Support enforcing nonempty values in ListOf (#268)

This adds empty value validation to ListOf.

src/everett/manager.py

@@ -40,18 +40,18 @@
     "ConfigDictEnv",
     "ConfigEnvFileEnv",
     "ConfigManager",
-    "ConfigOSEnv",
     "ConfigObjEnv",
-    "ListOf",
-    "Option",
+    "ConfigOSEnv",
     "config_override",
     "get_config_for_class",
     "get_runtime_config",
+    "ListOf",
+    "Option",
     "parse_bool",
     "parse_class",
     "parse_data_size",
-    "parse_time_period",
     "parse_env_file",
+    "parse_time_period",
 ]
 
 
@@ -534,7 +534,7 @@ def get_key_from_envs(envs: Iterable[Any], key: str) -> Union[str, NoValue]:
 
 
 class ListOf:
-    """Parse a comma-separated list of things.
+    """Parse a delimiter-separated list of things.
 
     After delimiting items, this strips the whitespace at the beginning and end
     of each string. Then it passes each string into the parser to get the final
@@ -550,6 +550,25 @@ class ListOf:
     >>> ListOf(str)('1, 2  ,3,4')
     ['1', '2', '3', '4']
 
+    ``ListOf`` defaults to using a comma as a delimiter, but supports other
+    delimiters:
+
+    >>> ListOf(str, delimiter=":")("/path/a/:/path/b/")
+    ['/path/a/', '/path/b/']
+
+    ``ListOf`` supports raising a configuration error when one of the values
+    is an empty string:
+
+    >>> ListOf(str, allow_empty=False)("a,,b")
+    Traceback (most recent call last):
+      ...
+    ValueError: 'a,,b' can not have empty values
+
+    The user will get a configuration error like this::
+
+        ValueError: 'a,,b' can not have empty values
+        NAMES requires a value parseable by <ListOf(str, delimiter=',', allow_empty=False)>
+
     Note: This doesn't handle quotes or backslashes or any complicated string
     parsing.
 
@@ -560,19 +579,32 @@ class ListOf:
 
     """
 
-    def __init__(self, parser: Callable, delimiter: str = ","):
+    def __init__(
+        self, parser: Callable, delimiter: str = ",", allow_empty: bool = True
+    ):
         self.sub_parser = parser
         self.delimiter = delimiter
+        self.allow_empty = allow_empty
 
     def __call__(self, value: str) -> list[Any]:
         parser = get_parser(self.sub_parser)
         if value:
-            return [parser(token.strip()) for token in value.split(self.delimiter)]
+            parsed_values = []
+            for token in value.split(self.delimiter):
+                token = token.strip()
+                if not token and not self.allow_empty:
+                    raise ValueError(f"{value!r} can not have empty values")
+                parsed_values.append(parser(token))
+            return parsed_values
         else:
             return []
 
     def __repr__(self) -> str:
-        return f"<ListOf({qualname(self.sub_parser)})>"
+        return (
+            f"<ListOf({qualname(self.sub_parser)}, "
+            + f"delimiter={self.delimiter!r}, "
+            + f"allow_empty={self.allow_empty!r})>"
+        )
 
 
 class ChoiceOf:

tests/test_manager.py

@@ -9,32 +9,32 @@
 
 from everett import (
     ConfigurationError,
-    InvalidValueError,
     ConfigurationMissingError,
+    InvalidValueError,
     NO_VALUE,
 )
 import everett.manager
 from everett.manager import (
+    ChoiceOf,
     ConfigDictEnv,
     ConfigEnvFileEnv,
     ConfigManager,
-    ConfigOSEnv,
     ConfigObjEnv,
-    ChoiceOf,
-    ListOf,
-    Option,
+    ConfigOSEnv,
     config_override,
     generate_uppercase_key,
     get_config_for_class,
     get_key_from_envs,
     get_parser,
     get_runtime_config,
     listify,
+    ListOf,
+    Option,
     parse_bool,
     parse_class,
     parse_data_size,
-    parse_time_period,
     parse_env_file,
+    parse_time_period,
     qualname,
 )
 
@@ -53,7 +53,7 @@
         # class method
         (ConfigManager.basic_config, "everett.manager.ConfigManager.basic_config"),
         # instance
-        (ListOf(bool), "<ListOf(bool)>"),
+        (ListOf(bool), "<ListOf(bool, delimiter=',', allow_empty=True)>"),
         # instance
         (ChoiceOf(int, ["1", "10", "100"]), "<ChoiceOf(int, ['1', '10', '100'])>"),
         # instance method
@@ -279,6 +279,7 @@ def test_ListOf():
     assert ListOf(int)("1,2,3") == [1, 2, 3]
     assert ListOf(str)("1  , 2, 3") == ["1", "2", "3"]
     assert ListOf(int, delimiter=":")("1:2") == [1, 2]
+    assert ListOf(str)("a,,b") == ["a", "", "b"]
 
 
 def test_ListOf_error():
@@ -288,7 +289,20 @@ def test_ListOf_error():
 
     assert str(exc_info.value) == (
         "ValueError: 'badbool' is not a valid bool value\n"
-        "BOOLS requires a value parseable by <ListOf(bool)>"
+        "BOOLS requires a value parseable by "
+        "<ListOf(bool, delimiter=',', allow_empty=True)>"
+    )
+
+
+def test_ListOf_allow_empty_error():
+    config = ConfigManager.from_dict({"names": "bob,,alice"})
+    with pytest.raises(InvalidValueError) as exc_info:
+        config("names", parser=ListOf(str, allow_empty=False))
+
+    assert str(exc_info.value) == (
+        "ValueError: 'bob,,alice' can not have empty values\n"
+        "NAMES requires a value parseable by "
+        "<ListOf(str, delimiter=',', allow_empty=False)>"
     )
 
 

tests/test_sphinxext.py

@@ -520,7 +520,7 @@ def test_option_parser(self, tmpdir, capsys):
                user_listof
 
                   Parser:
-                     *<ListOf(str)>*
+                     *<ListOf(str, delimiter=',', allow_empty=True)>*
 
                   Required:
                      Yes