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How can I install packages?

I can't install a package with the pip or pip3 command and I can't install it from the Python Interpreter in PyCharm too. I can't tap on the '+' at the Python Interpreter in PyCharm.
I get this error:
pip command not found

Here is what you can try.
Lets check python installation.
C:\Users\User>python --version
Python 3.10.5
C:\Users\User>
If you get the version as 3.x, then we are good, else goto your python installation directory and copy its path and then add it to your environment variable PATH.
Check PIP available or not.
C:\Users\User>pip --version
pip 22.3 from C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\pip (python 3.10)
C:\Users\User>
if above didn't work then add pip path to PATH environment variable which would be something similar to above output.
Now we are ready to install any package
Just do pip install package-name
for testing, you can try:
pip install numpy
If it still throws error, try below one
python -m pip install numpy
if it still throws error, then you might need to clean your python installation.

3 commands
sudo apt update
sudo apt install python3-pip
pip3 install <package_name>
or maybe
sudo apt install <package_name> -y

It looks like you don't have pip installed on $PATH. The following command will run pip without any adjustments to $PATH:
python3 -m pip install <some_module>
Pay attention to the -m, this stands for modules and runs the pip program inside the python3 interpreter (I don't know exact definition here, if anyone wishes to correct me I will update the answer)
However, if this doesn't work then you may not have pip installed, and can be installed by running:
python3 -m ensurepip --upgrade
Then, reload your shell via the following (replace bash with your shell):
exec bash
Running pip --version should now work

-bash: pip: command not found, but with `python3 -m pip --version` shows the latest pip version

I get -bash: pip: command not found when I try to use pip -V or any other command with pip / pip3, I used to easily install something using pip3 install .., now pip doesn't work for some reason, BUT if I will write python3 -m pip --version I will get pip 21.1.3 from /Users/work/Library/Python/3.9/lib/python/site-packages/pip (python 3.9) , this seems to be the latest version at the moment and I can no longer install using the pip3 install .. command, using python3 -m pip, what could be the problem? I want to use pip3 like before
Thanks to all!

It seems to me that you need update Path in system variables.
Just add "Python_location"\Scripts\ (for example: c:\Python3\Scripts) to Path

How to install mesa (python package) for use in python 3

I have installed mesa via:
$ pip install mesa
but it is automatically installing it into
/Users/MyName/Documents/User/lib/python2.7/site-packages/mesa/~
which means that when I try to run it with a Python 3 kernel, it can't find the module and I receive the error
ModuleNotFoundError: No module named 'mesa'
Could someone help me out? I'm assuming the problem is that it is automatically installed into the python 2.7 directory - how can i change this?
Thanks

To install packages for Python3 while exists Python2,
try this
python3 -m pip install xxx
or this
sudo apt install pip3
pip3 install xxx

You should use pip3 instead of pip:
pip3 install mesa
If you don't have pip3 install it using:
sudo apt-get update
sudo apt-get -y install python3-pip
If it doesn't work you can do it manually using curl:
curl "https://bootstrap.pypa.io/get-pip.py" -o "get-pip.py"
python get-pip.py
You can also execute it straight from python3:
python3 -m pip install mesa
It is always a good practice to set pip command to be equivalent to your python command. i.e, if python points to python3, you better change pip to point to pip3. Add alias pip='pip3' to your ~/.bash_profile file.

How to upgrade pip3?

I want to use python3.5 for development, but many times when I install the module for python 3.5, it always fails. The terminal tells me that a higher version is available, but it doesn't work when I upgrade it.

You are using pip3 to install flask-script which is associated with python 3.5. However, you are trying to upgrade pip associated with the python 2.7, try running pip3 install --upgrade pip.
It might be a good idea to take some time and read about virtual environments in Python. It isn't a best practice to install all of your packages to the base python installation. This would be a good start: http://docs.python-guide.org/en/latest/dev/virtualenvs/

To upgrade your pip3, try running:
sudo -H pip3 install --upgrade pip
Your pip may move from /bin to /usr/local/bin
To upgrade pip as well, you can follow it by:
sudo -H pip2 install --upgrade pip

Try this command:
pip3 install --upgrade setuptools pip

First decide which pip you want to upgrade, i.e. just pip or pip3.
Mostly it'll be pip3 because pip is used by the system, so I won't recommend upgrading pip.
The difference between pip and pip3 is that
NOTE: I'm referring to PIP that is at the BEGINNING of the command
line.
pip is used by python version 2, i.e. python2
and
pip3 is used by python version 3, i.e. python3
For upgrading pip3: # This will upgrade python3 pip.
pip3 install --upgrade pip
For upgrading pip: # This will upgrade python2 pip.
pip install --upgrade pip
This will upgrade your existing pip to the latest version.

