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 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/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 refused

During 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.

这个问题是由于没安装Redis服务器形成的。
解决办法：

sudo apt-get install redis-server

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