在ubuntu18.04环境下装tensorflow的时候，总是会被装到~/.local下，寻找时候很麻烦，于是决定使用虚拟环境。
pycharm可以直接创建新的虚拟环境，并且可以直接在pycharm里添加安装环境需要的库（文末会提到），不过在pycharm里操作可控性不好，而且安装tensorflow往往会出现错误，所以这里还是先不使用pycharm来创建虚拟环境。
ubuntu18.04 默认的python版本是3.6.5
先安装python3 的自带的虚拟环境创建工具：
sudo apt-get install python3-venv
然后创建一个名为venv4ML0的虚拟环境（最好到需要创建环境的路径操作）：
python3 -m venv venv4ML0
进入创建的该环境：
source ./venv4ML0/bin/activate
进入之后命令行头部会有标识：
(venv4ML0) yangguang@yangguang-Precision-Tower-7910:~/MachineLearning$
接下来直接安装tensorflow，有一些依赖库在安装tensorflow的时候会自动安装，例如numpy1.14.3，基本上自动安装的都是最新版，所以比自己装方便多了：
pip3 install tensorflow==1.8.0
不过装的时候容易出以下的错误：
(venv4ML0) yangguang@yangguang-Precision-Tower-7910:~/MachineLearning$ pip3 install tensorflow==1.8.0
Collecting tensorflow==1.8.0
Using cached https://files.pythonhosted.org/packages/22/c6/d08f7c549330c2acc1b18b5c1f0f8d9d2af92f54d56861f331f372731671/tensorflow-1.8.0-cp36-cp36m-manylinux1_x86_64.whl
Collecting tensorboard<1.9.0,>=1.8.0 (from tensorflow==1.8.0)
Using cached https://files.pythonhosted.org/packages/59/a6/0ae6092b7542cfedba6b2a1c9b8dceaf278238c39484f3ba03b03f07803c/tensorboard-1.8.0-py3-none-any.whl
Collecting astor>=0.6.0 (from tensorflow==1.8.0)
Using cached https://files.pythonhosted.org/packages/b2/91/cc9805f1ff7b49f620136b3a7ca26f6a1be2ed424606804b0fbcf499f712/astor-0.6.2-py2.py3-none-any.whl
Collecting wheel>=0.26 (from tensorflow==1.8.0)
Using cached https://files.pythonhosted.org/packages/81/30/e935244ca6165187ae8be876b6316ae201b71485538ffac1d718843025a9/wheel-0.31.1-py2.py3-none-any.whl
Collecting termcolor>=1.1.0 (from tensorflow==1.8.0)
Collecting numpy>=1.13.3 (from tensorflow==1.8.0)
Using cached https://files.pythonhosted.org/packages/71/90/ca61e203e0080a8cef7ac21eca199829fa8d997f7c4da3e985b49d0a107d/numpy-1.14.3-cp36-cp36m-manylinux1_x86_64.whl
Collecting absl-py>=0.1.6 (from tensorflow==1.8.0)
Requirement already satisfied: six>=1.10.0 in ./venv4ML0/lib/python3.6/site-packages (from tensorflow==1.8.0)
Requirement already satisfied: grpcio>=1.8.6 in ./venv4ML0/lib/python3.6/site-packages (from tensorflow==1.8.0)
Collecting gast>=0.2.0 (from tensorflow==1.8.0)
Requirement already satisfied: protobuf>=3.4.0 in ./venv4ML0/lib/python3.6/site-packages (from tensorflow==1.8.0)
Collecting werkzeug>=0.11.10 (from tensorboard<1.9.0,>=1.8.0->tensorflow==1.8.0)
Exception:
Traceback (most recent call last):
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 350, in _make_request
self._validate_conn(conn)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 837, in _validate_conn
conn.connect()
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connection.py", line 323, in connect
ssl_context=context)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/util/ssl_.py", line 324, in ssl_wrap_socket
return context.wrap_socket(sock, server_hostname=server_hostname)
File "/usr/lib/python3.6/ssl.py", line 407, in wrap_socket
_context=self, _session=session)
File "/usr/lib/python3.6/ssl.py", line 814, in __init__
self.do_handshake()
File "/usr/lib/python3.6/ssl.py", line 1068, in do_handshake
self._sslobj.do_handshake()
File "/usr/lib/python3.6/ssl.py", line 689, in do_handshake
self._sslobj.do_handshake()
socket.timeout: _ssl.c:817: The handshake operation timed out
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 594, in urlopen
chunked=chunked)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 353, in _make_request
self._raise_timeout(err=e, url=url, timeout_value=conn.timeout)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 313, in _raise_timeout
raise ReadTimeoutError(self, url, "Read timed out. (read timeout=%s)" % timeout_value)
requests.packages.urllib3.exceptions.ReadTimeoutError: HTTPSConnectionPool(host='pypi.org', port=443): Read timed out. (read timeout=15)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/basecommand.py", line 215, in main
