Fix: detype None env-vars to '' to fix #4600 (#4649)

* detype untyped and path env-vars to ""; path_to_str("") == None

* update env-var-tutorial, add news

* update test_expandvars, test_expand_path, test_register_custom_var_str

* trigger CI-rerun since unrelated osx test failed

Co-authored-by: Alexander Firbas <alexander.firbas@gmail.com>

docs/tutorial.rst

@@ -211,12 +211,20 @@ simple:
 
 * ``\w*PATH``: any variable whose name ends in PATH is a list of strings.
 * ``\w*DIRS``: any variable whose name ends in DIRS is a list of strings.
-* ``XONSH_HISTORY_SIZE``: this variable is an int.
-* ``CASE_SENSITIVE_COMPLETIONS``: this variable is a boolean.
+
+Futhermore, a number of predefined environment variables listed `here <envvars.html>`_ have a static type.
+For example,
+* ``XONSH_HISTORY_SIZE``: is an int, and
+* ``CASE_SENSITIVE_COMPLETIONS``: is a boolean.
 
 xonsh will automatically convert back and forth to untyped (string-only)
 representations of the environment as needed (mostly by subprocess commands).
-When in xonsh, you'll always have the typed version.  Here are a couple of
+When in xonsh, you'll always have the typed version.
+
+Variables that do not match the rules above are converted to strings using ``str``,
+except they are ``None``. In this case the empty string is used.
+
+Here are a couple of
 PATH examples:
 
 .. code-block:: xonshcon

news/fix_env_None.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* Environment-variables of no predefined type or path environment variables are now represented as strings via the empty string. 
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

tests/test_environ.py

@@ -443,7 +443,7 @@ def test_register_custom_var_bool(val, converted):
         (32, "32"),
         (0, "0"),
         (27.0, "27.0"),
-        (None, "None"),
+        (None, ""),
         ("lol", "lol"),
         ("false", "false"),
         ("no", "no"),

tests/test_tools.py

@@ -1627,8 +1627,8 @@ def test_expand_case_matching(inp, exp):
         ("$unk $foo $unk $spam $unk", "$unk bar $unk eggs $unk"),
         ("$an_int$spam$a_bool", "42eggsTrue"),
         ("$unk$an_int$spam$a_bool", "$unk42eggsTrue"),
-        ("bar$foo$spam$foo $an_int $none", "barbareggsbar 42 None"),
-        ("$unk bar$foo$spam$foo $an_int $none", "$unk barbareggsbar 42 None"),
+        ("bar$foo$spam$foo $an_int $none", "barbareggsbar 42 "),
+        ("$unk bar$foo$spam$foo $an_int $none", "$unk barbareggsbar 42 "),
         ("$foo/bar", "bar/bar"),
         ("$unk/$foo/bar", "$unk/bar/bar"),
         ("${'foo'} $spam", "bar eggs"),
@@ -1688,7 +1688,7 @@ def test_ensure_timestamp(inp, fmt, exp, xession):
         ("~/$foo", True, "/bar"),
         ("~/test/$a_bool", True, "/test/True"),
         ("~/test/$an_int", True, "/test/42"),
-        ("~/test/$none", True, "/test/None"),
+        ("~/test/$none", True, "/test/"),
         ("~/$foo", False, "/$foo"),
     ],
 )

xonsh/tools.py

@@ -1226,8 +1226,8 @@ def always_none(x):
 
 
 def ensure_string(x):
-    """Returns a string if x is not a string, and x if it already is."""
-    return str(x)
+    """Returns a string if x is not a string, and x if it already is. If x is None, the empty string is returned."""
+    return str(x) if x is not None else ""
 
 
 def is_path(x):
@@ -1242,11 +1242,10 @@ def is_env_path(x):
 
 def str_to_path(x):
     """Converts a string to a path."""
-    if x is None:
+    if x is None or x == "":
         return None
     elif isinstance(x, str):
-        # checking x is needed to avoid uncontrolled converting empty string to Path('.')
-        return pathlib.Path(x) if x else None
+        return pathlib.Path(x)
     elif isinstance(x, pathlib.Path):
         return x
     elif isinstance(x, EnvPath) and len(x) == 1:
@@ -1267,7 +1266,7 @@ def str_to_env_path(x):
 
 def path_to_str(x):
     """Converts a path to a string."""
-    return str(x)
+    return str(x) if x is not None else ""
 
 
 def env_path_to_str(x):