diff: add support for specifying a target

dvc/repo/diff.py

@@ -40,8 +40,9 @@ def diff(self, a_ref="HEAD", b_ref=None, *, target=None):
     By default, it compares the working tree with the last commit's tree.
 
     When a `target` path is given, it only shows that file's comparison.
+    It will silently fail when `target` is not found in any of the trees.
 
-    This implementation differs from `git diff`, since DVC doesn't have
+    This implementation differs from `git diff` since DVC doesn't have
     the concept of `index`, `dvc diff` would be the same as `dvc diff HEAD`.
     """
     outs = {}
@@ -52,11 +53,13 @@ def diff(self, a_ref="HEAD", b_ref=None, *, target=None):
             for stage in self.stages
             for out in stage.outs
         )
-
     old = outs[a_ref]
     new = outs[b_ref or "working tree"]
     delta = old ^ new
 
+    if target:
+        delta = set(x for x in delta if x.filename == target)
+
     if not delta:
         return
 

tests/func/test_diff.py

@@ -1,4 +1,9 @@
 import pytest
+import hashlib
+
+
+def _checksum(text):
+    return hashlib.md5(bytes(text, "utf-8")).hexdigest()
 
 
 def test_no_scm(tmp_dir, dvc):
@@ -13,7 +18,7 @@ def test_added(tmp_dir, scm, dvc):
     result = {
         "file": {
             "old": {},
-            "new": {"checksum": "1cb251ec0d568de6a929b520c4aed8d1", "size": 4},
+            "new": {"checksum": _checksum("text"), "size": 4},
             "diff": {"status": "added", "size": 4},
         }
     }
@@ -27,7 +32,7 @@ def test_deleted(tmp_dir, scm, dvc):
 
     result = {
         "file": {
-            "old": {"checksum": "1cb251ec0d568de6a929b520c4aed8d1", "size": 4},
+            "old": {"checksum": _checksum("text"), "size": 4},
             "new": {},
             "diff": {"status": "deleted", "size": 4},
         }
@@ -42,8 +47,8 @@ def test_modified(tmp_dir, scm, dvc):
 
     result = {
         "file": {
-            "old": {"checksum": "8b04d5e3775d298e78455efc5ca404d5", "size": 6},
-            "new": {"checksum": "a9f0e61a137d86aa9db53465e0801612", "size": 6},
+            "old": {"checksum": _checksum("first"), "size": 6},
+            "new": {"checksum": _checksum("second"), "size": 6},
             "diff": {"status": "modified", "size": 0},
         }
     }
@@ -56,10 +61,27 @@ def test_refs(tmp_dir, scm, dvc):
     tmp_dir.dvc_gen("file", "second", commit="second version")
     tmp_dir.dvc_gen("file", "third", commit="third version")
 
-    # dvc.diff("HEAD~1") --> (third, second)
-    # dvc.diff("HEAD~1", "HEAD~2") --> (second, first)
-    # dvc.diff("missing") --> error
-    pytest.skip("TODO: define output structure")
+    HEAD_2 = _checksum("first")
+    HEAD_1 = _checksum("second")
+    HEAD = _checksum("third")
+
+    assert dvc.diff("HEAD~1") == {
+        "file": {
+            "old": {"checksum": HEAD_1, "size": 5},
+            "new": {"checksum": HEAD, "size": 5},
+            "diff": {"status": "modified", "size": 0},
+        }
+    }
+
+    assert dvc.diff("HEAD~1", "HEAD~2") == {
+        "file": {
+            "old": {"checksum": HEAD_2, "size": 5},
+            "new": {"checksum": HEAD_1, "size": 5},
+            "diff": {"status": "modified", "size": 0},
+        }
+    }
+
+    pytest.skip('TODO: test dvc.diff("missing")')
 
 
 def test_target(tmp_dir, scm, dvc):
@@ -73,9 +95,15 @@ def test_target(tmp_dir, scm, dvc):
     scm.add(["foo.dvc", "bar.dvc"])
     scm.commit("uppercase")
 
-    # dvc.diff("HEAD~1", target="foo")
-    # dvc.diff("HEAD~1", target="missing") --> error
-    pytest.skip("TODO: define output structure")
+    assert dvc.diff("HEAD~1", target="foo") == {
+        "foo": {
+            "old": {"checksum": _checksum("foo"), "size": 3},
+            "new": {"checksum": _checksum("FOO"), "size": 3},
+            "diff": {"status": "modified", "size": 0},
+        }
+    }
+
+    assert not dvc.diff("HEAD~1", target="missing")
 
 
 def test_directories(tmp_dir, scm, dvc):