Develop Reference

Pip is using wrong path on macOS

pip is installing packages into wrong path (/opt/anaconda3/bin/pip), since I don't want to use anaconda as my interpreter, how can I change it? I am using macos.

It sounds like you don't want Conda to be the default Python in the system. In that case, read instructions to disable auto-activation. Otherwise, one can always explicitly use a particular interpreter with the syntax:
/path/to/my/python -m pip install pkgname

How to move all modules to new version of Python (from 3.6 to 3.7)

I just upgraded to python 3.7 and I realized that all my modules stuck with the previous version. Even Django is not recognised anymore. How can I do to transfer everything to the new version? I am a little lost right now, don't even know where the new version has been installed.
Edit:
When I do $ which python3.6 the terminal tells me it doesn't exist, but I have a python3.6 directory in /usr/local/lib/, where all modules are installed.
In the same directory /usr/local/lib/ I also have a python3.7 directory with some modules installed but many are missing. However when I search for the file python3.7 in my finder it doesn't appear. when I do $ which python3.7 the path is /usr/local/bin so not the same path as the directory.
Anyone sees what happened and knows how I can transfer all modules to python3.7?

Even if the old python version has been removed, it is possible to use the pip of the current python version with the --path option to list all the modules installed in the previous version.
For example, migrating all my user installed python modules from 3.7 to 3.8
pip freeze --path ~/.local/lib/python3.7/site-packages > requirements.txt
pip install --user -r requirements.txt
Incidentally, I always use pip install with --user and leave the system wide installations to the package manager of my linux distro.

It is safer to re-install all packages due to possible compatibility issues:
pip3.6 list | awk '{print $1}' | xargs -I{} pip3.7 install {}

in older version of Python --run the command
pip freeze > requirements.txt
download and install newer version on python.. change the PATH variable to the new version
and run the command
pip install -r requirements.txt

I'm not sure about all modules...but if you want to install a module specifically in python3.7, try this:
python3.7 -m pip install *module_name*

In some cases, we don't have the opportunity to pip freeze in old version--because I've already updated and old version have been purged! There are some measures I've taken to recover some of the packages but I'm NOT sure every package would work with this fix.(e.g. the packages built with wheels)
mv /your/path/to/python3.{6,7}/site-packages/
If the case is packages installed outside venv (in /usr/local/ or ~/.local), reinstall pip with get-pip.py, just to be safe.
If you are recovering a virtualenv. Activate your virtualenv and use my script
Most of your packages should work by now. If anything malfunctions, pip reinstall would works. If you still want it 100% works, pip freeze now.😉

I have an alternative
(Not sure if works outside Windows 10)
I'm currently migrating from 3.7 to 3.8 and the way I found to re-install my previous libraries was by using a script I had that updates all packages via pip install. (Assuming you installed your new Python version as your main version) This checks for all the packages I had and updates/install them in the new Python version.
Note: I prefer to run the script from the command line
Use the file explorer to go to the folder where you have the script;
Click on the path box, write "cmd" and press enter to open a command line from the folder where you are;
Write "python name_of_your_script.py" and press enter to run the command.
The script (adapted from this solution):
import pkg_resources
from subprocess import call
packages = [dist.project_name for dist in pkg_resources.working_set]
[call("pip install " + name + " --upgrade") for name in packages]

I faced a similar problem, now that I upgraded from python 3.7 to python 3.8 (new)
I installed Python 3.8, but the system kept the python37 subfolder with the already installed packages(...\Python37-32\Lib\site-packages) even with the Pyhton38 subfolder created, with the new python.exe.
Usually, it's possible to keep on using the old libraries on your new Python version, because the existent libraries installation folder are already registered in your local computer system path (*).
Even though, I've had problems to use some libraries (some worked in Jupyter Notebook but not in Spyder). I tried the alternatives others proposed to migrate libraries but it did not worked (maybe I did not
So I used brutal force solution.. Not elegant at all, but it worked:
remove the OLD python version folders from the system path or even remove the folder itself for good..
Folders: C:\Users\USERNAME\AppData\Roaming\Python\Python37
C:\Users\USERNAME\AppData\Local\Programs\Python\Python37
Reinstall the packages you need, preferably via Anaconda prompt.
python -mpip install library_name
OR
pip install --user --force-reinstall package_name
OR
pip install --user --force-reinstall package_name == specify_package_version
The libraries will be installed at c:\users\USERNAME\anaconda3\lib\site-packages and recognized by your new python version.
(*) to add the folder to the PATH: System properties --> environment variables --> click "Path"--> edit --> add folder name)

Pip installs to anaconda directory instead python's directory (Windows)

Whenever I try to install package with pip (using wheel or just regular pip install numpy ->e.g), pip installs new package to location where Anaconda holds its site-packages. How do I remove that? That started happening since I installed Anaconda which I use for some tasks as python interpreter, but now I need my regular python installation.

