Dry run mode

README.md

@@ -42,6 +42,10 @@ optional arguments:
   --no-multiline        convert only single line expressions
   -ll LINE_LENGTH, --line-length LINE_LENGTH
                         for expressions spanning multiple lines, convert only if the resulting single line will fit into the line length limit. Default value is 88 characters.
+  -d, --dry-run         Do not change the files in-place and print the diff
+                        instead. Note that this must be used in conjunction
+                        with '--fail-on-change' when used for linting
+                        purposes.
   -tc, --transform-concats
                         Replace string concatenations (defined as + operations involving string literals) with f-strings. Available only if flynt is installed with 3.8+
                         interpreter.

src/flynt/api.py

@@ -4,6 +4,7 @@
 import time
 import traceback
 from typing import Tuple, List
+from difflib import unified_diff
 
 import astor
 
@@ -77,8 +78,12 @@ def default_result():
         )
         return default_result()
 
-    with open(filename, "w", encoding="utf-8") as f:
-        f.write(new_code)
+    if state.dry_run:
+        for l in unified_diff(contents.split('\n'), new_code.split('\n'), fromfile=filename):
+            print(l)
+    else:
+        with open(filename, "w", encoding="utf-8") as f:
+            f.write(new_code)
 
     return True, changes, len(contents), len(new_code)
 

src/flynt/cli.py

@@ -52,6 +52,16 @@ def main():
         default=88,
     )
 
+    parser.add_argument(
+        "-d",
+        "--dry-run",
+        action="store_true",
+        default=False,
+        help="Do not change the files in-place and print the diff instead. "
+             "Note that this must be used in conjunction with '--fail-on-change' when "
+             "used for linting purposes."
+    )
+
     parser.add_argument(
         "-tc",
         "--transform-concats",
@@ -91,6 +101,9 @@ def main():
 
     args = parser.parse_args()
 
+    if args.dry_run:
+        print("Running flynt in dry-run mode. No files will be changed.")
+
     if args.transform_concats:
         if sys.version_info < (3, 8):
             raise Exception(
@@ -101,6 +114,7 @@ def main():
     state.aggressive = args.aggressive
     state.verbose = args.verbose
     state.quiet = args.quiet
+    state.dry_run = args.dry_run
 
     return fstringify(
         args.src,

src/flynt/state.py

@@ -3,6 +3,7 @@
 verbose = False
 quiet = False
 aggressive = False
+dry_run = False
 
 percent_candidates = 0
 percent_transforms = 0

test/integration/test_api.py

@@ -4,6 +4,7 @@
 import pytest
 
 from flynt import api
+from flynt import state
 from flynt.api import _fstringify_file
 
 
@@ -91,3 +92,17 @@ def broken_fstringify_by_line(*args, **kwargs):
 
     assert not modified
     assert content_after == content_before
+
+
+def test_dry_run(formattable_file, monkeypatch):
+    monkeypatch.setattr(state, "dry_run", True)
+    with open(formattable_file) as f:
+        content_before = f.read()
+
+    modified, _, _, _ = _fstringify_file(formattable_file, True, 1000)
+
+    with open(formattable_file) as f:
+        content_after = f.read()
+
+    assert modified
+    assert content_after == content_before