diff --git a/CHANGELOG.md b/CHANGELOG.md
index e4b81353..0852d66e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,19 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this
 project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+- Raise `ValueError` if `quote_mode` isn't one of `always`, `auto` or `never` in
+  `set_key` (#330 by [@bbc2]).
+- When writing a value to a .env file with `set_key` or `dotenv set <key> <value>` (#330
+  by [@bbc2]):
+  - Use single quotes instead of double quotes.
+  - Don't strip surrounding quotes.
+  - In `auto` mode, don't add quotes if the value is only made of alphanumeric characters
+    (as determined by `string.isalnum`).
+
 ## [0.17.1] - 2021-04-29
 
 ### Fixed
diff --git a/src/dotenv/main.py b/src/dotenv/main.py
index 16f22d2c..b85836a5 100644
--- a/src/dotenv/main.py
+++ b/src/dotenv/main.py
@@ -151,13 +151,16 @@ def set_key(dotenv_path, key_to_set, value_to_set, quote_mode="always", export=F
     If the .env path given doesn't exist, fails instead of risking creating
     an orphan .env somewhere in the filesystem
     """
-    value_to_set = value_to_set.strip("'").strip('"')
+    if quote_mode not in ("always", "auto", "never"):
+        raise ValueError("Unknown quote_mode: {}".format(quote_mode))
 
-    if " " in value_to_set:
-        quote_mode = "always"
+    quote = (
+        quote_mode == "always"
+        or (quote_mode == "auto" and not value_to_set.isalnum())
+    )
 
-    if quote_mode == "always":
-        value_out = '"{}"'.format(value_to_set.replace('"', '\\"'))
+    if quote:
+        value_out = "'{}'".format(value_to_set.replace("'", "\\'"))
     else:
         value_out = value_to_set
     if export:
diff --git a/tests/test_cli.py b/tests/test_cli.py
index bc6b8d47..d2558234 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -73,10 +73,11 @@ def test_unset_non_existent_value(cli, dotenv_file):
 @pytest.mark.parametrize(
     "quote_mode,variable,value,expected",
     (
-        ("always", "HELLO", "WORLD", 'HELLO="WORLD"\n'),
-        ("never", "HELLO", "WORLD", 'HELLO=WORLD\n'),
-        ("auto", "HELLO", "WORLD", 'HELLO=WORLD\n'),
-        ("auto", "HELLO", "HELLO WORLD", 'HELLO="HELLO WORLD"\n'),
+        ("always", "a", "x", "a='x'\n"),
+        ("never", "a", "x", 'a=x\n'),
+        ("auto", "a", "x", "a=x\n"),
+        ("auto", "a", "x y", "a='x y'\n"),
+        ("auto", "a", "$", "a='$'\n"),
     )
 )
 def test_set_quote_options(cli, dotenv_file, quote_mode, variable, value, expected):
@@ -92,8 +93,8 @@ def test_set_quote_options(cli, dotenv_file, quote_mode, variable, value, expect
 @pytest.mark.parametrize(
     "dotenv_file,export_mode,variable,value,expected",
     (
-        (".nx_file", "true", "HELLO", "WORLD", "export HELLO=\"WORLD\"\n"),
-        (".nx_file", "false", "HELLO", "WORLD", "HELLO=\"WORLD\"\n"),
+        (".nx_file", "true", "a", "x", "export a='x'\n"),
+        (".nx_file", "false", "a", "x", "a='x'\n"),
     )
 )
 def test_set_export(cli, dotenv_file, export_mode, variable, value, expected):
diff --git a/tests/test_main.py b/tests/test_main.py
index b927d7f2..f36f7340 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -28,18 +28,18 @@ def test_set_key_no_file(tmp_path):
 @pytest.mark.parametrize(
     "before,key,value,expected,after",
     [
-        ("", "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'c", (True, "a", "b'c"), 'a="b\'c"\n'),
-        ("", "a", "b\"c", (True, "a", "b\"c"), 'a="b\\\"c"\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'),
-        ("a=b\nc=d", "a", "e", (True, "a", "e"), 'a="e"\nc=d'),
-        ("a=b\nc=d\ne=f", "c", "g", (True, "c", "g"), 'a=b\nc="g"\ne=f'),
-        ("a=b\n", "c", "d", (True, "c", "d"), 'a=b\nc="d"\n'),
+        ("", "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'c", (True, "a", "b'c"), "a='b\\'c'\n"),
+        ("", "a", "b\"c", (True, "a", "b\"c"), "a='b\"c'\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"),
+        ("a=b\nc=d", "a", "e", (True, "a", "e"), "a='e'\nc=d"),
+        ("a=b\nc=d\ne=f", "c", "g", (True, "c", "g"), "a=b\nc='g'\ne=f"),
+        ("a=b\n", "c", "d", (True, "c", "d"), "a=b\nc='d'\n"),
     ],
 )
 def test_set_key(dotenv_file, before, key, value, expected, after):