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(Dumb noob) Will changing directory in cmd.exe prior to installing a third party module, cause that module to be stored in that directory?

I want to install python, and then some additional modules, to my computer (windows).
I've been told that the best way to install python and packages is to make a folder in your drive somewhere BEFORE installing anything, (ex. 'Python') then pointing all your downloads (using cd in the command line) to this folder so that everything is in once place, and you don't have to go on a wild goose chase to make sure everything you want access to is in your PATH when you go to import modules etc.
Do I have the right idea?
I have had trouble importing modules in the past because they were not in the Path.
Will changing the directory in the command line before typing:
pip install somemodule
cause that module to be saved to where I just changed the directory to?

pip always installs the libraries in a fixed directory, usually in the user folder. You can check this by the command pip show <installed-package-name>
So you can use any package you already installed to get the pip directory. The location might vary based on your python version and env name.
Example: c:\users\<user>\appdata\roaming\python\python37\site-packages

how to set different PYTHONPATH variables for python3 and python2 respectively

I want to add a specific library path only to python2. After adding export PYTHONPATH="/path/to/lib/" to my .bashrc, however, executing python3 gets the error: Your PYTHONPATH points to a site-packages dir for Python 2.x but you are running Python 3.x!
I think it is due to that python2 and python3 share the common PYTHONPATH variable.
So, can I set different PYTHONPATH variables respectively for python2 and python3. If not, how can I add a library path exclusively to a particular version of python?

PYTHONPATH is somewhat of a hack as far as package management is concerned. A "pretty" solution would be to package your library and install it.
This could sound more tricky than it is, so let me show you how it works.
Let us assume your "package" has a single file named wow.py and you keep it in /home/user/mylib/wow.py.
Create the file /home/user/mylib/setup.py with the following content:
from setuptools import setup
setup(name="WowPackage",
packages=["."],
)
That's it, now you can "properly install" your package into the Python distribution of your choice without the need to bother about PYTHONPATH. As far as "proper installation" is concerned, you have at least three options:
"Really proper". Will copy your code to your python site-packages directory:
$ python setup.py install
"Development". Will only add a link from the python site-packages to /home/user/mylib. This means that changes to code in your directory will have effect.
$ python setup.py develop
"User". If you do not want to write to the system directories, you can install the package (either "properly" or "in development mode") to /home/user/.local directory, where Python will also find them on its own. For that, just add --user to the command.
$ python setup.py install --user
$ python setup.py develop --user
To remove a package installed in development mode, do
$ python setup.py develop -u
or
$ python setup.py develop -u --user
To remove a package installed "properly", do
$ pip uninstall WowPackage
If your package is more interesting than a single file (e.g. you have subdirectories and such), just list those in the packages parameter of the setup function (you will need to list everything recursively, hence you'll use a helper function for larger libraries). Once you get a hang of it, make sure to read a more detailed manual as well.
In the end, go and contribute your package to PyPI -- it is as simple as calling python setup.py sdist register upload (you'll need a PyPI username, though).

You can create a configuration file mymodule.pth under lib/site-packages (on Windows) or lib/pythonX.Y/site-packages (on Unix and Macintosh), then add one line containing the directory to add to python path.
From docs.python2 and docs.python3:
A path configuration file is a file whose name has the form name.pth and exists in one of the four directories mentioned above; its contents are additional items (one per line) to be added to sys.path. Non-existing items are never added to sys.path, and no check is made that the item refers to a directory rather than a file. No item is added to sys.path more than once. Blank lines and lines beginning with # are skipped. Lines starting with import (followed by space or tab) are executed.

I found that there is no way to modify PYTHONPATH that is only for python2 or only for python3. I had to use a .pth file.
What I had to do was:
make sure directory is created in my home: $HOME/.local/lib/python${MAJOR_VERSION}.${MINOR_VERSION}/site-packages
create a .pth file in that directory
test that your .pth file is work
done
For more info on `.pth. file syntax and how they work please see: python2 docs and python3 docs.
(.pth files in a nutshell: when your python interpreter starts it will look in certain directories and see the .pth file, open those files, parse the files, and add those directories to your sys.path (i.e. the same behavior as PYTHONPATH) and make any python modules located on those directories available for normal importing.)

If you don't want to bother with moving/adding documents in lib/site-packages, try adding two lines of code in the python2.7 script you would like to run (below.)
import sys
sys.path = [p for p in sys.path if p.startswith(r'C:\Python27')]
This way, PYTHONPATH will be updated (ignore all python3.x packages) every time you run your code.

Access a Python Package from local git repository

I have a local git repository on my machine, let's say under /develop/myPackage.
I'm currently developing it as a python package (a Django app) and I would like to access it from my local virtualenv. I've tried to include its path in my PYTHONPATH (I'm on a Mac)
export PATH="$PATH:/develop/myPackage"
The directory already contains a __init__.py within its root and within each subdirectory.
No matter what I do but I can't get it work, python won't see my package.
The alternatives are:
Push my local change to github and install the package within my virtualenv from there with pip
Activate my virtualenv and install the package manually with python setup.py install
Since I often need to make changes to my code the last two solution would require too much work all the time even for a small change.
Am I doing something wrong? Would you suggest a better solution?

Install it in editable mode from your local path:
pip install -e /develop/MyPackage
This actually symlinks the package within your virtualenv so you can keep on devving and testing.