If you have Python 3 installed but you see that which pip points to your Anaconda installation, try using pip3 instead - if it is available then you will see that which pip3 points to your Pythons installation path instead of your Anaconda path. Same with which python3.

Instead of just writing pip instal ... in the command line, which apparently points to your Anaconda installation, you can navigate (using the cd command) to your Python installation and invoke the pip.exe file located somewhere there.
I guess you could try renaming one of pip.exe files (the one in Anaconda or the one in Python) to something else (e.g. pipanadonda.exe), and then you will be able to call them separately from the command line.

Package installed by Conda, Python cannot find it

I try to install Theano by Anaconda. It works, but when I enter the python -i, import theano shows No module named 'theano'. Do I need to switch another interpreter of Python, how? Also, for the packages installed by conda, if I don't double install them, can I find in Python? How is Python related to Python by Anaconda? Thanks!!!

I had have a similar issue, trying to install folium. If you are using the Anaconda:
When you install using conda install -c conda-forge folium, the package will be placed in:
./anaconda3/envs/[name env]/lib/python3.7/site-packages/folium
When you install using pip (with a anaconda env activated), pip install folium, the package will be placed in:
./anaconda3/lib/python3.7/site-packages/folium
Python use first the sites-packages as the target directory of manually built python packages. When you build and install python packages from source (using distutils, probably by executing python setup.py install ), you will find the installed modules in site-packages by default.
In this case you have two places: /anaconda3/lib/python3.7/site-packages/ and /anaconda3/envs/[name env]/lib/python3.7/site-packages/.
First the modules will be available as default in /anaconda3/lib/python3.7/site-packages/. Sometimes (and I really don't know why) the modules inside sites-packages conda env are not available to import automatically without export the PATH.
So, to solve this issue, you have 2 options:
Installing using pip install folium and import folium (don't need install by conda install), or
After conda install , run conda init, close the terminal and open a new one. So, try to import again.
Here are some tips about use a pip in a conda-environment.

You can refer to a specific version of python by using the following at the first line of your .py file
This is for python 2.7
#!/usr/bin/env python2.7
This is for python 3
#!/usr/bin/env python3
As other users already pointed out you need to check if your module is included in your sys path. Use code:
import sys
print(sys.path)
If not you can include this in your sys.path by using the command:
sys.path.append('/path/to/the/folder/of/your/module/file')
or place it in default PYTHONPATH itself.
Other great answers:
https://stackoverflow.com/a/19305076/5381704

The problem is that in the code editor you are using, you are running the default interpreter. Based on your code editor, change the python interpreter to the conda interpreter and it will work.

In my case that happened because conda screwed up the environment variables. Instead of using env-specific python and pip, it used the globally installed ones.
Solution:
conda deactivate your-env
conda activate your-env

In my workstation, I was able to solve No module named <module name> error using two different ways.
First method, I solved this temporarily by:
(1) Open a Terminal
(2) $ conda activate <Conda environment name>
(3) $ export PYTHONPATH=/home/<user name>/anaconda3/envs/<Conda environment name>/lib/<Python package version>/site-packages:$PYTHONPATH
It is a temporary solution. Whenever you run your virtual environment, you have to do this.
My runtime environment:
    OS: Unbuntu 18.04
    Conda version: 4.8.2
    Conda-build version: 3.18,11
    Python version 3.7.6.final.0
Second method, I removed the
alias python=/usr/bin/python3.6 line in bashrc file.
Somehow this line blocks using Python tools installed in Anaconda Virtual Environment if the Python version in the Virtual Environment is different.

How to configure pip to install packages for python 2.x and python 3.x separately

I am running Windows x64 bit.
I downloaded the Pyro4 package via pip install Pyro4. It downloaded the packages successfully and they are all present in my "C:\Python34\Scripts" folder as I've kept Python3.4 as default.
Now when I went to the that "C:\Python27\Scripts" folder, the Pyro4 package is not to be found. This is as expected, but I would like to work on both Python 2.7 and 3.4 as Pyro4 is compatible in both.
How do I change my pip command to download the package to Python 2.7's installation scripts directory?

First make a new environment variable:
Go to your system properties
Under Advanced tab click Environment Variables...
Under System variables section click New...
Variable name: (whatever you can remember for example p27s)
Variable value: your python 2.7 scripts folder ("C:\Python27\Scripts\")
From now on whenever you want to install a package for python 2.7 you can do it this way: %your_variable_name%pip install package_name
For example: C:>%p27s%pip install Pyro4
This way you can install any package for python 2.7 and use default pip for python 3.4

You will need to go to your environment variables in the control panel and change the path from C:\Python34\Scripts to C:\Python27\Scripts. After that change, when you type 'python' in the command prompt it will be using Python 2.7. Next, install pip like you initially did.

Not exactly what you're asking, but you could check out Anaconda which allows you switch python environments really easily.
https://www.continuum.io/downloads
Edit
Just to clarify
if you were to use this method, you can have named environments say python27 and python34
then you can just:
activate python27
pip install module
activate python34
pip install module
and it will install them to both environments.