The Problem
You use pip (the Python 2 one). Now you want to upgrade pip (the Python 3 one). After that, pip is the Python 3 one.
The solution
Use pip2 and pip3. This way it is explicit.
If you want to use pip, just check where it is (which pip) and change the link. For example:
$ which pip
/usr/local/bin/pip
$ pip --version
pip 9.0.1 from /usr/local/lib/python3.5/dist-packages (python 3.5)
$ which pip2
/usr/local/bin/pip2
$ sudo rm /usr/local/bin/pip
$ sudo ln -s /usr/local/bin/pip2 /usr/local/bin/pip
$ pip --version
pip 9.0.1 from /usr/local/lib/python2.7/dist-packages (python 2.7)

for Python 3:
python3 -m pip install --upgrade pip
for Python 2:
python2 -m pip install --upgrade pip

What worked for me was the following command:
python -m pip install --upgrade pip

pip3 install --upgrade pip worked for me

In Ubuntu 18.04, below are the steps that I followed.
python3 -m pip install --upgrade pip
For some reason you will be getting an error, and that be fixed by making bash forget the wrongly referenced locations using the following command.
hash -r pip

If you have 2 versions of Python (eg: 2.7.x and 3.6), you need do:
add the path of 2.x to system PATH
add the path of 3.x to system PATH
pip3 install --upgrade pip setuptools wheel
for example, in my .zshrc file:
export PATH=/usr/local/Cellar/python#2/2.7.15/bin:/usr/local/Cellar/python/3.6.5/bin:$PATH
You can exec command pip --version and pip3 --version check the pip from the special version. Because if don't add Python path to $PATH, and exec pip3 install --upgrade pip setuptools wheel, your pip will be changed to pip from python3, but the pip should from python2.x

This worked for me (mac)
sudo curl https://bootstrap.pypa.io/get-pip.py | python

If you try to run
sudo -H pip3 install --upgrade pip3
you will get the following error:
WARNING: You are using pip version 19.2.3, however version 21.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
but if you upgrade using the suggested command:
pip install --upgrade pip
then, the legacy pip will be upgraded, so what I did is the following:
which pip3
and I located my pip3 installation (just in case the following command wouldn't upgrade the legacy pip. Then i changed to that directory and upgraded pip3 using the following commands: (your directory could be different)
cd /Library/Frameworks/Python.framework/Versions/3.8/bin
sudo -H pip3 install --upgrade pip
after this:
pip --version
will still show the legacy version, while
pip3 --version
will show pip 21.0.1

How do I update/upgrade pip itself from inside my virtual environment?

I'm able to update pip-managed packages, but how do I update pip itself? According to pip --version, I currently have pip 1.1 installed in my virtualenv and I want to update to the latest version.
What's the command for that? Do I need to use distribute or is there a native pip or virtualenv command? I've already tried pip update and pip update pip with no success.

pip is just a PyPI package like any other; you could use it to upgrade itself the same way you would upgrade any package:
pip install --upgrade pip
On Windows the recommended command is:
python -m pip install --upgrade pip

The more safe method is to run pip though a python module:
python -m pip install -U pip
On windows there seem to be a problem with binaries that try to replace themselves, this method works around that limitation.

In my case my pip version was broken so the update by itself would not work.
Fix:
(inside virtualenv):easy_install -U pip

I tried all of these solutions mentioned above under Debian Jessie. They don't work, because it just takes the latest version compile by the debian package manager which is 1.5.6 which equates to version 6.0.x. Some packages that use pip as prerequisites will not work as a results, such as spaCy (which needs the option --no-cache-dir to function correctly).
So the actual best way to solve these problems is to run get-pip.py downloaded using wget, from the website or using curl as follows:
wget https://bootstrap.pypa.io/get-pip.py -O ./get-pip.py
python ./get-pip.py
python3 ./get-pip.py
This will install the current version which at the time of writing this solution is 9.0.1 which is way beyond what Debian provides.
$ pip --version
pip 9.0.1 from /home/myhomedir/myvirtualenvdir/lib/python2.7/dist-packages (python 2.7)
$ pip3 --version
pip 9.0.1 from /home/myhomedir/myvirtualenvdir/lib/python3.4/site-packages (python 3.4)

In case you are using venv any update to pip install will result in upgrading the system pip instead of the venv pip. You need to upgrade the pip bootstrapping packages as well.
python3 -m pip install --upgrade pip setuptools wheel

for windows,
go to command prompt
and use this command
python -m pip install -–upgrade pip
Dont forget to restart the editor,to avoid any error
you can check the version of the pip by
pip --version
if you want to install any particular version of pip , for example version 18.1 then use this command,
python -m pip install pip==18.1

pip install --upgrade pip
In UBUNTU 18.04 I got the following error when I execute the above command:
ERROR: Could not install packages due to an EnvironmentError: [Errno 13] Permission denied: '/usr/local/bin/pip'
Consider using the `--user` option or check the permissions.
The below command solves my problem:
pip install --upgrade pip --user

Upgrading pip using 'pip install --upgrade pip' does not always work because of the dreaded cert issue: There was a problem confirming the ssl certificate: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version
I like to use the one line command for virtual envs:
curl https://bootstrap.pypa.io/get-pip.py | python -
Or if you want to install it box wide you will need
curl https://bootstrap.pypa.io/get-pip.py | sudo python -
you can give curl a -s flag if you want to silence the output when running in an automation script.

