api: open: Raise ValueError if rev is used in wrong mode.

Closes #7405

Co-authored-by: Peter Rowlands (변기호) <[email protected]>

dvc/api.py

@@ -90,6 +90,7 @@ def open(  # noqa, pylint: disable=redefined-builtin
         mode (str, optional): Specifies the mode in which the file is opened.
             Defaults to "r" (read).
             Mirrors the namesake parameter in builtin `open()`_.
+            Only reading `mode` is supported.
         encoding(str, optional): `Codec`_ used to decode the file contents.
             Defaults to None.
             This should only be used in text mode.
@@ -104,6 +105,7 @@ def open(  # noqa, pylint: disable=redefined-builtin
 
     Raises:
         AttributeError: If this method is not used as a context manager.
+        ValueError: If non-read `mode` is used.
 
     Examples:
 
@@ -178,6 +180,8 @@ def open(  # noqa, pylint: disable=redefined-builtin
         https://docs.python.org/3/glossary.html#term-file-object
 
     """
+    if "r" not in mode:
+        raise ValueError("Only reading `mode` is supported.")
 
     args = (path,)
     kwargs = {

tests/unit/test_api.py

@@ -0,0 +1,21 @@
+import pytest
+
+from dvc import api
+
+
+def test_open_raises_error_if_no_context(tmp_dir, dvc):
+    tmp_dir.dvc_gen("foo", "foo-text")
+
+    with pytest.raises(
+        AttributeError, match="should be used in a with statement."
+    ):
+        fd = api.open("foo")
+        fd.read()
+
+
+def test_open_rev_raises_error_on_wrong_mode(tmp_dir, dvc):
+    tmp_dir.dvc_gen("foo", "foo-text")
+
+    with pytest.raises(ValueError, match="Only reading `mode` is supported."):
+        with api.open("foo", mode="w"):
+            pass