redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused.

$ pip install redis>>> import redis>>> conn = redis.Redis()>>> conn.keys('*')Traceback (most recent call last):  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 484, in connect    sock = self._connect()  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 541, in _connect    raise err  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 529, in _connect    sock.connect(socket_address)ConnectionRefusedError: [Errno 111] Connection refusedDuring handling of the above exception, another exception occurred:Traceback (most recent call last):  File "/home/henry/.local/lib/python3.5/site-packages/redis/client.py", line 667, in execute_command    connection.send_command(*args)  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 610, in send_command    self.send_packed_command(self.pack_command(*args))  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 585, in send_packed_command    self.connect()  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 489, in connect    raise ConnectionError(self._error_message(e))redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused.During handling of the above exception, another exception occurred:Traceback (most recent call last):  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 484, in connect    sock = self._connect()  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 541, in _connect    raise err  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 529, in _connect    sock.connect(socket_address)ConnectionRefusedError: [Errno 111] Connection refusedDuring handling of the above exception, another exception occurred:Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "/home/henry/.local/lib/python3.5/site-packages/redis/client.py", line 1032, in keys    return self.execute_command('KEYS', pattern)  File "/home/henry/.local/lib/python3.5/site-packages/redis/client.py", line 673, in execute_command    connection.send_command(*args)  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 610, in send_command    self.send_packed_command(self.pack_command(*args))  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 585, in send_packed_command    self.connect()  File "/home/henry/.local/lib/python3.5/site-packages/redis/connection.py", line 489, in connect    raise ConnectionError(self._error_message(e))redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused.

這個問題是因為沒安裝Redis伺服器造成的。
解決辦法：

sudo apt-get install redis-server

參考文獻：
1. Rescue : Connection refused - Unable to connect to Redis on localhost:6379。