fix: write set_key values containing single quotes as double-quoted

CHANGELOG.md

@@ -10,6 +10,7 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### Fixed
 
 - An unquoted empty value followed by an inline comment (e.g. `KEY= # comment`) is now parsed as an empty string instead of the comment text by [@Noethix55555] in [#663]
+- `set_key` now writes values containing `'` double-quoted, so the resulting line is valid shell and can be `source`d by [@SAY-5] in [#647]
 
 ## [1.2.3] - 2026-08-16
 
@@ -446,6 +447,7 @@ os.PathLike]` instead of just `os.PathLike` (#347 by [@bbc2]).
 [#606]: https://github.com/theskumar/python-dotenv/pull/606
 [#638]: https://github.com/theskumar/python-dotenv/pull/638
 [#640]: https://github.com/theskumar/python-dotenv/pull/640
+[#647]: https://github.com/theskumar/python-dotenv/pull/647
 [#663]: https://github.com/theskumar/python-dotenv/pull/663
 [#680]: https://github.com/theskumar/python-dotenv/pull/680
 [790c5c0]: https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311
@@ -489,6 +491,7 @@ os.PathLike]` instead of just `os.PathLike` (#347 by [@bbc2]).
 [@mgorny]: https://github.com/mgorny
 [@naorlivne]: https://github.com/naorlivne
 [@Noethix55555]: https://github.com/Noethix55555
+[@SAY-5]: https://github.com/SAY-5
 [@qnighy]: https://github.com/qnighy
 [@rabinadk1]: https://github.com/rabinadk1
 [@randomseed42]: https://github.com/randomseed42

src/dotenv/main.py

@@ -216,12 +216,18 @@ def set_key(
     )
 
     if quote:
-        # The single-quoted-value parser decodes `\\` and `\'`, so both have to
-        # be escaped here for the value to survive a write/read round-trip.
-        # Backslashes first, otherwise the backslash added by the quote
-        # escaping would be escaped in turn.
-        escaped = value_to_set.replace("\\", "\\\\").replace("'", "\\'")
-        value_out = f"'{escaped}'"
+        if "'" in value_to_set:
+            # A single quote cannot be escaped inside a single-quoted shell
+            # value, so values containing one are written double-quoted to
+            # keep the file source-able.
+            escaped = value_to_set.replace("\\", "\\\\").replace('"', '\\"')
+            value_out = f'"{escaped}"'
+        else:
+            # The single-quoted-value parser decodes `\\`, so backslashes have
+            # to be escaped here for the value to survive a write/read
+            # round-trip.
+            escaped = value_to_set.replace("\\", "\\\\")
+            value_out = f"'{escaped}'"
     else:
         value_out = value_to_set
     if export:

tests/test_main.py

@@ -29,10 +29,11 @@ def test_set_key_no_file(tmp_path):
     [
         ("", "a", "", (True, "a", ""), "a=''\n"),
         ("", "a", "b", (True, "a", "b"), "a='b'\n"),
-        ("", "a", "'b'", (True, "a", "'b'"), "a='\\'b\\''\n"),
+        ("", "a", "'b'", (True, "a", "'b'"), "a=\"'b'\"\n"),
         ("", "a", '"b"', (True, "a", '"b"'), "a='\"b\"'\n"),
-        ("", "a", "b'c", (True, "a", "b'c"), "a='b\\'c'\n"),
+        ("", "a", "b'c", (True, "a", "b'c"), 'a="b\'c"\n'),
         ("", "a", 'b"c', (True, "a", 'b"c'), "a='b\"c'\n"),
+        ("", "a", 'I\'m "in"', (True, "a", 'I\'m "in"'), 'a="I\'m \\"in\\""\n'),
         ("a=b", "a", "c", (True, "a", "c"), "a='c'\n"),
         ("a=b\n", "a", "c", (True, "a", "c"), "a='c'\n"),
         ("a=b\n\n", "a", "c", (True, "a", "c"), "a='c'\n\n"),
@@ -42,7 +43,7 @@ def test_set_key_no_file(tmp_path):
         ("a=b", "c", "d", (True, "c", "d"), "a=b\nc='d'\n"),
         ("", "a", "b\\c", (True, "a", "b\\c"), "a='b\\\\c'\n"),
         ("", "a", "b\\", (True, "a", "b\\"), "a='b\\\\'\n"),
-        ("", "a", "b\\'c", (True, "a", "b\\'c"), "a='b\\\\\\'c'\n"),
+        ("", "a", "b\\'c", (True, "a", "b\\'c"), 'a="b\\\\\'c"\n'),
     ],
 )
 def test_set_key(dotenv_path, before, key, value, expected, after):