fix multiline strings representation in cli yaml dumper

Author: mr-mixas

nested_diff/cli.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 #
-# Copyright 2019-2022 Michael Samoglyadov
+# Copyright 2019-2023 Michael Samoglyadov
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -472,12 +472,19 @@ def __init__(self, **kwargs):
 
         import yaml
         try:
-            from yaml import CSafeDumper as YamlDumper
+            from yaml import CSafeDumper as ImportedYamlDumper
         except ImportError:
-            from yaml import SafeDumper as YamlDumper
+            from yaml import SafeDumper as ImportedYamlDumper
+
+        class _YamlDumper(ImportedYamlDumper):
+            def represent_scalar(self, tag, value, style=None):
+                if isinstance(value, str) and '\n' in value:
+                    return super().represent_scalar(tag, value, style='|')
+
+                return super().represent_scalar(tag, value, style=style)
 
         self.yaml = yaml
-        self.yaml_dumper = YamlDumper
+        self.yaml_dumper = _YamlDumper
         self.opts = self.get_opts(kwargs)
 
     def encode(self, data):

tests/cli/shared.multiline_strings.json

@@ -0,0 +1,6 @@
+{
+   "text_with_single_line": "one",
+   "text_with_specific_symbols": "one:two\n\\nthree",
+   "text_with_trailing_lines": "one\n\n\n",
+   "text_with_two_lines": "one\ntwo"
+}

tests/cli/test_diff_tool.py

@@ -556,3 +556,18 @@ def test_values_yaml(capsys, expected, rpath):
     assert exit_code == 1
 
     assert captured.out == expected
+
+
+def test_values_yaml_multiline_strings(capsys, expected, rpath):
+    exit_code = nested_diff.diff_tool.App(args=(
+        rpath('shared.multiline_strings.json'),
+        rpath('shared.multiline_strings.json'),
+        '-U', '1',
+        '--values', 'yaml',
+    )).run()
+
+    captured = capsys.readouterr()
+    assert captured.err == ''
+    assert exit_code == 0
+
+    assert captured.out == expected

tests/cli/test_diff_tool.test_values_yaml_multiline_strings.exp

@@ -0,0 +1,11 @@
+  text_with_single_line: one
+  text_with_specific_symbols: |-
+    one:two
+    \nthree
+  text_with_trailing_lines: |+
+    one
+  
+  
+  text_with_two_lines: |-
+    one
+    two