解决pip超时的问题

pip下载包太久超时的话，会出现如下情况：

Collecting pip
  Downloading pip-9.0.1-py2.py3-none-any.whl (1.3MB)
    0% |                                | 4.1kB 17.7MB/s eta 0:00:01Exception:
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/basecommand.py", line 223, in main
    status = self.run(options, args)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/commands/install.py", line 282, in run
    requirement_set.prepare_files(finder)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/req/req_set.py", line 334, in prepare_files
    functools.partial(self._prepare_file, finder))
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/req/req_set.py", line 321, in _walk_req_to_install
    more_reqs = handler(req_to_install)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/req/req_set.py", line 491, in _prepare_file
    session=self.session)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/download.py", line 825, in unpack_url
    session,
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/download.py", line 673, in unpack_http_url
    from_path, content_type = _download_http_url(link, session, temp_dir)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/download.py", line 886, in _download_http_url
    _download_url(resp, link, content_file)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/download.py", line 621, in _download_url
    for chunk in progress_indicator(resp_read(4096), 4096):
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/utils/ui.py", line 133, in iter
    for x in it:
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/download.py", line 586, in resp_read
    decode_content=False):
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/_vendor/requests/packages/urllib3/response.py", line 307, in stream
    data = self.read(amt=amt, decode_content=decode_content)
  File "/Library/Python/2.7/site-packages/pip-7.1.0-py2.7.egg/pip/_vendor/requests/packages/urllib3/response.py", line 267, in read
    raise ReadTimeoutError(self._pool, None, 'Read timed out.')
ReadTimeoutError: HTTPSConnectionPool(host='pypi.python.org', port=443): Read timed out.

解决方法有3种：
1.手动换源（推荐）

手动指定源：

在 pip 后面跟-i 来指定源，比如用豆瓣的源来安装 web.py 框架：

pip install numpy -i https://pypi.doubanio.com/simple/

注意后面要有 /simple/ 目录

推荐的源除了豆瓣之外，还推荐清华大学的tuna源：

pip install numpy -i https://pypi.tuna.tsinghua.edu.cn/simple/

2.更改默认设置（不推荐）

需要创建或修改配置文件（ linux 的文件在~/.pip/pip.conf ， windows 在%HOMEPATH%\pip\pip.ini ），修改内容为：

[global]
index-url = http://mirrors.aliyun.com/pypi/simple/
[install]
trusted-host=mirrors.aliyun.com

如果不加后面的 install

则需要每次在命令后面加上 --trusted-host

3. 手动设置延时

需要看包的大小，有时可以下载下来，有时会卡住

pip --default-timeout=100 install numpy