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Does `pip install -U pip -r requirements.txt` upgrade pip before installing the requirements?

It seems to be common practice to set up a Python virtual environment using some variant of the following:
python -m venv venv && source ./venv/bin/activate
python -m pip install -U pip -r requirements.txt
What I hope the above command does is:
Upgrade pip first
Run the installation of the packages in requirements.txt
However, what actually seems to happen is:
Collects all packages, including newest version of pip
Installs them all together
The original/outdated version of pip is what actually runs the installs
And the new version of pip is not used until after this command
Question(s)
Is it possible to have pip upgrade itself and then install a requirements file, in one command?
Would this infer any particular benefits?
Should I switch to the following?
python -m venv venv && source ./venv/bin/activate
python -m pip install -U pip
python -m pip install -r requirements.txt
What is the optimal method to install requirements files?
I see people sometimes installing/upgrading wheel and setuptools as well

The answers to your questions are:
No. pip doesn't currently treat itself as a special dependency, so it doesn't know to install then execute itself, which is what it would need to do to overcome the problems you observed.
Updating pip in a separate step is indeed the recommended way to proceed.
You may from time to time see pip issue a message advising that a newer version is available. This happens a lot if you create them from a python with an outdated pip.

I had a situation similar to yours, I needed to first upgrade pip and then install a bunch of libraries in a lab that had 20 PCs. What I did was writing all the librarie's name in a requirements.txt file, then create a .bat file with two commands:
`python -m pip install --upgrade pip`
`pip install -r requirements.txt`
The first command for upgrading pip and the second one for installing all the libraries listed in the requirements.txt file.

Python update package version in requirements.txt

I have a requirement.txt file with the list of python package to install. One of the packages is psycopg2==2.6.2 I need to update this package to psycopg2==2.7. I tried to install by pip3 install psycopg2 But it doesn't affect requirement.txt file. Can you please point me in the right direction?

Notice that running pip3 install psycopg2 doesn't respect the requirements.txt file. To upgrade this package you need to use -U option:
pip3 install -U psycopg2
which is a shorthand for:
pip3 install --upgrade psycopg2
After that, you can update your requirements.txt with the following command:
pip freeze > requirements.txt
If you're looking for a solution to automatically update the requirements.txt file after you upgrade package/packages, you can use pip-upgrader.
Installation:
pip install pip-upgrader
Usage:
pip-upgrade
The above command auto-discovers the requirements file and prompts for selecting upgrades. You can also specify a path to the requirements file or/and specify a package to upgrade:
pip-upgrade /path/to/requirements.txt -p psycopg2

As you've discovered, pip doesn't update the requirements file. So the workflow you'd likely want to use is:
Update the version of psycopg2 in your requirements file from 2.6.2 to 2.7
Run pip install with the upgrade flag
pip3 install -U -r requirements.txt
If you're familiar with tools like npm that do update the version in the catalog file, you may be interested in using pipenv, which manages your dependencies and the virtual environment for you, much like npm does.
If you don't know the latest version of your package, then use pip to figure it out:
$ pip list --outdated | grep psycopg2
psycopg2 (2.7.3.2) - Latest: 2.7.4 [wheel]

you can try:
pip install --upgrade --force-reinstall -r requirements.txt
You can also ignore installed package and install the new one :
pip install --ignore-installed -r requirements.txt

How can I revert changes done to python packages?

I'm debugging an issue on a staging, and I've added a bunch of logging statements to a 3rd party package. Once I'm done with that, I'd like to get them back to their original state. In ruby, I could do a gem pristine lib_name and that would restore the lib to it's original source code.
It might be relevant to mention that I'm modifying code that was installed with sudo pip install some_pkg.
What's the usual way of reverting any changes done to a lib?

