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How do I make pip use my current python installation? [duplicate]

Is there any way to make pip play well with multiple versions of Python? For example, I want to use pip to explicitly install things to either my site 2.5 installation or my site 2.6 installation.
For example, with easy_install, I use easy_install-2.{5,6}.
And, yes — I know about virtualenv, and no — it's not a solution to this particular problem.

The current recommendation is to use python -m pip, where python is the version of Python you would like to use. This is the recommendation because it works across all versions of Python, and in all forms of virtualenv. For example:
# The system default python:
$ python -m pip install fish
# A virtualenv's python:
$ .env/bin/python -m pip install fish
# A specific version of python:
$ python-3.6 -m pip install fish
Previous answer, left for posterity:
Since version 0.8, Pip supports pip-{version}. You can use it the same as easy_install-{version}:
$ pip-2.5 install myfoopackage
$ pip-2.6 install otherpackage
$ pip-2.7 install mybarpackage
EDIT: pip changed its schema to use pipVERSION instead of pip-VERSION in version 1.5. You should use the following if you have pip >= 1.5:
$ pip2.6 install otherpackage
$ pip2.7 install mybarpackage
Check https://github.com/pypa/pip/pull/1053 for more details
References:
https://github.com/pypa/pip/issues/200
http://www.pip-installer.org/docs/pip/en/0.8.3/news.html#id4
https://pip.pypa.io/en/stable/news/#v0-8 or
https://web.archive.org/web/20140310013920/http://www.pip-installer.org:80/docs/pip/en/0.8.3/news.html#id4

On Windows, you can execute the pip module using a given Python version through the Python launcher, py.exe, if you chose to install it during Python 3 setup.
py -3 -m pip install packagename
py -2 -m pip install packagename
You can be even more specific and request an exact sub-version of Python:
py -3.6 -m pip install packagename
To get a list of all installed Python versions available through the launcher, run:
py --list
Alternatively, you can launch the desired Python executable directly:
C:/path/to/specific/python.exe -m pip install packagename

/path/to/python2.{5,6} /path/to/pip install PackageName doesn't work?
For this to work on any python version that doesn't have pip already installed you need to download pip and do python*version* setup.py install. For example python3.3 setup.py install. This resolves the import error in the comments. (As suggested by #hbdgaf)

I had python 2.6 installed by default (Amazon EC2 AMI), but needed python2.7 plus some external packages for my application. Assuming you already installed python2.7 alongside with default python (2.6 in my case). Here is how to install pip and packages for non-default python2.7
Install pip for your python version:
curl -O https://bootstrap.pypa.io/get-pip.py
python27 get-pip.py
Use specific pip version to install packages:
pip2.7 install mysql-connector-python --allow-external mysql-connector-python

It worked for me in windows this way:
I changed the name of python files python.py and pythonw.exe to python3.py pythonw3.py
Then I just ran this command in the prompt:
python3 -m pip install package

Other answers show how to use pip with both 2.X and 3.X Python, but does not show how to handle the case of multiple Python distributions (eg. original Python and Anaconda Python).
I have a total of 3 Python versions: original Python 2.7 and Python 3.5 and Anaconda Python 3.5.
Here is how I install a package into:
Original Python 3.5:
/usr/bin/python3 -m pip install python-daemon
Original Python 2.7:
/usr/bin/python -m pip install python-daemon
Anaconda Python 3.5:
python3 -m pip install python-daemon
or
pip3 install python-daemon
Simpler, as Anaconda overrides original Python binaries in user environment.
Of course, installing in anaconda should be done with conda command, this is just an example.
Also, make sure that pip is installed for that specific python.You might need to manually install pip. This works in Ubuntu 16.04:
sudo apt-get install python-pip
or
sudo apt-get install python3-pip

From here: https://docs.python.org/3/installing/
Here is how to install packages for various versions that are installed at the same time linux, mac, posix:
python2 -m pip install SomePackage # default Python 2
python2.7 -m pip install SomePackage # specifically Python 2.7
python3 -m pip install SomePackage # default Python 3
python3.4 -m pip install SomePackage # specifically Python 3.4
python3.5 -m pip install SomePackage # specifically Python 3.5
python3.6 -m pip install SomePackage # specifically Python 3.6
On Windows, use the py Python launcher in combination with the -m switch:
py -2 -m pip install SomePackage # default Python 2
py -2.7 -m pip install SomePackage # specifically Python 2.7
py -3 -m pip install SomePackage # default Python 3
py -3.4 -m pip install SomePackage # specifically Python 3.4

