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I'm confused by pip installations

I have recently begun having troubles using pip to install python packages. I have always used pip but never really understood how it actually works, my experience with it is basically limited to "pip install pkg".
Recently when trying to install openCV on my machine, I followed a few guides that involved changing paths etc. Since making these changes I have been having trouble using pip to install packages correctly.
Now when I run "pip3 install pkg", the install runs fine without any errors. When I try to import the module in python however, python cannot find the package. If I run "pip3 list" in the terminal I get a list of modules that is different to running help('modules') within python.
I think pip is installing the packages to a different location than my version of python is referencing when importing modules?
Is there a way I can change where pip installs to? What did it mean to change paths and how can I avoid this in the future?
Thanks in advance.
EDIT: I should mention that running "python3 -m pip install pkg" installs the packages correctly.

Because you have 2 versions of python installed, the best solution is to install and use virtualenv
A Virtual Environment is a tool to keep all dependencies required by different projects and python versions in separate places. It solves the problem you mentioned and keeps your site-packages directory manageable.

Tox installs the wrong version of pip to it's virtual env

I am using tox to manage some testing environments. I have a dependency (backports.ssl-match-hostname) that I cannot download using the latest version of pip, so I need to revert back to pip 8.0.3 to allow the install to work.
I have included the 8.0.3 version of pip inside my tox.ini file for dependencies.
deps=
pip==8.0.3
However, when I run
source .tox/py27/bin/activate
and enter the virtual testing environment, and then run
pip --version
I end up with
8.1.2
However, outside of my tox environment, when I run the same command, I get
8.0.3
Is there anything special that tox does when grabbing pip? Why am I not able to specify the version of pip that I want to use as a dependency?
EDIT : to add to this, it seems as though I am able to grab the dependency pip==8.0.3, but for the other dependencies, they are still running from the command launched with pip==8.1.2
So, I need to be able to grab pip==8.0.3 first, and then once installed, grab everything else. Still unsure why tox is starting with pip==8.1.2

This was apparently the result of the "virtualenvs" python package containing a pre-selected group of python packages that it refers to, one of which was the latest and greatest pip.
I don't know if this is the preferred way of doing this, but I found success by running
pip uninstall virtualenv
And then reinstalling with the version that worked
pip install virtualenv==15.0.1
With the "correct" version of virtualenv in place, I was able to run my tox command
source .tox/py27/bin/activate
and see the desired version of pip
pip --version
pip 8.0.3

A workaround for this is here: https://github.com/pypa/pip/issues/3666
Although to make it work I had to write "pip install pip==8.1.1" in my script. So to recap:
Add a pip.sh script to your project:
#!/bin/bash
pip install pip==8.1.1
pip install "$#"
Add to your tox.ini:
install_command = {toxinidir}/pip.sh {opts} {packages}

I've recently hit this problem. I've had it for a while but it just didn't register because I had such occasional failures with Python 2/3 code. Another way that this can happen is, if like me, you change the virtualenv between different Python versions and don't clean up.
Check /bin or /Scripts to see whether python2 points to python. If the virtualenv is Python 3 then this will mean that python2 actually calls Python 3. Vice versa, of course, if you the virtualenv is Python 2 and you want to test Python 3 code.

New versions of virtualenv reach out to download the latest pip, setuptools, and wheel -- you can disable this behavior when running through tox with the tox-virtualenv-no-download package See: https://github.com/asottile/tox-virtualenv-no-download#wait-why

Can I have my pip user-installed package be preferred over system?

