Add support for SCRIPT EXISTS subcommand

I've added support for the SCRIPT EXISTS subcommand in the SCRIPTS
command handler. The subcommand takes in a sequence of sha1 hash values
of Lua scripts and checks if scripts with those sha1 hash values are
currently registered with Redis.

The command will return a redis List with the integers 1 and 0 to
indicate which sha1 hash is known to Redis. In accordance with Redis,
this subcommand returns an empty List if no hash values were passed as
arguments to the subcommand.

README.rst

@@ -310,7 +310,6 @@ scripting
 ---------
 
  * script debug
- * script exists
  * script flush
  * script kill
 

fakeredis/_server.py

@@ -2316,7 +2316,7 @@ def time(self):
         return [str(now_s).encode(), str(now_us).encode()]
 
     # Script commands
-    # TODO: script exists, script flush
+    # TODO: script flush
     # (script debug and script kill will probably not be supported)
 
     def _convert_redis_arg(self, lua_runtime, value):
@@ -2475,6 +2475,8 @@ def script(self, subcmd, *args):
             sha1 = hashlib.sha1(script).hexdigest().encode()
             self._server.script_cache[sha1] = script
             return sha1
+        elif casematch(subcmd, b'exists'):
+            return [int(sha1 in self._server.script_cache) for sha1 in args]
         else:
             raise SimpleError(BAD_SUBCOMMAND_MSG.format('SCRIPT'))
 

test/test_fakeredis.py

@@ -4486,6 +4486,26 @@ def test_script(r):
     assert result == b'42'
 
 
+def test_script_exists(r):
+    # test response for no arguments by bypassing the py-redis command
+    # as it requires at least one argument
+    assert raw_command(r, "SCRIPT EXISTS") == []
+
+    # use single character characters for non-existing scripts, as those
+    # will never be equal to an actual sha1 hash digest
+    assert r.script_exists("a") == [0]
+    assert r.script_exists("a", "b", "c", "d", "e", "f") == [0, 0, 0, 0, 0, 0]
+
+    sha1_one = r.script_load("return 'a'")
+    assert r.script_exists(sha1_one) == [1]
+    assert r.script_exists(sha1_one, "a") == [1, 0]
+    assert r.script_exists("a", "b", "c", sha1_one, "e") == [0, 0, 0, 1, 0]
+
+    sha1_two = r.script_load("return 'b'")
+    assert r.script_exists(sha1_one, sha1_two) == [1, 1]
+    assert r.script_exists("a", sha1_one, "c", sha1_two, "e", "f") == [0, 1, 0, 1, 0, 0]
+
+
 @fake_only
 def test_lua_log(r, caplog):
     logger = fakeredis._server.LOGGER