I ran into this issue myself recently and found that I wasn't getting the right pip for Python 3, on my Linux system that also has Python 2.
First you must ensure that you have installed pip for your python version:
For Python 2:
sudo apt-get install python-pip
For Python 3:
sudo apt-get install python3-pip
Then to install packages for one version of Python or the other, simply use the following for Python 2:
pip install <package>
or for Python 3:
pip3 install <package>

pip is also a python package. So the easiest way to install modules to a specific python version would be below
python2.7 /usr/bin/pip install foo
or
python2.7 -m pip install foo

So apparently there are multiple versions of easy_install and pip. It seems to be a big mess. Anyway, this is what I did to install Django for Python 2.7 on Ubuntu 12.10:
$ sudo easy_install-2.7 pip
Searching for pip
Best match: pip 1.1
Adding pip 1.1 to easy-install.pth file
Installing pip-2.7 script to /usr/local/bin
Using /usr/lib/python2.7/dist-packages
Processing dependencies for pip
Finished processing dependencies for pip
$ sudo pip-2.7 install django
Downloading/unpacking django
Downloading Django-1.5.1.tar.gz (8.0Mb): 8.0Mb downloaded
Running setup.py egg_info for package django
warning: no previously-included files matching '__pycache__' found under directory '*'
warning: no previously-included files matching '*.py[co]' found under directory '*'
Installing collected packages: django
Running setup.py install for django
changing mode of build/scripts-2.7/django-admin.py from 644 to 755
warning: no previously-included files matching '__pycache__' found under directory '*'
warning: no previously-included files matching '*.py[co]' found under directory '*'
changing mode of /usr/local/bin/django-admin.py to 755
Successfully installed django
Cleaning up...
$ python
Python 2.7.3 (default, Sep 26 2012, 21:51:14)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import django
>>>

On Linux, Mac OS X and other POSIX systems, use the versioned Python commands in combination with the -m switch to run the appropriate copy of pip:
python2.7 -m pip install SomePackage
python3.4 -m pip install SomePackage
(appropriately versioned pip commands may also be available)
On Windows, use the py Python launcher in combination with the -m switch:
py -2.7 -m pip install SomePackage # specifically Python 2.7
py -3.4 -m pip install SomePackage # specifically Python 3.4
if you get an error for py -3.4 then try:
pip install SomePackage

Installation of multiple versions of Python and respective Packages.
Python version on the same windows machine : 2.7 , 3.4 and 3.6
Installation of all 3 versions of Python :
Installed the Python 2.7 , 3.4 and 3.6 with the below paths
PATH for all 3 versions of Python :
Made sure the PATH variable ( in System Variables ) has below paths included - C:\Python27\;C:\Python27\Scripts;C:\Python34\;C:\Python34\Scripts;C:\Python36\;C:\Python36\Scripts\;
Renaming the executables for versions :
Changed the python executable name in C:\Python36 and C:\Python34 to python36 and python34 respectively.
Checked for the command prompt with all versions :
Installing the packages separately for each version

If you have multiple versions as well as multiple architectures (32 bit, 64 bit) you will need to add a -32 or -64 at the end of your version.
For windows, go to cmd and type py --list and it will produce the versions you have installed. The list will look like the following:
Installed Pythons found by py Launcher for Windows
-3.7-64 *
-3.7-32
-3.6-32
The full command as an example will be:
py -3.6-32 -m pip install (package)
If you want to get more indepth, to install a specific version of a package on a specific version of python, use ==(version) after the package. As an example,
py -3.6-32 -m pip install opencv-python==4.1.0.25

