Reformatting more than expected

[anakinxc]

Describe the bug

Darker yields more reformat changes than expected. See repro steps for details.

To Reproduce
Steps to reproduce the behavior:

Let's have a code snippet with inconsistent format

def test_fun():
  with open("bla") as f:
    content = str("""
    whatever
    """)

    A = 1
    B = 2
    C = 3

  return

def main():
  print("hello")  
  test_fun()

if __name__ == "__main__":
    main()

Apply some changes

diff --git a/scripts/test.py b/scripts/test.py
index d50207d076..29c1af2247 100644
--- a/scripts/test.py
+++ b/scripts/test.py
@@ -2,7 +2,7 @@
 def test_fun():
   with open("bla") as f:
     content = str("""
-    whatever
+    whatever//
     """)

     A = 1

Run darker with

python -m darker --check --diff scripts/test.py

Check darker's output diff

--- test.py	2024-12-17 03:08:39.433662 +0000
+++ test.py	2024-12-17 03:09:47.115313 +0000
@@ -1,17 +1,19 @@
+def test_fun():
+    with open("bla") as f:
+        content = str(
+            """
+    whatever//
+    """
+        )

-def test_fun():
-  with open("bla") as f:
-    content = str("""
-    whatever//
-    """)
+        A = 1
+        B = 2
+        C = 3

-    A = 1
-    B = 2
-    C = 3
+    return

-  return

 def main():
   print("hello")
   test_fun()

Expected behavior

Darker should only reformat issues in the string literal that I modified.

Screenshots
If applicable, add copy/paste the output or attach a screenshots to help explain your problem.

Environment (please complete the following information):

• OS: Ubuntu 22.04
• Python version 3.12.7
• Git version 2.45.2
• Darker version 2.1.1
• Black version 24.10.0
• other reformatter versions [e.g. isort==5.10.1, flynt==1.0.1]

Additional context
Add any other context about the problem here.

[akaihola]

Hi @anakinxc, thanks for the report!

Unfortunately this is by design. The algorithm Darker uses to narrow down the set of lines to reformat is based on diff chunks. In your example case the corrected indentation forms a contiguous chunk of changed lines, and Darker is unable to figure out the smallest valid set of lines to reformat.

There has been some exploration of improvement to this behavior, but unfortunately the amount of work would be massive (have Darker understand Python syntax), and some blocking questions couldn't be resolved. See #388 for earlier discussions, this particular comment for another example of indentation reformatting, and #221 for the failed attempt to solve this.