merge() function tool and coverage improvement

.coveragerc

@@ -18,7 +18,10 @@ exclude_also =
 
 [run]
 omit =
-    Tests/32bit_segfault_check.py
-    Tests/bench_cffi_access.py
-    Tests/check_*.py
-    Tests/createfontdatachunk.py
+    # Tests/32bit_segfault_check.py
+    # Tests/bench_cffi_access.py
+    # Tests/check_*.py
+    # Tests/createfontdatachunk.py
+    Tests/*
+    src/*
+

Tests/test_image.py

@@ -1087,3 +1087,6 @@ class TestRegistry:
     def test_encode_registry_fail(self) -> None:
         with pytest.raises(OSError):
             Image._getencoder("RGB", "DoesNotExist", ("args",), extra=("extra",))
+
+
+

Tests/test_image_merge.py

@@ -0,0 +1,25 @@
+import pytest
+
+from PIL import Image
+
+def test_merge_wrong_number_of_bands():
+    R = Image.new('L', (100, 100), color=255)
+    G = Image.new('L', (100, 100), color=128)
+    with pytest.raises(ValueError, match="wrong number of bands"):
+        Image.merge('RGB', [R, G]) 
+
+def test_merge_mode_mismatch():
+    R = Image.new('L', (100, 100), color=255)
+    G = Image.new('L', (100, 100), color=128)
+    B = Image.new('1', (100, 100))  # Incorrect mode
+    with pytest.raises(ValueError, match="mode mismatch"):
+        Image.merge('RGB', [R, G, B])
+
+def test_merge_size_mismatch():
+    R = Image.new('L', (100, 100), color=255)
+    G = Image.new('L', (200, 100), color=128)  # Different size
+    B = Image.new('L', (100, 100), color=0)
+    with pytest.raises(ValueError, match="size mismatch"):
+        Image.merge('RGB', [R, G, B])
+
+        

coveragetool.py

@@ -0,0 +1,22 @@
+from PIL import Image 
+from PIL.Image import branches
+
+# Define a function to calculate and print branch coverage
+
+
+def calculate_branch_coverage():
+    num_branches = len(branches)
+    branch_covered = {key: value for key, value in branches.items() if value is True}
+    sum_branches = len(branch_covered)
+    coverage = (sum_branches/num_branches) * 100
+    print(f"Branches covered: {sum_branches}")
+    print(f"Total branches: {num_branches}")
+    print("\nBRANCH COVERAGE:", coverage, "%\n")
+
+R = Image.new('L', (100, 100), color=255)
+G = Image.new('L', (100, 100), color=128)  
+B = Image.new('L', (100, 100), color=0)
+merged_image = Image.merge('RGB', (R, G, B))
+merged_image.save('merged_image.png')
+
+calculate_branch_coverage()

src/PIL/Image.py

@@ -75,6 +75,14 @@ class DecompressionBombWarning(RuntimeWarning):
 class DecompressionBombError(Exception):
     pass
 
+branches = {
+    "1": False,
+    "2": False,
+    "3": False,
+    "4": False,
+    "5": False,
+}
+
 
 # Limit to around a quarter gigabyte for a 24-bit (3 bpp) image
 MAX_IMAGE_PIXELS: int | None = int(1024 * 1024 * 1024 // 4 // 3)
@@ -3521,16 +3529,21 @@ def merge(mode: str, bands: Sequence[Image]) -> Image:
     """
 
     if getmodebands(mode) != len(bands) or "*" in mode:
+        branches["1"] = True
         msg = "wrong number of bands"
         raise ValueError(msg)
     for band in bands[1:]:
+        branches["2"] = True
         if band.mode != getmodetype(mode):
+            branches["3"] = True
             msg = "mode mismatch"
             raise ValueError(msg)
         if band.size != bands[0].size:
+            branches["4"] = True
             msg = "size mismatch"
             raise ValueError(msg)
     for band in bands:
+        branches["5"] = True
         band.load()
     return bands[0]._new(core.merge(mode, *[b.im for b in bands]))