Here is my take on the problem. Works for Python3. The main features are:
Each Python version is compiled from source
All versions are installed locally
Does not mangle your system's default Python installation in any way
Each Python version is isolated with virtualenv
Prerequisites: If you are using some bare-bones thin client with no extra turf installed, you should run this first (in ubuntu 18.04 at least, extra packages added for convenience):
sudo apt-get update
sudo apt-get install software-properties-common
sudo apt-add-repository universe
sudo apt-get update
sudo apt-get install -y build-essential cmake
sudo apt-get install -y zlib1g zlib1g-dev libsqlite3-dev \
openssl libssl-dev libffi-dev unzip pciutils net-tools \
libblas-dev gfortran libblas3
The steps are as follows:
If you have several extra python versions installed in some other way, get rid of them, e.g., remove $HOME/.local/lib/python3.x, etc. (also the globally installed ones). Don't touch your system's default python3 version though.
Download source for different python versions under the following directory structure:
$HOME/
python_versions/ : download Python-*.tgz packages here and "tar xvf" them. You'll get directories like this:
Python-3.4.8/
Python-3.6.5/
Python-3.x.y/
...
At each "Python-3.x.y/" directory, do the following (do NOT use "sudo" in any of the steps!):
mkdir root
./configure --prefix=$PWD/root
make -j 2
make install
virtualenv --no-site-packages -p root/bin/python3.x env
At "python_versions/" create files like this:
env_python3x.bash:
#!/bin/bash
echo "type deactivate to exit"
source $HOME/python_versions/Python-3.x.y/env/bin/activate
Now, anytime you wish to opt for python3.x, do
source $HOME/python_versions/env_python3x.bash
to enter the virtualenv
While in the virtualenv, install your favorite python packages with
pip install --upgrade package_name
To exit the virtualenv and python version just type "deactivate"
UPDATE
It seems that --no-site-packages is deprecated. There's an easy fix for this: Once you have activated the virtualenv, just point the HOME env variable to somewhere else than your actual home directory, i.e.:
export HOME=some/where/else
A nice way to do this in general is:
Create virtualenv
Activate virtualenv
If you want to "recycle" existing libraries to your virtualenv, softlink them from your existing install, i.e.
ln -s $HOME/.local/lib/python3.6/site-packages/numpy $PWD/venv/lib/python3.6/site-packages/
Do export PYTHONPATH=, export HOME=/some/other/dir
Now you should have custom-isolated virtualenv.
UPDATE 2 / SUDO
Wan't to force sudo to use your virtualenv?
Defaults secure_path="/home/USENAME/Python-3.x.y/env/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"
Defaults env_keep += "VIRTUAL_ENV"
Defaults env_keep += "PYTHONPATH"
Now try "sudo python3 --version" and magic should happen
UPDATE 3 / DOCKER
Enable virtualenv inside your docker (of course, you have built it in your docker image):
ENV VIRTUAL_ENV=/home/USER/Python-3.x.y/env
ENV PYTHONPATH=
ENV PATH="$VIRTUAL_ENV/bin:$PATH"

You can use one of the following commands:
pip2 install SomePackage
pip3 install SomePackage
python2 -m pip install SomePackage
python3 -m pip install SomePackage
And of course, make sure that you have the correct version of pip installed
sudo apt-get install python-pip
sudo apt-get install python3-pip
I haven't used these commands myself but, some answers above suggests using them to specify exactly the version of python you want to use
pip-2.7 install SomePackage
python-3.6 -m pip install SomePackage

For python 3 and Windows OS, I always use this syntax to install packages on different version:
First I always use Git Bash Command Prompt.
Here an example installing urllib package.
Default Python version:(The normal pip command)
pip install urllib3
For the other versions
py -3.8 -m pip install urllib3
py => for python
-3.8 => for the version (I'm using the 3.8.7 version) but if you're using the 3.7.7 version it will be "-3.7"
-m : just because or for modify
pip install urllib3 : the normal pip command

Most of the answers here address the issue but I want to add something what was continually confusing me with regard to creating an alternate installation of python in the /usr/local on CentOS 7. When I installed there, it appeared like pip was working since I could use pip2.7 install and it would install modules. However, what I couldn't figure out was why my newly installed version of python wasn't seeing what I was installing.
It turns out in CentOS 7 that there is already a python2.7 and a pip2.7 in the /usr/bin folder. To install pip for your new python distribution, you need to specifically tell sudo to go to /usr/local/bin
sudo /usr/local/bin/python2.7 -m ensurepip
This should get pip2.7 installed in your /usr/local/bin folder along with your version of python. The trick is that when you want to install modules, you either need to modify the sudo $PATH variable to include /usr/local/bin or you need to execute
sudo /usr/local/bin/pip2.7 install <module>
if you want to install a new module. It took me forever to remember that sudo wasn't immediately seeing /usr/local/bin.

Simple and recent
On windows
1- Supposed that you have a different version of Python installed in your system. To check use the following command to check:
> py --list
-3.10-64 *
-3.7-64
2- Set your preferred default version:
by setting the PY_PYTHON environment variable (e.g. PY_PYTHON=3.7).
by settting the py.ini file usually located on C:\Users\<your user name>\AppData\Local if not create one. For example, setting PY_PYTHON=3 and PY_PYTHON3=3.7 environment variables are equivalent to the INI file containing:
[defaults]
python=3
python3=3.7
3- check by typing again py --list:
> py --list
-3.10-64
-3.7-64 *
4- If you would like to run virtual environment with spesific version of python and pip see this post.
On Linux (Ubuntu)
An easy approach for managing different python versions on Linux is update-alternatives command. This command giving us the ability to switch between many versions of the same software easily.
command format:update-alternatives --install link name path priority, the name is the generic name for the master link, the link is the name of its symlink, the path is the alternative being introduced for the master link, and priority is the priority of the alternatives group.
Usage: Suppose you installed two versions of python (python3.10 , python3.7). Now by running this command you will link the command name (python3) to different versions of python and assign a priority number. A higher priority number means a higher priority.
$ update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
$ update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.10 2
List installed versions of python with this command:
$ update-alternatives --list python3
/usr/bin/python3.7
/usr/bin/python3.10
Switching between versions: Just manually select the priority number of the desired python version after running following command.
$ update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/python3.10 2 auto mode
* 1 /usr/bin/python3.7 1 manual mode
2 /usr/bin/python3.10 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 2