To get this to work for me I had to drill down in the Python directory using the Python command prompt (on WIN10 from VS CODE). In my case it was in my AppData\Local\Programs\Python\python35-32 directory. From there now I ran the command...
python -m pip install --upgrade pip
This worked and I'm good to go.

For linux
python3 -m pip install --upgrade pip
For windows:
Type Command Prompt in the Windows search box
In the Command Prompt, type cd\
Press Enter, and you’ll see the drive name C:\>
Locate your Python application path, which is the folder where you originally installed Python
Here is an example of a Python application path:
C:\Users\Ron\AppData\Local\Programs\Python\Python39
Once you retrieved the Python application path, type the following command in the Command Prompt:
cd followed by your Python application path
For our example:
C:\>cd C:\Users\Ron\AppData\Local\Programs\Python\Python39
Press Enter
Type python -m pip install --upgrade pip and press Enter

In my case this worked from the terminal command line in Debian Stable
python3 -m pip install --upgrade pip

Open Command Prompt with Administrator Permissions, and repeat the command:
python -m pip install --upgrade pip

pip version 10 has an issue. It will manifest as the error:
ubuntu#mymachine-:~/mydir$ sudo pip install --upgrade pip
Traceback (most recent call last):
File "/usr/bin/pip", line 9, in <module>
from pip import main
ImportError: cannot import name main
The solution is to be in the venv you want to upgrade and then run:
sudo myvenv/bin/pip install --upgrade pip
rather than just
sudo pip install --upgrade pip

I was in a similar situation and wanted to update urllib3 package.
What worked for me was:
pip3 install --upgrade --force-reinstall --ignore-installed urllib3==1.25.3

On my lap-top with Windows 7 the right way to install latest version of pip is:
python.exe -m pip install --upgrade pip

First, do this:
sudo apt install python3-pip python-setuptools-doc
Then, as a non-root user (NEVER, never run pip* as root!):
# N.B. bash shell works for this, I have never tested with other shells!
. ....your_virtualenv_folder/bin/activate
pip3 install -U pip
Note: -U is a synonym for --upgrade, as far as I know.

I had installed Python in C:\Python\Python36 so I went to the Windows command prompt and typed cd C:\Python\Python36 to get to the right directory. Then entered the python -m install --upgrade pip all good!

Single Line Python Program
The best way I have found is to write a single line program that downloads and runs the official get-pip script. See below for the code.
The official docs recommend using curl to download the get-pip script, but since I work on windows and don't have curl installed I prefer using python itself to download and run the script.
Here is the single line program that can be run via the command line using Python 3:
python -c "import urllib.request; exec(urllib.request.urlopen('https://bootstrap.pypa.io/get-pip.py').read())"
This line gets the official "get-pip.py" script as per the installation notes and executes the script with the "exec" command.
For Python2 you would replace "urllib.request" with "urllib2":
python -c "import urllib2; exec(urllib2.urlopen('https://bootstrap.pypa.io/get-pip.py').read())"
Precautions
It's worth noting that running any python script blindly is inherently dangerous. For this reason, the official instructions recommend downloading the script and inspecting it before running.
That said, many people don't actually inspect the code and just run it. This one-line program makes that easier.

I had a similar problem on a raspberry pi.
The problem was that http requires SSL and so I needed to force it to use https to get around this requirement.
sudo pip install --upgrade pip --index-url=https://pypi.python.org/simple
or
sudo pip-3.2 --upgrade pip --index-url=https://pypi.python.org/simple/

Head to your command prompt and type the following:
python -m pip install --upgrade pip

While updating pip in virtual env use full path in python command
Envirnments folder struture
myenv\scripts\python
h:\folderName\myenv\scripts\python -m pip install --upgrade pip

Very Simple. Just download pip from https://bootstrap.pypa.io/get-pip.py . Save the file in some forlder or dekstop. I saved the file in my D drive.Then from your command prompt navigate to the folder where you have downloaded pip. Then type there
python -get-pip.py

In linux
I will update with this code
sudo -H pip3 install --upgrade pip