Normalize option values additionally by type

Now that the contract has narrowed for `utils.normalize_paths()`
and `utils.parse_comma_separated_list()`, `Option.normalize()` must
handle when the option value is either a singular value or a sequence
(i.e., `list`) of values.

The paths where `Option.normalize()` is called are:

1. options/config.py: `MergedConfigParser.parse_*_config()`
    There are 3 paths wehre eventually `_normalize_value()` is called.
2. options/manager.py: `OptionManager.parse_args()`

For (1), values are coming from the `configparser` module.  For (2),
values are coming from `optparse.OptionParser`.

Rudimentary investigation seems to implicate that
`optparse.OptionParser.parse_args()` always returns values in a `list`
because it accumulates values.  However, for `configparser`, the values
are a string due to the key-value nature of the INI format.

Given that `Option.normalize()` is the convergence point where
normalization of an option occurs, it is acceptable for the method to
also handle the parsing comma-separated values and path normalization by
the option value's type.

1 files changed, 1 insertions, 3 deletions

diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index 5ed78e1..619c83d 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -39,9 +39,7 @@ def parse_comma_separated_list(value, regexp=COMMA_SEPARATED_LIST_RE):
     :rtype:
         list
     """
-    assert isinstance(  # nosec (for bandit)
-        value, (string_types, type(None))
-    ), value
+    assert isinstance(value, string_types), value  # nosec (for bandit)
 
     if not value:
         return []