified_diff(*, file_input: str, file_output: str, file_path: Optional[
         tofiledate=str(datetime.now()),
     )
     for line in unified_diff_lines:
-        sys.stdout.write(line)
+        output.write(line)
 
 
 def ask_whether_to_apply_changes_to_file(file_path: str) -> bool:

tests/test_api.py

@@ -1,4 +1,5 @@
 """Tests the isort API module"""
+from io import StringIO
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -41,3 +42,10 @@ def test_check_file(tmpdir) -> None:
 def test_sorted_imports_multiple_configs() -> None:
     with pytest.raises(ValueError):
         api.sort_code_string("import os", config=Config(line_length=80), line_length=80)
+
+
+def test_diff_stream() -> None:
+    output = StringIO()
+    assert api.sort_stream(StringIO("import b\nimport a\n"), output, show_diff=True)
+    output.seek(0)
+    assert "import a\n import b\n" in output.read()