DRAFT: Apply pyupgrade.

mypy.ini

@@ -90,6 +90,9 @@ ignore_missing_imports = True
 [mypy-pytest.*]
 ignore_missing_imports = True
 
+[mypy-pyupgrade.*]
+ignore_missing_imports = True
+
 [mypy-setuptools.*]
 ignore_missing_imports = True
 

setup.cfg

@@ -51,6 +51,8 @@ isort =
     isort>=5.0.1
 color =
     Pygments>=2.4.0
+pyupgrade =
+    pyupgrade>=2.31.0
 test =
     # NOTE: remember to keep `constraints-oldest.txt` in sync with these
     airium>=0.2.3

src/darker/syntax_upgrade.py

@@ -0,0 +1,29 @@
+"""Wrapper for applying `pyupgrade` on Python source code"""
+
+from darker.utils import TextDocument
+
+try:
+    from pyupgrade import _main as pyupgrade_main
+except ImportError:
+    # `pyupgrade` is an optional dependency. Prevent the `ImportError` if it's missing.
+    pyupgrade_main = None
+
+
+__all__ = ["apply_pyupgrade", "pyupgrade_main"]
+
+
+def apply_pyupgrade(content: TextDocument) -> TextDocument:
+    """Upgrade syntax to newer version of Python using `pyupgrade`
+
+    :param content: The Python source code to upgrade
+    :return: The upgraded Python source code
+
+    """
+    # pylint: disable=protected-access
+    min_version = (3, 6)
+    result = pyupgrade_main._fix_plugins(
+        content.string, pyupgrade_main.Settings(min_version=min_version)
+    )
+    result = pyupgrade_main._fix_tokens(result, min_version)
+    result = pyupgrade_main._fix_py36_plus(result, min_version=min_version)
+    return TextDocument.from_str(result)