Context: Archlinux
Action:
Install python2-pip:
sudo pacman -S python2-pip
You now have pip2.7:
sudo pip2.7 install boto
Test (in my case I needed 'boto'):
Run the following commands:
python2
import boto
Success: No error.
Exit: Ctrl+D

for example, if you set other versions (e.g. 3.5) as default and want to install pip for python 2.7:
download pip at https://pypi.python.org/pypi/pip (tar)
unzip tar file
cd to the file’s directory
sudo python2.7 setup.py install

If you have both python3.6 and python3.7 installed and want to use pip with python3.7 by default, here's what you should do:
First make sure you have pip installed for python3.7
python3.7 -m pip install -U pip
Now pip3.7 must be available, so we edit .bashrc
nano ~/.bashrc
adding the following line to it
alias pip=pip3.7
In order for the changes to take effect type in the shell:
source ~/.bashrc
Now if you type:
pip --version
you should get:
pip 20.1.1 from /usr/local/lib/python3.7/dist-packages/pip (python 3.7)
which means, if you use, for example:
pip install <package>
it would install the <package> for python3.7

Another possible way could be using conda and pip. Some time you probably want to use just one of those, but if you really need to set up a particular version of python I combine both.
I create a starting conda enviroment with the python I want. As in here https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html. Alternatively you could set up the whole enviroment just using conda.
conda create -n myenv python=3.6.4
Then activate your enviroment with the python you like. This command could change depending on the OS.
source activae myenv
Now you have your python active then you could continue using conda but if you need/want to use pip:
python -m pip -r requirements.txt
Here you have a possible way.

You can go to for example C:\Python2.7\Scripts and then run cmd from that path. After that you can run pip2.7 install yourpackage...
That will install package for that version of Python.

This is probably the completely wrong thing to do (I'm a python noob), but I just went in and edited the pip file
#!/usr/bin/env python3 <-- I changed this line.
# -*- coding: utf-8 -*-
import re
import sys
from pip._internal import main
if __name__ == '__main__':
sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv[0])
sys.exit(main())

To use multiple versions of pip, just type
pip{version} and run command
Example: for python 3.10
pip3.10
pip3.10 list
pip3.10
for Python 3.7
pip3.7
pip3.7 list

For windows specifically:
\path\to\python.exe -m pip install PackageName works.

for Blender:
/usr/bin $ python3.7 -m pip install irc

Some useful information for debugging this is the pip debug command. It shows the location of the python interpreter that it is attached to in the 1st line (after the warning).
$ pip debug
WARNING: This command is only meant for debugging. Do not use this with automation for parsing and getting these details, since the output and options of this command may change without notice.
pip version: pip 21.2.4 from /data/akshay/anaconda3/lib/python3.9/site-packages/pip (python 3.9)
sys.version: 3.9.12 (main, Apr 5 2022, 06:56:58)
sys.executable: /data/akshay/anaconda3/bin/python

How to install packages in specific installation of Python? [duplicate]

Is there any way to make pip play well with multiple versions of Python? For example, I want to use pip to explicitly install things to either my site 2.5 installation or my site 2.6 installation.
For example, with easy_install, I use easy_install-2.{5,6}.
And, yes — I know about virtualenv, and no — it's not a solution to this particular problem.

The current recommendation is to use python -m pip, where python is the version of Python you would like to use. This is the recommendation because it works across all versions of Python, and in all forms of virtualenv. For example:
# The system default python:
$ python -m pip install fish
# A virtualenv's python:
$ .env/bin/python -m pip install fish
# A specific version of python:
$ python-3.6 -m pip install fish
Previous answer, left for posterity:
Since version 0.8, Pip supports pip-{version}. You can use it the same as easy_install-{version}:
$ pip-2.5 install myfoopackage
$ pip-2.6 install otherpackage
$ pip-2.7 install mybarpackage
EDIT: pip changed its schema to use pipVERSION instead of pip-VERSION in version 1.5. You should use the following if you have pip >= 1.5:
$ pip2.6 install otherpackage
$ pip2.7 install mybarpackage
Check https://github.com/pypa/pip/pull/1053 for more details
References:
https://github.com/pypa/pip/issues/200
http://www.pip-installer.org/docs/pip/en/0.8.3/news.html#id4
https://pip.pypa.io/en/stable/news/#v0-8 or
https://web.archive.org/web/20140310013920/http://www.pip-installer.org:80/docs/pip/en/0.8.3/news.html#id4