status = self.run(options, args)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/commands/install.py", line 353, in run
wb.build(autobuilding=True)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/wheel.py", line 749, in build
self.requirement_set.prepare_files(self.finder)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/req/req_set.py", line 380, in prepare_files
ignore_dependencies=self.ignore_dependencies))
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/req/req_set.py", line 554, in _prepare_file
require_hashes
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/req/req_install.py", line 278, in populate_link
self.link = finder.find_requirement(self, upgrade)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/index.py", line 465, in find_requirement
all_candidates = self.find_all_candidates(req.name)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/index.py", line 423, in find_all_candidates
for page in self._get_pages(url_locations, project_name):
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/index.py", line 568, in _get_pages
page = self._get_page(location)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/index.py", line 683, in _get_page
return HTMLPage.get_page(link, session=self.session)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/index.py", line 792, in get_page
"Cache-Control": "max-age=600",
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 501, in get
return self.request('GET', url, **kwargs)
File "/home/yangguang/MachineLearning/venv4ML0/lib/python3.6/site-packages/pip/download.py", line 386, in request
return super(PipSession, self).request(method, url, *args, **kwargs)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 488, in request
resp = self.send(prep, **send_kwargs)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 630, in send
history = [resp for resp in gen] if allow_redirects else []
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 630, in <listcomp>
history = [resp for resp in gen] if allow_redirects else []
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 190, in resolve_redirects
**adapter_kwargs
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/sessions.py", line 609, in send
r = adapter.send(request, **kwargs)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/CacheControl-0.11.7-py2.py3-none-any.whl/cachecontrol/adapter.py", line 47, in send
resp = super(CacheControlAdapter, self).send(request, **kw)
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/requests-2.12.4-py2.py3-none-any.whl/requests/adapters.py", line 423, in send
timeout=timeout
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 643, in urlopen
_stacktrace=sys.exc_info()[2])
File "/home/yangguang/MachineLearning/venv4ML0/share/python-wheels/urllib3-1.19.1-py2.py3-none-any.whl/urllib3/util/retry.py", line 315, in increment
total -= 1
TypeError: unsupported operand type(s) for -=: 'Retry' and 'int'
个人觉得可能是网络的问题，遇到这种情况就单独装出错处的lib，笔者出现过多次出错，但是重复安装就成功了：
pip3 install Werkzeug==0.14.1
然后再一遍：
pip3 install tensorflow==1.8.0
可能还会有其他同样的问题，同样处理，直到pip3 install tensorflow==1.8.0运行成功！
这样tensorflow就装好了，可以使用hello world测试一下，此处不做说明。
现在打开pycharm，File->Setting->Project Interpreter->设置->Existing environment->选择该虚拟环境路径 bin下的python3即可，导入虚拟环境后pycharm显示：
可以直接点击右边的+进行添加新的库，也可以一开始就在这里安装虚拟环境里的tensorflow，有兴趣可以尝试，不过我这样操作的体验并不好。
测试：
安装一些其他依赖的模块：
pip3 install matplotlib --upgrade
pip3 install scikit-learn --upgrade
pip3 install scikit-image --upgrade
使用完环境退出虚拟环境：
deactivate
使用这种方法创建虚拟环境不会导入系统python环境里pip安装的包（ubuntu18.04会将pip安装的包存在~/.local中），默认使用最干净的环境。
可以使用pip freeze > requirements.txt 将当前环境中的包的名称及版本导出到文本中
然后在另一个环境中 pip install -r requirements.txt 安装，虚拟环境会自动创建和系统环境里包的链接，不需要重新下载一遍。
如果从系统环境中导出，会有很多没有用的lib，可以在requirement.txt里删除，或者自己手动编辑一个requirement.txt文件。
如果同时需要python2的开发环境，可以再创建一个python2的虚拟环境，这个虚拟环境和python3的虚拟环境完全隔离：
sudo apt-get install python-virtualenv
virtualenv -p /usr/bin/python2 py2env4ML
之后的操作和python3的虚拟环境一样。