Only write file if there have been changes.

This avoids unnecessary changes to the mtime

formate/__init__.py

@@ -314,7 +314,6 @@ def reformat_file(
 
 	if ret:
 		click.echo(r.get_diff(), color=resolve_color_default(colour))
-
-	r.to_file()
+		r.to_file()
 
 	return ret

tests/test_integration.py

@@ -79,14 +79,26 @@ def test_integration(
 
 	config = load_toml(tmp_pathplus / "formate.toml")
 
+	st = (tmp_pathplus / "code.py").stat()
+	assert st == st
+
 	assert reformat_file(tmp_pathplus / "code.py", config) == 1
 	advanced_file_regression.check_file(tmp_pathplus / "code.py")
 	check_out(capsys.readouterr(), advanced_data_regression)
 
+	# mtime should have changed
+	new_st = (tmp_pathplus / "code.py").stat()
+	assert new_st.st_mtime != st.st_mtime
+	assert new_st != st
+
 	# Calling a second time shouldn't change anything
 	assert reformat_file(tmp_pathplus / "code.py", config) == 0
 	advanced_file_regression.check_file(tmp_pathplus / "code.py")
 
+	# mtime should be the same
+	assert (tmp_pathplus / "code.py").stat().st_mtime == new_st.st_mtime
+	assert (tmp_pathplus / "code.py").stat() == new_st
+
 
 def test_integration_pyproject(
 		tmp_pathplus: PathPlus,
@@ -128,6 +140,9 @@ def test_reformatter_class(
 	with pytest.raises(ValueError, match=r"'Reformatter.run\(\)' must be called first!"):
 		r.get_diff()
 
+	st = (tmp_pathplus / "code.py").stat()
+	assert st == st
+
 	assert r.run() == 1
 	r.to_file()
 
@@ -139,6 +154,11 @@ def test_reformatter_class(
 	assert not captured.out
 	assert not captured.err
 
+	# mtime should have changed
+	new_st = (tmp_pathplus / "code.py").stat()
+	assert new_st.st_mtime != st.st_mtime
+	assert new_st != st
+
 	# Calling a second time shouldn't change anything
 	r = Reformatter(tmp_pathplus / "code.py", config)
 	assert r.run() == 0