I would like to figure out a "fool-proof" installation instruction to put in the README of a Python project, call it footools, such that other people in our group can install the newest SVN version of it on their laptops and their server accounts.
The problem is getting the user-installed libs to be used by Python when they call the scripts installed by pip. E.g., we're using a server that has an old version of footools in /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/.
If I do python2.7 setup.py install --user and run the main entry script, it uses the files in /Users/unhammer/Library/Python/2.7/lib/python/site-packages/. This is what I want, but setup.py alone doesn't install dependencies.
If I (revert the installation and) instead do pip-2.7 install --user . and run the main entry script, it uses the old files in /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/ – that's not what I want.
If I (revert the installation and) instead do pip-2.7 install --user -e . and run the main entry script, it uses the files in . – that's not what I want, the user should be able to remove the source dir (and be able to svn up without that affecting their install).
I could use (and recommend other people to use) python2.7 setup.py install --user – but then they have to first do
pip-2.7 install -U --user -r requirements.txt -e .
pip-2.7 uninstall -y footools
in order to get the dependencies installed (since pip has no install --only-deps option). That's rather verbose though.
What is setup.py doing that pip is not doing here?
(Edited to make it clear I'm looking for simpler+safer installation instructions.)

Install virtualenvwrapper. I allows setting up separate python environments to alleviate any conflicts you might be having. Here is a tutorial for installing and using virtualenv.
Related:
https://virtualenvwrapper.readthedocs.org/en/latest/

Console scripts generated by pip in the process of installation should use user installed versions of libraries as according to PEP 370:
The user site directory is added before the system site directories
but after Python's search paths and PYTHONPATH. This setup allows the
user to install a different version of a package than the system
administrator but it prevents the user from accidently overwriting a
stdlib module. Stdlib modules can still be overwritten with
PYTHONPATH.
Sidenote
Setuptools use hack by inserting code in easy_install.pth file which is placed in site-packages directory. This code makes packages installed with setuptools come before other packages in sys.path so they shadow other packages with the same name. This is referred to as sys.path modification in the table comparing setuptools and pip. This is the reason console scripts use user installed libraries when you install with setup.py install instead of using pip.
Taking all of the above into account the reason for what you observe might be caused by:
PYTHONPATH pointing to directories with system-wide installed libraries
Having system-wide libraries installed using sudo python.py install (...)
Having OS influence sys.path construction in some way
In the first case either clearing PYTHONPATH or adding path to user installed library to the beginning of PYTHONPATH should help.
In the second case uninstalling system-wide libraries and installing them with distro package manager instead might help (please note that you never should use sudo with pip or setup.py to install Python packages).
In the third case it's necessary to find out how does OS influence sys.path construction and if there's some way of placing user installed libraries before system ones.
You might be interested in reading issue pip list reports wrong version of package installed both in system site and user site where I asked basically the same question as you:
Does it mean that having system wide Python packages installed with easy_install thus having them use sys.path manipulation breaks scripts from user bin directory? If so is there any workaround?
Last resort solution would be to manually place directory/directories with user installed libraries in the beginning of sys.path from your scripts before importing these libraries.
Having said that if your users do not need direct access to source code I would propose packaging your app together with all dependencies using tool like pex or Platter into self-contained bundle.

Difference between 'python setup.py install' and 'pip install'

I have an external package I want to install into my python virtualenv from a tar file.
What is the best way to install the package?
I've discovered 2 ways that can do it:
Extract the tar file, then run python setup.py install inside of the extracted directory.
pip install packagename.tar.gz from example # 7 in https://pip.pypa.io/en/stable/reference/pip_install/#examples
Is if there is any difference doing them in these 2 ways.