On Linux:
Just type the following command in a terminal (with pip, pip2 or pip3, accordingly to the Python version you're targetting):
sudo -H pip install --upgrade --force-reinstall some_pkg
On Windows:
Open an admin terminal, and run the following command (ditto):
pip install --upgrade --force-reinstall some_pkg

Try this
pip install -r requirements.txt --force --upgrade

I suppose you have one "requriemnt.txt" file which you want to revert back packages to:
#remove all currently installed packages
pip freeze > remove.txt
pip uninstall -r remove.txt
#re-insatll all packages
pip install -r requriement.txt
This ensures that you also remove all unwanted packages

pip - installation of sub-dependencies overrides other packages on requirements.txt

my requirements file is like that :
https://github.com/sontek/pyramid_webassets/archive/38b0b9f9f4e36dc22b3a5c10eabf4d9228d97740.zip#egg=pyramid_webassets-0.0
https://github.com/miracle2k/webassets/archive/334d55c6bcfd091cb2d984777daf943acde0d364.zip#egg=webassets-0.8.dev
when running pip install -r requirements.txt I want it to install the specific version of pyramid_webassets, and then the specific webassets version (0.8.dev)
the problem is that pyramid_webassets have the webassets as sub-dependency, and it installs the latest of this package.
so the output of pip freeze is
Chameleon==2.14
Mako==0.9.1
MarkupSafe==0.18
PasteDeploy==1.5.2
WebOb==1.3.1
argparse==1.2.1
pyramid==1.4.5
pyramid-webassets==0.0
repoze.lru==0.6
translationstring==1.1
venusian==1.0a8
webassets==0.9
wsgiref==0.1.2
zope.deprecation==4.1.0
zope.interface==4.0.5
you might notice that webassets version is the latest (0.9) though I specified the version I want (0.8.dev).
I tried to reorder the list, adding the --upgrade flag- nothing helped.
any idea how can I install it and still having the required version of webassets?
Thanks.
soultion:
I found this commend useful:
cat requirements.txt | xargs -L1 pip install
that will install one by one the packages orderly
but we should add --upgrade for the last package so it'll upgrade it.

use pip install option to not install package dependencies
$ pip install --no-deps -r requirements.txt
Doing a pip freeze afterwards
gottfried#sascha-Latitude-XT2:~/venv$ bin/pip freeze
argparse==1.2.1
pyramid-webassets==0.0
webassets==0.8.dev
wsgiref==0.1.2
References
pip cookbook - Ensuring repeatability

What happens when you move webassets higher then pyramid_webassets on the list?

How can I use a pip requirements file to uninstall as well as install packages?

I have a pip requirements file that changes during development.
Can pip be made to uninstall packages that do not appear in the requirements file as well as installing those that do appear? Is there a standard method?
This would allow the pip requirements file to be the canonical list of packages - an 'if and only if' approach.
Update: I suggested it as a new feature at https://github.com/pypa/pip/issues/716

This should uninstall anything not in requirements.txt:
pip freeze | grep -v -f requirements.txt - | grep -v '^#' | xargs pip uninstall -y
Although this won't work quite right with packages installed with -e, i.e. from a git repository or similar. To skip those, just filter out packages starting with the -e flag:
pip freeze | grep -v -f requirements.txt - | grep -v '^#' | grep -v '^-e ' | xargs pip uninstall -y
Then, obviously:
pip install -r requirements.txt
Update for 2016:
You probably don't really want to actually use the above approach, though. Check out pip-tools and pip-sync which accomplish what you are probably looking to do in a much more robust way.
https://github.com/nvie/pip-tools
Update for May, 2016:
You can now also use pip uninstall -r requirements.txt, however this accomplishes basically the opposite - it uninstalls everything in requirements.txt
Update for May, 2019:
Check out pipenv or Poetry. A lot has happened in the world of package management that makes this sort of question a bit obsolete. I'm actually still quite happily using pip-tools, though.

You can now pass the -r requirements.txt argument to pip uninstall.
pip uninstall -r requirements.txt -y
At least as of pip 8.1.2, pip help uninstall shows:
...
Uninstall Options:
-r, --requirement <file> Uninstall all the packages listed in the given requirements file. This option can be
used multiple times.
...