On Windows, you can execute the pip module using a given Python version through the Python launcher, py.exe, if you chose to install it during Python 3 setup.
py -3 -m pip install packagename
py -2 -m pip install packagename
You can be even more specific and request an exact sub-version of Python:
py -3.6 -m pip install packagename
To get a list of all installed Python versions available through the launcher, run:
py --list
Alternatively, you can launch the desired Python executable directly:
C:/path/to/specific/python.exe -m pip install packagename

/path/to/python2.{5,6} /path/to/pip install PackageName doesn't work?
For this to work on any python version that doesn't have pip already installed you need to download pip and do python*version* setup.py install. For example python3.3 setup.py install. This resolves the import error in the comments. (As suggested by #hbdgaf)

I had python 2.6 installed by default (Amazon EC2 AMI), but needed python2.7 plus some external packages for my application. Assuming you already installed python2.7 alongside with default python (2.6 in my case). Here is how to install pip and packages for non-default python2.7
Install pip for your python version:
curl -O https://bootstrap.pypa.io/get-pip.py
python27 get-pip.py
Use specific pip version to install packages:
pip2.7 install mysql-connector-python --allow-external mysql-connector-python

It worked for me in windows this way:
I changed the name of python files python.py and pythonw.exe to python3.py pythonw3.py
Then I just ran this command in the prompt:
python3 -m pip install package

Other answers show how to use pip with both 2.X and 3.X Python, but does not show how to handle the case of multiple Python distributions (eg. original Python and Anaconda Python).
I have a total of 3 Python versions: original Python 2.7 and Python 3.5 and Anaconda Python 3.5.
Here is how I install a package into:
Original Python 3.5:
/usr/bin/python3 -m pip install python-daemon
Original Python 2.7:
/usr/bin/python -m pip install python-daemon
Anaconda Python 3.5:
python3 -m pip install python-daemon
or
pip3 install python-daemon
Simpler, as Anaconda overrides original Python binaries in user environment.
Of course, installing in anaconda should be done with conda command, this is just an example.
Also, make sure that pip is installed for that specific python.You might need to manually install pip. This works in Ubuntu 16.04:
sudo apt-get install python-pip
or
sudo apt-get install python3-pip

From here: https://docs.python.org/3/installing/
Here is how to install packages for various versions that are installed at the same time linux, mac, posix:
python2 -m pip install SomePackage # default Python 2
python2.7 -m pip install SomePackage # specifically Python 2.7
python3 -m pip install SomePackage # default Python 3
python3.4 -m pip install SomePackage # specifically Python 3.4
python3.5 -m pip install SomePackage # specifically Python 3.5
python3.6 -m pip install SomePackage # specifically Python 3.6
On Windows, use the py Python launcher in combination with the -m switch:
py -2 -m pip install SomePackage # default Python 2
py -2.7 -m pip install SomePackage # specifically Python 2.7
py -3 -m pip install SomePackage # default Python 3
py -3.4 -m pip install SomePackage # specifically Python 3.4

I ran into this issue myself recently and found that I wasn't getting the right pip for Python 3, on my Linux system that also has Python 2.
First you must ensure that you have installed pip for your python version:
For Python 2:
sudo apt-get install python-pip
For Python 3:
sudo apt-get install python3-pip
Then to install packages for one version of Python or the other, simply use the following for Python 2:
pip install <package>
or for Python 3:
pip3 install <package>

pip is also a python package. So the easiest way to install modules to a specific python version would be below
python2.7 /usr/bin/pip install foo
or
python2.7 -m pip install foo

So apparently there are multiple versions of easy_install and pip. It seems to be a big mess. Anyway, this is what I did to install Django for Python 2.7 on Ubuntu 12.10:
$ sudo easy_install-2.7 pip
Searching for pip
Best match: pip 1.1
Adding pip 1.1 to easy-install.pth file
Installing pip-2.7 script to /usr/local/bin
Using /usr/lib/python2.7/dist-packages
Processing dependencies for pip
Finished processing dependencies for pip
$ sudo pip-2.7 install django
Downloading/unpacking django
Downloading Django-1.5.1.tar.gz (8.0Mb): 8.0Mb downloaded
Running setup.py egg_info for package django
warning: no previously-included files matching '__pycache__' found under directory '*'
warning: no previously-included files matching '*.py[co]' found under directory '*'
Installing collected packages: django
Running setup.py install for django
changing mode of build/scripts-2.7/django-admin.py from 644 to 755
warning: no previously-included files matching '__pycache__' found under directory '*'
warning: no previously-included files matching '*.py[co]' found under directory '*'
changing mode of /usr/local/bin/django-admin.py to 755
Successfully installed django
Cleaning up...
$ python
Python 2.7.3 (default, Sep 26 2012, 21:51:14)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import django
>>>