On the surface, both do the same thing: doing either python setup.py install or pip install <PACKAGE-NAME> will install your python package for you, with a minimum amount of fuss.
However, using pip offers some additional advantages that make it much nicer to use.
pip will automatically download all dependencies for a package for you. In contrast, if you use setup.py, you often have to manually search out and download dependencies, which is tedious and can become frustrating.
pip keeps track of various metadata that lets you easily uninstall and update packages with a single command: pip uninstall <PACKAGE-NAME> and pip install --upgrade <PACKAGE-NAME>. In contrast, if you install a package using setup.py, you have to manually delete and maintain a package by hand if you want to get rid of it, which could be potentially error-prone.
You no longer have to manually download your files. If you use setup.py, you have to visit the library's website, figure out where to download it, extract the file, run setup.py... In contrast, pip will automatically search the Python Package Index (PyPi) to see if the package exists there, and will automatically download, extract, and install the package for you. With a few exceptions, almost every single genuinely useful Python library can be found on PyPi.
pip will let you easily install wheels, which is the new standard of Python distribution. More info about wheels.
pip offers additional benefits that integrate well with using virtualenv, which is a program that lets you run multiple projects that require conflicting libraries and Python versions on your computer. More info.
pip is bundled by default with Python as of Python 2.7.9 on the Python 2.x series, and as of Python 3.4.0 on the Python 3.x series, making it even easier to use.
So basically, use pip. It only offers improvements over using python setup.py install.
If you're using an older version of Python, can't upgrade, and don't have pip installed, you can find more information about installing pip at the following links:
Official instructions on installing pip for all operating systems
Instructions on installing pip on Windows (including solutions to common problems)
Instructions on installing pip for Mac OX
pip, by itself, doesn't really require a tutorial. 90% of the time, the only command you really need is pip install <PACKAGE-NAME>. That said, if you're interested in learning more about the details of what exactly you can do with pip, see:
Quickstart guide
Official documentation.
It is also commonly recommended that you use pip and virtualenv together. If you're a beginner to Python, I personally think it'd be fine to start of with just using pip and install packages globally, but eventually I do think you should transition to using virtualenv as you tackle more serious projects.
If you'd like to learn more about using pip and virtualenv together, see:
Why you should be using pip and virtualenv
A non-magical introduction to Pip and Virtualenv for Python beginners
Virtual Environments

python setup.py install is the analog of make install: it’s a limited way to compile and copy files to destination directories. This doesn’t mean that it’s the best way to really install software on your system.
pip is a package manager, which can install, upgrade, list and uninstall packages, like familiar package managers including: dpkg, apt, yum, urpmi, ports etc. Under the hood, it will run python setup.py install, but with specific options to control how and where things end up installed.
In summary: use pip.

The question is about the preferred method to install a local tarball containing a python package, NOT about the advantage of uploading package to an indexing service like PyPi.
As lest I know some software distributor does not upload their package to PyPi, instead asking developers to download package from their website and install.
python setup.py install
This can work but not recommended. It's not necessary to unwrap the tarball file and go into it to run setup.py file.
pip install ../path/to/packagename.tar.gz
This is the way designed and preferred. Concise and align with PyPi-style packages.
More information about pip install can be found here: https://pip.readthedocs.io/en/stable/reference/pip_install/

Working with many different modules in python

I am new to python just a few weeks back i started using python(Classic Noob-Disclaimer)
Now whenever i install a module by copying the unzipped folder in site-packages under Lib and running the source install by using "c:\python27\lib\site-packages\tweepy-1.2\setup.py install" in command prompt it installs without any errors.
But now when i make a python script (*.py)
and store it on the desktop it wont work
and it gives out an error "No module found"
but when i store it in the same folder as the source it works perfectly.
also if i open the IDLE GUI it also returns the same error.
But this doesnt happen with the win32com module which i use for TTS.
I missing something..but i cudnt find the answer to it.
Plz help me!
i need to use many of these modules..they work great differently but not together as the modules are always missing!

Copying an unzipped folder to site-packages does not install a Python package.
To install manually, unzip the package to a temporary directory, then run:
python setup.py install
in this directory, after that you can remove the directory.
To download and install a pure Python package automatically, run:
pip install tweepy
if you have pip installed.
The simplest way to install Python packages that have C extensions is to use binary installers (*.exe, *.msi files).

To avoid all this use VirtualEnv
Virtualenv is a tool to create isolated Python environments.
The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these applications? If you install everything into /usr/lib/python2.7/site-packages (or whatever your platform's standard location is), it's easy to end up in a situation where you unintentionally upgrade an application that shouldn't be upgraded.
Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application.
Also, what if you can't install packages into the global site-packages directory? For instance, on a shared host.

The easiest way to install python packages is by using pip. First you need to install pip as explained here if you use windows. Then you can query some packages, from command line, for example
> pip search twitter
Then to install certain packages, just use pip something like this:
> pip install tweepy