[python] pip or pip3 to install packages for Python 3?

I have a Macbook with OS X El Captain. I think that Python 2.7 comes preinstalled on it. However, I installed Python 3.5 too. When I started using Python 3, I read that if I want to install a package, I should type:

pip3 install some_package

Anyway, now when I use

pip install some_package

I get some_package installed for Python 3. I mean I can import it and use it without problems. Moreover, when I type just pip3 in Terminal, I got this message about the usage:

Usage:   
  pip <command> [options]

which is the same message I get when I type just pip.

Does it mean that in previos versions, things were different, and now pip and pip3 can be used interchangeably? If so, and for the sake of argument, how can I install packages for Python 2 instead of Python 3?

When you install python3, pip3 gets installed. And if you don't have another python installation(like python2.7) then a link is created which points pip to pip3.

So pip is a link to to pip3 if there is no other version of python installed(other than python3). pip generally points to the first installation.

Given an activated Python 3.6 virtualenv in somepath/venv, the following aliases resolved the various issues on a macOS Sierra where pip insisted on pointing to Apple's 2.7 Python.

alias pip='python somepath/venv/lib/python3.6/site-packages/pip/__main__.py'

This didn't work so well when I had to do sudo pip as the root user doesn't know anything about my alias or the virtualenv, so I had to add an extra alias to handle this as well. It's a hack, but it works, and I know what it does:

alias sudopip='sudo somepath/venv/bin/python somepath/venv/lib/python3.6/site-packages/pip/__main__.py'

background:

pip3 did not exist to start (command not found) with and which pip would return /opt/local/Library/Frameworks/Python.framework/Versions/2.7/bin/pip, the Apple Python.

Python 3.6 was installed via macports.

After activation of the 3.6 virtualenv I wanted to work with, which python would return somepath/venv/bin/python

Somehow pip install would do the right thing and hit my virtualenv, but pip list would rattle off Python 2.7 packages.

For Python, this is batting way beneath my expectations in terms of beginner-friendliness.

In my system, I use the update alternatives.

sudo update-alternatives --install /usr/bin/pip pip /usr/bin/pip3 1
sudo update-alternatives --install /usr/bin/pip pip /usr/bin/pip2 2

If I want to switch between them I use the following command.

sudo update-alternatives --config pip

Note: The 1st line is enough if you have only pip3 installed and not pip2.

This is a tricky subject. In the end, if you invoke pip it will invoke either pip2 or pip3, depending on how you set your system up.

On my Windows instance - and I do not fully understand my environment - using pip3 to install the kaggle-cli package worked - whereas pip did not. I was working in a conda environment and the environments appear to be different.

(fastai) C:\Users\redact\Downloads\fast.ai\deeplearning1\nbs>pip --version

pip 9.0.1 from C:\ProgramData\Anaconda3\envs\fastai\lib\site-packages (python 3.6)

(fastai) C:\Users\redact\Downloads\fast.ai\deeplearning1\nbs>pip3 --version

pip 9.0.1 from c:\users\redact\appdata\local\programs\python\python36\lib\site-packages (python 3.6)

If you installed Python 2.7, I think you could use pip2 and pip2.7 to install packages specifically for Python 2, like

pip2 install some_pacakge

or

pip2.7 install some_package

And you may use pip3 or pip3.5 to install pacakges specifically for Python 3.

By illustration:

pip --version
  pip 19.0.3 from /usr/lib/python3.7/site-packages/pip (python 3.7)

pip3 --version
  pip 19.0.3 from /usr/lib/python3.7/site-packages/pip (python 3.7)

python --version
  Python 3.7.3

which python
  /usr/bin/python

ls -l '/usr/bin/python'
  lrwxrwxrwx 1 root root 7 Mar 26 14:43 /usr/bin/python -> python3

which python3
  /usr/bin/python3

ls -l /usr/bin/python3
  lrwxrwxrwx 1 root root 9 Mar 26 14:43 /usr/bin/python3 -> python3.7

ls -l /usr/bin/python3.7
  -rwxr-xr-x 2 root root 14120 Mar 26 14:43 /usr/bin/python3.7

Thus, my in my default system python (Python 3.7.3), pip is pip3.

I think pip, pip2 and pip3 are not soft links to the same executable file path. Note these commands and results in my Linux terminal:

mrz@mrz-pc ~ $ ls -l `which pip`
-rwxr-xr-x 1 root root 292 Nov 10  2016 /usr/bin/pip
mrz@mrz-pc ~ $ ls -l `which pip2`
-rwxr-xr-x 1 root root 283 Nov 10  2016 /usr/bin/pip2
mrz@mrz-pc ~ $ ls -l `which pip3`
-rwxr-xr-x 1 root root 293 Nov 10  2016 /usr/bin/pip3
mrz@mrz-pc ~ $ pip -V
pip 9.0.1 from /home/mrz/.local/lib/python2.7/site-packages (python 2.7)
mrz@mrz-pc ~ $ pip2 -V
pip 8.1.1 from /usr/lib/python2.7/dist-packages (python 2.7)
mrz@mrz-pc ~ $ pip3 -V
pip 9.0.1 from /home/mrz/.local/lib/python3.5/site-packages (python 3.5)

As you see they exist in different paths.

pip3 always operates on the Python3 environment only, as pip2 does with Python2. pip operates in whichever environment is appropriate to the context. For example, if you are in a Python3 venv, pip will operate on the Python3 environment.

If you had python 2.x and then installed python3, your pip will be pointing to pip3. you can verify that by typing pip --version which would be the same as pip3 --version.

On your system you have now pip, pip2 and pip3.

If you want you can change pip to point to pip2 instead of pip3.