On Linux, Mac OS X and other POSIX systems, use the versioned Python commands in combination with the -m switch to run the appropriate copy of pip:
python2.7 -m pip install SomePackage
python3.4 -m pip install SomePackage
(appropriately versioned pip commands may also be available)
On Windows, use the py Python launcher in combination with the -m switch:
py -2.7 -m pip install SomePackage # specifically Python 2.7
py -3.4 -m pip install SomePackage # specifically Python 3.4
if you get an error for py -3.4 then try:
pip install SomePackage

Installation of multiple versions of Python and respective Packages.
Python version on the same windows machine : 2.7 , 3.4 and 3.6
Installation of all 3 versions of Python :
Installed the Python 2.7 , 3.4 and 3.6 with the below paths
PATH for all 3 versions of Python :
Made sure the PATH variable ( in System Variables ) has below paths included - C:\Python27\;C:\Python27\Scripts;C:\Python34\;C:\Python34\Scripts;C:\Python36\;C:\Python36\Scripts\;
Renaming the executables for versions :
Changed the python executable name in C:\Python36 and C:\Python34 to python36 and python34 respectively.
Checked for the command prompt with all versions :
Installing the packages separately for each version

If you have multiple versions as well as multiple architectures (32 bit, 64 bit) you will need to add a -32 or -64 at the end of your version.
For windows, go to cmd and type py --list and it will produce the versions you have installed. The list will look like the following:
Installed Pythons found by py Launcher for Windows
-3.7-64 *
-3.7-32
-3.6-32
The full command as an example will be:
py -3.6-32 -m pip install (package)
If you want to get more indepth, to install a specific version of a package on a specific version of python, use ==(version) after the package. As an example,
py -3.6-32 -m pip install opencv-python==4.1.0.25

Here is my take on the problem. Works for Python3. The main features are:
Each Python version is compiled from source
All versions are installed locally
Does not mangle your system's default Python installation in any way
Each Python version is isolated with virtualenv
Prerequisites: If you are using some bare-bones thin client with no extra turf installed, you should run this first (in ubuntu 18.04 at least, extra packages added for convenience):
sudo apt-get update
sudo apt-get install software-properties-common
sudo apt-add-repository universe
sudo apt-get update
sudo apt-get install -y build-essential cmake
sudo apt-get install -y zlib1g zlib1g-dev libsqlite3-dev \
openssl libssl-dev libffi-dev unzip pciutils net-tools \
libblas-dev gfortran libblas3
The steps are as follows:
If you have several extra python versions installed in some other way, get rid of them, e.g., remove $HOME/.local/lib/python3.x, etc. (also the globally installed ones). Don't touch your system's default python3 version though.
Download source for different python versions under the following directory structure:
$HOME/
python_versions/ : download Python-*.tgz packages here and "tar xvf" them. You'll get directories like this:
Python-3.4.8/
Python-3.6.5/
Python-3.x.y/
...
At each "Python-3.x.y/" directory, do the following (do NOT use "sudo" in any of the steps!):
mkdir root
./configure --prefix=$PWD/root
make -j 2
make install
virtualenv --no-site-packages -p root/bin/python3.x env
At "python_versions/" create files like this:
env_python3x.bash:
#!/bin/bash
echo "type deactivate to exit"
source $HOME/python_versions/Python-3.x.y/env/bin/activate
Now, anytime you wish to opt for python3.x, do
source $HOME/python_versions/env_python3x.bash
to enter the virtualenv
While in the virtualenv, install your favorite python packages with
pip install --upgrade package_name
To exit the virtualenv and python version just type "deactivate"
UPDATE
It seems that --no-site-packages is deprecated. There's an easy fix for this: Once you have activated the virtualenv, just point the HOME env variable to somewhere else than your actual home directory, i.e.:
export HOME=some/where/else
A nice way to do this in general is:
Create virtualenv
Activate virtualenv
If you want to "recycle" existing libraries to your virtualenv, softlink them from your existing install, i.e.
ln -s $HOME/.local/lib/python3.6/site-packages/numpy $PWD/venv/lib/python3.6/site-packages/
Do export PYTHONPATH=, export HOME=/some/other/dir
Now you should have custom-isolated virtualenv.
UPDATE 2 / SUDO
Wan't to force sudo to use your virtualenv?
Defaults secure_path="/home/USENAME/Python-3.x.y/env/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"
Defaults env_keep += "VIRTUAL_ENV"
Defaults env_keep += "PYTHONPATH"
Now try "sudo python3 --version" and magic should happen
UPDATE 3 / DOCKER
Enable virtualenv inside your docker (of course, you have built it in your docker image):
ENV VIRTUAL_ENV=/home/USER/Python-3.x.y/env
ENV PYTHONPATH=
ENV PATH="$VIRTUAL_ENV/bin:$PATH"

