在CentOS 7上安装Python 2.7以及virtualenv环境

通过Yum安装（EPEL源）

yum -y update
yum -y install python-pip

通过pip脚本安装

• 安装升级pip - 这个方法是通用安装pip方法，适合所有python环境

参考 PyPA » pip 20.1.1 documentation » Installation

注意：如果系统没有setuptools和wheel，则get-pip.py会自动安装

# First get the script:
curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py

# Then execute it using Python 2.7 and/or Python 3.6:
python2.7 get-pip.py
#python3.6 get-pip.py

# With pip installed you can now do things like this:
pip2.7 install [packagename]
pip2.7 install --upgrade [packagename]
pip2.7 uninstall [packagename]

如果系统过于陈旧，可能存在ssl证书报错：

Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirming the ssl certificate: HTT
PSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLEr
ror(CertificateError("hostname 'pypi.org' doesn't match either of 'www.python.org', '*.python.org', 'docs
.python.org', 'downloads.python.org', 'pypi.python.org'",),)) - skipping

解决方法参考 Pip: Could not fetch URL for pypi and issue confirming the ssl certificate

• 安装虚拟环境virtualenv

# Install virtualenv for Python 2.7 and create a sandbox called "venv2"
pip2.7 install virtualenv  # need root privilege
virtualenv venv2    # normal user's privilege

# Activate the venv2 sandbox:
source venv2/bin/activate
# Check the Python version in the sandbox (it should be Python 2.7.14):
python --version
# Deactivate the sandbox:
deactivate

我在执行 virtualenv venv2 时候遇到一个报错

Traceback (most recent call last):
  File "/bin/virtualenv", line 7, in <module>
    from virtualenv.__main__ import run_with_catch
  File "/usr/lib/python2.7/site-packages/virtualenv/__init__.py", line 3, in <module>
    from .run import cli_run, session_via_cli
  File "/usr/lib/python2.7/site-packages/virtualenv/run/__init__.py", line 7, in <module>
    from ..app_data import make_app_data
  File "/usr/lib/python2.7/site-packages/virtualenv/app_data/__init__.py", line 9, in <module>
    from platformdirs import user_data_dir
ImportError: No module named platformdirs

这个报错原因是因为系统太陈旧，pip版本甚至无法升级

··· You are using pip version 7.1.0, however version 21.3.1 is available. You should consider upgrading via the 'pip install --upgrade pip' command. Collecting pip Using cached https://files.pythonhosted.org/packages/da/f6/c83229dcc3635cdeb51874184241a9508ada15d8baa3 37a41093fab58011/pip-21.3.1.tar.gz Complete output from command python setup.py egg_info: Traceback (most recent call last): File "", line 20, in File "/tmp/pip-build-wfciDf/pip/setup.py", line 7 def read(rel_path: str) -> str: ^ SyntaxError: invalid syntax

----------------------------------------

Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-wfciDf/pip

所以改为手工升级

```bash
curl https://bootstrap.pypa.io/pip/2.7/get-pip.py -o get-pip.py
python get-pip.py

参考

• How to Install Pip on CentOS 7