Rewrite pool reset_on_return parsing using a util function

Choosing a util.symbol() based on a user parameter is about to have
another use case added as part of #4623, so add a generalized solution
ahead of it.

Change-Id: I420631f81af2ffc655995b9cce9ff2ac618c16d7

2 files changed, 45 insertions, 10 deletions

diff --git a/lib/sqlalchemy/pool/base.py b/lib/sqlalchemy/pool/base.py
index 761127e83..2325e7faa 100644
--- a/lib/sqlalchemy/pool/base.py
+++ b/lib/sqlalchemy/pool/base.py
@@ -191,16 +191,16 @@ class Pool(log.Identified):
         self._recycle = recycle
         self._invalidate_time = 0
         self._pre_ping = pre_ping
-        if reset_on_return in ("rollback", True, reset_rollback):
-            self._reset_on_return = reset_rollback
-        elif reset_on_return in ("none", None, False, reset_none):
-            self._reset_on_return = reset_none
-        elif reset_on_return in ("commit", reset_commit):
-            self._reset_on_return = reset_commit
-        else:
-            raise exc.ArgumentError(
-                "Invalid value for 'reset_on_return': %r" % reset_on_return
-            )
+        self._reset_on_return = util.symbol.parse_user_argument(
+            reset_on_return,
+            {
+                reset_rollback: ["rollback", True],
+                reset_none: ["none", None, False],
+                reset_commit: ["commit"],
+            },
+            "reset_on_return",
+            resolve_symbol_names=False,
+        )
 
         self.echo = echo
 
diff --git a/lib/sqlalchemy/util/langhelpers.py b/lib/sqlalchemy/util/langhelpers.py
index 0e0e3f4df..12fc5c0e8 100644
--- a/lib/sqlalchemy/util/langhelpers.py
+++ b/lib/sqlalchemy/util/langhelpers.py
@@ -1368,6 +1368,41 @@ class symbol(object):
         finally:
             symbol._lock.release()
 
+    @classmethod
+    def parse_user_argument(
+        cls, arg, choices, name, resolve_symbol_names=False
+    ):
+        """Given a user parameter, parse the parameter into a chosen symbol.
+
+        The user argument can be a string name that matches the name of a
+        symbol, or the symbol object itself, or any number of alternate choices
+        such as True/False/ None etc.
+
+        :param arg: the user argument.
+        :param choices: dictionary of symbol object to list of possible
+         entries.
+        :param name: name of the argument.   Used in an :class:`.ArgumentError`
+         that is raised if the parameter doesn't match any available argument.
+        :param resolve_symbol_names: include the name of each symbol as a valid
+         entry.
+
+        """
+        # note using hash lookup is tricky here because symbol's `__hash__`
+        # is its int value which we don't want included in the lookup
+        # explicitly, so we iterate and compare each.
+        for sym, choice in choices.items():
+            if arg is sym:
+                return sym
+            elif resolve_symbol_names and arg == sym.name:
+                return sym
+            elif arg in choice:
+                return sym
+
+        if arg is None:
+            return None
+
+        raise exc.ArgumentError("Invalid value for '%s': %r" % (name, arg))
+
 
 _creation_order = 1