You can use one of the following commands:
pip2 install SomePackage
pip3 install SomePackage
python2 -m pip install SomePackage
python3 -m pip install SomePackage
And of course, make sure that you have the correct version of pip installed
sudo apt-get install python-pip
sudo apt-get install python3-pip
I haven't used these commands myself but, some answers above suggests using them to specify exactly the version of python you want to use
pip-2.7 install SomePackage
python-3.6 -m pip install SomePackage

For python 3 and Windows OS, I always use this syntax to install packages on different version:
First I always use Git Bash Command Prompt.
Here an example installing urllib package.
Default Python version:(The normal pip command)
pip install urllib3
For the other versions
py -3.8 -m pip install urllib3
py => for python
-3.8 => for the version (I'm using the 3.8.7 version) but if you're using the 3.7.7 version it will be "-3.7"
-m : just because or for modify
pip install urllib3 : the normal pip command

Most of the answers here address the issue but I want to add something what was continually confusing me with regard to creating an alternate installation of python in the /usr/local on CentOS 7. When I installed there, it appeared like pip was working since I could use pip2.7 install and it would install modules. However, what I couldn't figure out was why my newly installed version of python wasn't seeing what I was installing.
It turns out in CentOS 7 that there is already a python2.7 and a pip2.7 in the /usr/bin folder. To install pip for your new python distribution, you need to specifically tell sudo to go to /usr/local/bin
sudo /usr/local/bin/python2.7 -m ensurepip
This should get pip2.7 installed in your /usr/local/bin folder along with your version of python. The trick is that when you want to install modules, you either need to modify the sudo $PATH variable to include /usr/local/bin or you need to execute
sudo /usr/local/bin/pip2.7 install <module>
if you want to install a new module. It took me forever to remember that sudo wasn't immediately seeing /usr/local/bin.

Simple and recent
On windows
1- Supposed that you have a different version of Python installed in your system. To check use the following command to check:
> py --list
-3.10-64 *
-3.7-64
2- Set your preferred default version:
by setting the PY_PYTHON environment variable (e.g. PY_PYTHON=3.7).
by settting the py.ini file usually located on C:\Users\<your user name>\AppData\Local if not create one. For example, setting PY_PYTHON=3 and PY_PYTHON3=3.7 environment variables are equivalent to the INI file containing:
[defaults]
python=3
python3=3.7
3- check by typing again py --list:
> py --list
-3.10-64
-3.7-64 *
4- If you would like to run virtual environment with spesific version of python and pip see this post.
On Linux (Ubuntu)
An easy approach for managing different python versions on Linux is update-alternatives command. This command giving us the ability to switch between many versions of the same software easily.
command format:update-alternatives --install link name path priority, the name is the generic name for the master link, the link is the name of its symlink, the path is the alternative being introduced for the master link, and priority is the priority of the alternatives group.
Usage: Suppose you installed two versions of python (python3.10 , python3.7). Now by running this command you will link the command name (python3) to different versions of python and assign a priority number. A higher priority number means a higher priority.
$ update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
$ update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.10 2
List installed versions of python with this command:
$ update-alternatives --list python3
/usr/bin/python3.7
/usr/bin/python3.10
Switching between versions: Just manually select the priority number of the desired python version after running following command.
$ update-alternatives --config python3
There are 2 choices for the alternative python3 (providing /usr/bin/python3).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/python3.10 2 auto mode
* 1 /usr/bin/python3.7 1 manual mode
2 /usr/bin/python3.10 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 2

Context: Archlinux
Action:
Install python2-pip:
sudo pacman -S python2-pip
You now have pip2.7:
sudo pip2.7 install boto
Test (in my case I needed 'boto'):
Run the following commands:
python2
import boto
Success: No error.
Exit: Ctrl+D

for example, if you set other versions (e.g. 3.5) as default and want to install pip for python 2.7:
download pip at https://pypi.python.org/pypi/pip (tar)
unzip tar file
cd to the file’s directory
sudo python2.7 setup.py install

If you have both python3.6 and python3.7 installed and want to use pip with python3.7 by default, here's what you should do:
First make sure you have pip installed for python3.7
python3.7 -m pip install -U pip
Now pip3.7 must be available, so we edit .bashrc
nano ~/.bashrc
adding the following line to it
alias pip=pip3.7
In order for the changes to take effect type in the shell:
source ~/.bashrc
Now if you type:
pip --version
you should get:
pip 20.1.1 from /usr/local/lib/python3.7/dist-packages/pip (python 3.7)
which means, if you use, for example:
pip install <package>
it would install the <package> for python3.7

Another possible way could be using conda and pip. Some time you probably want to use just one of those, but if you really need to set up a particular version of python I combine both.
I create a starting conda enviroment with the python I want. As in here https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html. Alternatively you could set up the whole enviroment just using conda.
conda create -n myenv python=3.6.4
Then activate your enviroment with the python you like. This command could change depending on the OS.
source activae myenv
Now you have your python active then you could continue using conda but if you need/want to use pip:
python -m pip -r requirements.txt
Here you have a possible way.

