What's the Difference Between "Pip Install" and "Python -M Pip Install"

What is the difference between pip install and pip download?

I found the answer in the online docs for pip download.

IMO, some of this should be stated in the help (i.e. in the description). Otherwise, how would one know how to use it, using pip help download?

pip download replaces the --download option to pip install, which is
now deprecated and will be removed in pip 10.

pip download does the same resolution and downloading as pip install,
but instead of installing the dependencies, it collects the downloaded
distributions into the directory provided (defaulting to the current
directory). This directory can later be passed as the value to pip
install --find-links to facilitate offline or locked down package
installation.

pip download with the --platform, --python-version, --implementation,
and --abi options provides the ability to fetch dependencies for an
interpreter and system other than the ones that pip is running on.
--only-binary=:all: is required when using any of these options. It is important to note that these options all default to the current
system/interpreter, and not to the most restrictive constraints (e.g.
platform any, abi none, etc). To avoid fetching dependencies that
happen to match the constraint of the current interpreter (but not
your target one), it is recommended to specify all of these options if
you are specifying one of them. Generic dependencies (e.g. universal
wheels, or dependencies with no platform, abi, or implementation
constraints) will still match an over- constrained download
requirement.

Difference between installing by pip and installing globally

These are 2 separate package managers, that sometimes don't play well with each other.

# linux system level as root
(sudo) apt-get install

# inside of an more isolated python folder structure, that does not interface with the system level packages
(venv) pip install

You may* be able to install with all of the build tools:

sudo apt-get install python-pip python-dev build-essential 
pip install --upgrade pip

Is there a difference between brew install and pip install?

well, packages for OSX may include packages for python.

pip is a packager for the python world - you should only ever be able to install python-things with it; homebrew is a package manager targetted at OSX; it doesn't impose any restrictions onto what software you can install with it - since python is a subset of software.

installing things with brew will install them into /usr/local/;

installing things with pip will fetch packages from the Python Package Index, and it will install them in a place where your python interpreter will find them: either into your home directory (e.g. ~/.local/lib/python2.7/site-packages/) or in some global search-path of your python interpreter (e.g. /usr/local/lib/python2.7/dist-packages/)

if you have installed the python interpreter via brew, then chances are high that any python-package installed via brew will be usable out of the box.

What is the difference between pip install and sudo pip install?

pip install

Will run pip install as the current user

sudo pip install

Will run pip install with the security privileges of another user, root for example.

You normally need to use sudo to install a package on a system.

You may want to read linux-101-introduction-to-sudo

difference between pip install -r pip-requires VS setup.py install

pip install -r pip-requires will only install the dependencies as mentioned in the pip-requires file. setup.py install, will install the project itself along with the dependencies as mentioned in install_requires within setup.py

Whats the difference between `pip install pyqt5` and `sudo apt-get install python3-pyqt5`? [here pyQt5 itself is more a placeholder than anything]

apt installs binary .deb packages from apt repositories into system. pip installs binary wheels or source packages from pypi into system or into virtual environment. Python developers publish packages into pypi, you have opportunity to install from apt only if linux distribution maintainers (for some reason) took trouble of packaging for apt. apt packages are likely more stable, pip packages are likely more recent.