It's not a feature of pip, no. If you really want such a thing, you could write a script to compare the output of pip freeze with your requirements.txt, but it would likely be more hassle than it's worth.
Using virtualenv, it is easier and more reliable to just create a clean environment and (re)install from requirements.txt, like:
deactivate
rm -rf venv/
virtualenv venv/
source venv/bin/activate
pip install -r requirements.txt

The short answer is no, you can't do that with pip.

Here's a simple solution that works:
pip uninstall $(pip freeze) -y

This is an old question (but a good one), and things have changed substantially since it was asked.
There's an offhand reference to pip-sync in another answer, but deserves its own answer, because it solves precisely the OP's problem.
pip-sync takes a requirements.txt file as input, and "trues up" your current Python environment so that it matches exactly what's in that requirements.txt. This includes removing any packages that are present in your env but absent from requirements.txt.
Example: Suppose we want our env to contain (only) 3 libraries: libA, libB, and libC, like so:
> cat requirements.txt
libA==1.0
libB==1.1
libC==1.2
But our env currently contains libC and libD:
> pip freeze
libC==1.2
libD==1.3
Running pip-sync will result in this, which was our desired final state:
> pip-sync requirements.txt
> pip freeze
libA==1.0
libB==1.1
libC==1.2

Stephen's proposal is a nice idea, but unfortunately it doesn't work
if you include only direct requirements in your file, which sounds
cleaner to me.
All dependencies will be uninstalled,
including even distribute, breaking down pip itself.
Maintaining a clean requirements file while version tracking a virtual environment
Here is how I try to version-track my virtual environment.
I try to maintain a minimal requirements.txt, including only
the direct requirements, and not even mentioning version constraints where
I'm not sure.
But besides, I keep, and include in version tracking (say git),
the actual status of my virtualenv in a venv.pip file.
Here is a sample workflow:
setup virtualenv workspace, with version tracking:
mkdir /tmp/pip_uninstalling
cd /tmp/pip_uninstalling
virtualenv venv
. venv/bin/activate
initialize version tracking system:
git init
echo venv > .gitignore
pip freeze > venv.pip
git add .gitignore venv.pip
git commit -m "Python project with venv"
install a package with dependencies, include it in requirements file:
echo flask > requirements.txt
pip install -r requirements.txt
pip freeze > venv.pip
Now start building your app, then commit and start a new branch:
vim myapp.py
git commit -am "Simple flask application"
git checkout -b "experiments"
install an extra package:
echo flask-script >> requirements.txt
pip install -r requirements.txt
pip freeze > venv.pip
... play with it, and then come back to earlier version
vim manage.py
git commit -am "Playing with flask-script"
git checkout master
Now uninstall extraneous packages:
pip freeze | grep -v -f venv.pip | xargs pip uninstall -y
I suppose the process can be automated with git hooks, but let's not go off topic.
Of course, it makes sense then to use some package caching system
or local repository like pip2pi

you can create a new file with all installed packages
pip freeze > uninstall.txt
and then uninstall all of those
pip uninstall -r uninstall.txt -y
and then finally re-install the packages you had in your original requirements.txt file
pip install -r requirements.txt

Piggybacking off #stephen-j-fuhry here is a powershell equivalent I use:
pip freeze | ? { $_ -notmatch ((gc req.txt) -join "|") }

While this doesn't directly answer the question, a better alternative to requirements.txt now is using a Pipfile. This functions similarly to a Ruby Gemfile. Currently, you need to make use of the pipenv tool but hopefully this will eventually be incorporated into pip. This provides the pipenv clean command which does what you want.
(Note that you can import an existing requirements.txt with pipenv install -r requirements.txt. After this you should have a Pipfile and the requirements.txt can be removed.)

It is possible now using:
pip uninstall -r requirements.txt