The example you show above uses PATH, and not PYTHONPATH. Generally, the search path used by python is partially predicated on the PYTHONPATH environment variable (PATH has little use for this case.)
Try this:
export PYTHONPATH=$PYTHONPATH:/develop/myPackage
Though in reality, you likely want it to be pointing to the directory that contains your package (so you can do 'import myPackage', rather than importing things within the package. That being said, you likely want:
export PYTHONPATH=$PYTHONPATH:/develop/
Reference the python docs here for more information about Python's module/package search path: http://docs.python.org/2/tutorial/modules.html#the-module-search-path
By default, Python uses the packages that it was installed with as it's default path, and as a result PYTHONPATH is unset in the environment.

Python "setup.py develop": is it possible to create ".egg-info" folder not in source code folder?

Python has ability to "pseudoinstall" a package by running it's setup.py script with develop instead of install. This modifies python environment so package can be imported from it's current location (it's not copied into site-package directory). This allows to develop packages that are used by other packages: source code is modified in place and changes are available to rest of python code via simple import.
All works fine except that setup.py develop command creates an .egg-info folder with metadata at same level as setup.py. Mixing source code and temporary files is not very good idea - this folder need to be added into "ignore" lists of multiple tools starting from vcs and ending backup systems.
Is it possible to use setup.py develop but create .egg-info directory in some other place, so original source code is not polluted by temporary directory and files?

setup.py develop creates a python egg, in-place; it does not [modify the] python environment so package can be imported from it's current location. You still have to either add it's location to the python search path or use the directory it is placed in as the current directory.
It is the job of the develop command to create an in-place egg, which may include compiling C extensions, running the 2to3 python conversion process to create Python3 compatible code, and to provide metadata other python code may be relying on. When you install the package as an egg in your site-packages directory, the same metadata is included there as well. The data is certainly not temporary (it is extracted from your setup.py file for easy parsing by other tools).
The intent is that you can then rely on that metadata when using your package in a wider system that relies on the metadata being present, while still developing the package. For example, in a buildout development deployment, we often use mr.developer to automate the process of fetching the source code for a given package when we need to work on it, which builds it as a develop egg and ties it into the deployment while we work on the code.
Note that the .egg-info directory serves a specific purpose: to signal to other tools in the setuptools eco-system that your package is installed and available. If your package is a dependency of another egg in your setup, then that dependency is satisfied. pip and easy_install and buildout will not try and fetch the egg from somewhere else instead.
Apart from creating the .egg-info directory, the only other thing the command does, is to build extensions, in-place. So the command you are looking for instead is:
setup.py build_ext --inplace
This will do the exact same thing as setup.py develop but leave out the .egg-info directory. It also won't generate the .pth file.
There is no way of generating only the .pth file and leave out the .egg-info directory generation.
Technically speaking, setup.py develop will also check if you have the setuptools site.py file installed to support namespaced packages, but that's not relevant here.

The good manner is to keep all source files inside special directory which name is your project name (programmers using other languages keep their code inside src directory). So if your setup.py file is inside myproject directory then you should keep the files at myproject/myproject. This method keeps your sources separated from other files regardless what happen in main directory.
My suggestion would be to use whitelist instead of blacklist -- tell the tools to ignore all files excluding these which are inside myproject directory. I think that this is the simplest way not to touch your ignore lists too often.

Try the --install-dir option. You may also want to use --build-dir to change building dir.

Python: using downloaded modules

I am new to Python and mostly used my own code. But so now I downloaded a package that I need for some problem I have.
Example structure:
root\
externals\
__init__.py
cowfactory\
__init__.py
cow.py
milk.py
kittens.py
Now the cowfactory's __init__.py does from cowfactory import cow. This gives an import error.
I could fix it and change the import statement to from externals.cowfactory import cow but something tells me that there is an easier way since it's not very practical.
An other fix could be to put the cowfactory package in the root of my project but that's not very tidy either.
I think I have to do something with the __init__.py file in the externals directory but I am not sure what.

Inside the cowfactory package, relative imports should be used such as from . import cow. The __init__.py file in externals is not necessary. Assuming that your project lies in root\ and cowfactory is the external package you downloaded, you can do it in two different ways:
Install the external module
External Python packages usually come with a file "setup.py" that allows you to install it. On Windows, it would be the command "setup.py bdist_wininst" and you get a EXE installer in the "dist" directory (if it builds correctly). Use that installer and the package will be installed in the Python installation directory. Afterwards, you can simply do an import cowfactory just like you would do import os.
If you have pip or easy_install installed: Many external packages can be installed with them (pip even allows easy uninstallation).
Use PYTHONPATH for development
If you want to keep all dependencies together in your project directory, then keep all external packages in the externals\ folder and add the folder to the PYTHONPATH. If you're using the command line, you can create a batch file containing something like
set PYTHONPATH=%PYTHONPATH%:externals
yourprogram.py
I'm actually doing something similar, but using PyDev+Eclipse. There, you can change the "Run configurations" to include the environment variable PYTHONPATH with the value "externals". After the environment variable is set, you can simply import cowfactory in your own modules. Note how that is better than from external import cowfactory because in the latter case, it wouldn't work anymore once you install your project (or you'd have to install all external dependencies as a package called "external" which is a bad idea).
Same solutions of course apply to Linux, as well, but with different commands.

generally, you would use easy_install our pip to install it for you in the appropriate directory. There is a site-packages directory on windows where you can put the package if you can't use easy_install for some reason. On ubuntu, it's /usr/lib/pythonX.Y/dist-packages. Google for your particular system. Or you can put it anywhere on your PYTHONPATH environment variable.
As a general rule, it's good to not put third party libs in your programs directory structure (although there are differing opinions on this vis a vis source control). This keeps your directory structure as minimalist as possible.

The easiest way is to use the enviroment variable $PYTHONPATH. You set it before running your scripts as follows:
export $PYTHONPATH=$PYTHONPATH:/root/externals/
You can add as many folders as you want (provided their separate by :) and python will look in all those folders when importing.