You can go to for example C:\Python2.7\Scripts and then run cmd from that path. After that you can run pip2.7 install yourpackage...
That will install package for that version of Python.

This is probably the completely wrong thing to do (I'm a python noob), but I just went in and edited the pip file
#!/usr/bin/env python3 <-- I changed this line.
# -*- coding: utf-8 -*-
import re
import sys
from pip._internal import main
if __name__ == '__main__':
sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv[0])
sys.exit(main())

To use multiple versions of pip, just type
pip{version} and run command
Example: for python 3.10
pip3.10
pip3.10 list
pip3.10
for Python 3.7
pip3.7
pip3.7 list

For windows specifically:
\path\to\python.exe -m pip install PackageName works.

for Blender:
/usr/bin $ python3.7 -m pip install irc

Some useful information for debugging this is the pip debug command. It shows the location of the python interpreter that it is attached to in the 1st line (after the warning).
$ pip debug
WARNING: This command is only meant for debugging. Do not use this with automation for parsing and getting these details, since the output and options of this command may change without notice.
pip version: pip 21.2.4 from /data/akshay/anaconda3/lib/python3.9/site-packages/pip (python 3.9)
sys.version: 3.9.12 (main, Apr 5 2022, 06:56:58)
sys.executable: /data/akshay/anaconda3/bin/python

Pip for python2 while python3 pip exists

I have both python2 and python3 in my system. But when I try :
python -m pip install sklearn
bash shows :
/usr/bin/python: No module named pip
And
pip --version
Yields :
pip 20.0.2 from /usr/lib/python3/dist-packages/pip (python 3.8)
Python versions are Python 2.7.18 and Python 3.8.2.
How to correct this error by having two pip versions?
This is a similar question, but did not solve the issue.
OS is Ubuntu.

Thanks to HK boy
The following works, it installs pip explicitly for the python version specified :
curl -O https://bootstrap.pypa.io/get-pip.py
sudo python get-pip.py
I already had pip3 before hand, so to install packages for python3, pip3 install <package> is enough.

How to have python 2 and 3 in virtualenv

I have a virtualenv (python2api) made in python2.7 on Ubuntu 16.04. In the virtualenv I installed python3.5:
$ virtualenv -p python3.5 python2api
Then I installed pip3:
$ sudo apt-get install python3-pip
But when I run 'which pip' it shows that pip3 installed outside of the virtualenv and any pip3 packages I install go to '/usr/lib/python2.7' instead of '/var/env/python2api/lib/python3.5/site-packages/'.
(python2api) user#comp:/var/env/python2api/lib$ which pip
/var/env/python2api/bin/pip
(python2api) user#comp:/var/env/python2api/lib$ which pip3
/usr/bin/pip3
Is there a way to make pip3 install packages in the virtualenv? It seems like only python2 or only python3 packages can exist in the virtualenv.

A virtualenv encapsulates one version of Python.
You can't use it to manage more than one version, and Python 2.x and Python 3.x are separate versions here.
Use two separate virtualenvs, one for each Python version.

how to use python2.7 pip instead of default pip

I just installed python 2.7 and also pip to the 2.7 site package.
When I get the version with:
pip -V
It shows:
pip 1.3.1 from /usr/lib/python2.6/site-packages (python 2.6)
How do I use the 2.7 version of pip located at:
/usr/local/lib/python2.7/site-packages

There should be a binary called "pip2.7" installed at some location included within your $PATH variable.
You can find that out by typing
which pip2.7
This should print something like '/usr/local/bin/pip2.7' to your stdout. If it does not print anything like this, it is not installed. In that case, install it by running
$ wget https://bootstrap.pypa.io/pip/2.7/get-pip.py
$ sudo python2.7 get-pip.py
Now, you should be all set, and
which pip2.7
should return the correct output.

An alternative is to call the pip module by using python2.7, as below:
python2.7 -m pip <commands>
For example, you could run python2.7 -m pip install <package> to install your favorite python modules. Here is a reference: https://stackoverflow.com/a/50017310/4256346.
In case the pip module has not yet been installed for this version of python, you can run the following:
python2.7 -m ensurepip
Running this command will "bootstrap the pip installer". Note that running this may require administrative privileges (i.e. sudo). Here is a reference: https://docs.python.org/2.7/library/ensurepip.html and another reference https://stackoverflow.com/a/46631019/4256346.

as noted here, this is what worked best for me:
sudo apt-get install python3 python3-pip python3-setuptools
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 10

pip has now dropped support for python2, therefore you can't use python2 pip
You can't find python2-pip in apt-get anymore, and you won't get pip when installing python2 from source
You can still install python modules using apt-get. To install a python prepend ‘python-’ to the module name
apt-get install python-six # install six