"ensurepip" --- Bootstrapping the "pip" installer
*************************************************

3.4 新版功能.

======================================================================

The "ensurepip" package provides support for bootstrapping the "pip"
installer into an existing Python installation or virtual environment.
This bootstrapping approach reflects the fact that "pip" is an
independent project with its own release cycle, and the latest
available stable version is bundled with maintenance and feature
releases of the CPython reference interpreter.

In most cases, end users of Python shouldn't need to invoke this
module directly (as "pip" should be bootstrapped by default), but it
may be needed if installing "pip" was skipped when installing Python
(or when creating a virtual environment) or after explicitly
uninstalling "pip".

注解: This module *does not* access the internet. All of the
  components needed to bootstrap "pip" are included as internal parts
  of the package.

参见:

  安装 Python 模块
     The end user guide for installing Python packages

  **PEP 453**: Explicit bootstrapping of pip in Python installations
     The original rationale and specification for this module.


Command line interface
======================

The command line interface is invoked using the interpreter's "-m"
switch.

The simplest possible invocation is:

   python -m ensurepip

This invocation will install "pip" if it is not already installed, but
otherwise does nothing. To ensure the installed version of "pip" is at
least as recent as the one bundled with "ensurepip", pass the "--
upgrade" option:

   python -m ensurepip --upgrade

By default, "pip" is installed into the current virtual environment
(if one is active) or into the system site packages (if there is no
active virtual environment). The installation location can be
controlled through two additional command line options:

* "--root <dir>": Installs "pip" relative to the given root
  directory rather than the root of the currently active virtual
  environment (if any) or the default root for the current Python
  installation.

* "--user": Installs "pip" into the user site packages directory
  rather than globally for the current Python installation (this
  option is not permitted inside an active virtual environment).

By default, the scripts "pipX" and "pipX.Y" will be installed (where
X.Y stands for the version of Python used to invoke "ensurepip"). The
scripts installed can be controlled through two additional command
line options:

* "--altinstall": if an alternate installation is requested, the
  "pipX" script will *not* be installed.

* "--default-pip": if a "default pip" installation is requested, the

     "pip" script will be installed in addition to the two regular
     scripts.

Providing both of the script selection options will trigger an
exception.


Module API
==========

"ensurepip" exposes two functions for programmatic use:

ensurepip.version()

   Returns a string specifying the bundled version of pip that will be
   installed when bootstrapping an environment.

ensurepip.bootstrap(root=None, upgrade=False, user=False, altinstall=False, default_pip=False, verbosity=0)

   Bootstraps "pip" into the current or designated environment.

   *root* specifies an alternative root directory to install relative
   to. If *root* is "None", then installation uses the default install
   location for the current environment.

   *upgrade* indicates whether or not to upgrade an existing
   installation of an earlier version of "pip" to the bundled version.

   *user* indicates whether to use the user scheme rather than
   installing globally.

   By default, the scripts "pipX" and "pipX.Y" will be installed
   (where X.Y stands for the current version of Python).

   If *altinstall* is set, then "pipX" will *not* be installed.

   If *default_pip* is set, then "pip" will be installed in addition
   to the two regular scripts.

   Setting both *altinstall* and *default_pip* will trigger
   "ValueError".

   *verbosity* controls the level of output to "sys.stdout" from the
   bootstrapping operation.

   Raises an auditing event "ensurepip.bootstrap" with argument
   "root".

   注解: The bootstrapping process has side effects on both
     "sys.path" and "os.environ". Invoking the command line interface
     in a subprocess instead allows these side effects to be avoided.

   注解: The bootstrapping process may install additional modules
     required by "pip", but other software should not assume those
     dependencies will always be present by default (as the
     dependencies may be removed in a future version of "pip").
