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Running pip install twice to see changes ("developer mode") -- second install fails but first works

I am wondering how to use pip to develop a Python package which is going through many revisions rapidly. My work flow is to write C++ code, compile and install with pip install and test my code.
Then, I would like to change some underlying C++ code, recompile and reinstall with pip, test the new feature, change something else, go back etc. until my package is ready.
Why did pip install ./cmake_example work well the first time but when making changes to the code, reinstalling with recompiling produced an error? I just re-ran the command pip install ./cmake_example.
I changed a single line of C++ code in an innocuous way (adding +1 in the 'add' function just to see if I can change code and recompile) and the code compiled fine in my IDE without pip.
My basic idea was to use pip following this method to avoid having to hackishly insert my shared object into some python directory each time I make a change.
I used the cmake_example from pybind here and followed the steps and did pip install ./cmake_example and it worked very well. I ran the example fine in a Python console.
Then, I changed some code (just added +1 to the adding function), so nothing substantial and wanted to re-install the package.
I then got this error:
Building wheels for collected packages: cmake-example
Building wheel for cmake-example (pyproject.toml) ... error
error: subprocess-exited-with-error
× Building wheel for cmake-example (pyproject.toml) did not run successfully.
│ exit code: 1
╰─> [65 lines of output]
running bdist_wheel
running build
running build_ext
CMake Error at CMakeLists.txt:2 (project):
Running
'/tmp/pip-build-env-q_8_pyjk/overlay/bin/ninja' '--version'
failed with:
No such file or directory
and
ERROR: Could not build wheels for cmake-example, which is required to
install pyproject.toml-based projects
I have tried pip uninstall cmake_example and then reinstall but to no avail. The upgrade function did not work either. Does pip change something in the project folder?
PS: The makers of pybind11 also provide a scikit example here where removing the cache for repeated builds is not necessary. The cmake_example seems to be intended for legacy projects that do not use the cmake extension scikit. So if the structure of the cmake_example is essential to the project, removal of the cache is the only way to go.

I found that deleting the build directory inside the cmake_example directory resolved the problem and pip install ./cmake_example worked again as it did the first time. You can combine the two commands:
rm -rf ./cmake_example/build && pip install ./cmake_example
Looking a little closer, (for me) it was sufficient to delete cmake_example/temp.linux-x86_64-3.8/CMakeCache.txt. I suspect the lines
//Program used to build from build.ninja files.
CMAKE_MAKE_PROGRAM:FILEPATH=/tmp/pip-build-env-yqopewjn/overlay/bin/ninja
inside CMakeCache.txt mean that cmake caches the path to the ninja executable, but the second time around the temporary path is different, so it can no longer be found where cmake expects it.

Error installing PyMVPA2

I'm getting the following error when I try to install using pip install for PyMVPA2 for Python. I have installed other libraries without any problems before. I would appreciate if anything could take a look at the errors:
C:\Users\usr>pip install pymvpa2
Collecting pymvpa2
Using cached pymvpa2-2.4.2.tar.gz
Complete output from command python setup.py egg_info:
running egg_info
running build_src
build_src
building extension "mvpa2.clfs.libsmlrc.smlrc" sources
building extension "mvpa2.clfs.libsvmc._svmc" sources
creating build
creating build\src.win32-2.7
creating build\src.win32-2.7\mvpa2
creating build\src.win32-2.7\mvpa2\clfs
creating build\src.win32-2.7\mvpa2\clfs\libsvmc
swig.exe++: mvpa2\clfs\libsvmc\svmc.i
swig.exe -python -I3rd\libsvm -c++ -I3rd\libsvm -o build\src.win32- 2.7\mvpa2\clfs\libsvmc\svmc_wrap.cpp -outdir build\src.win32-2.7\mvpa2\clfs\libsvmc mvpa2\clfs\libsvmc\svmc.i
error: command 'swig.exe' failed: No such file or directory
----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in c:\users\usr\appdata\local\temp\pip-build-v_zzkd\pymvpa2\

As the error states you either don't have swig installed or it is not on your path. You are obviously on Windows so you need to download the swig executable from the swig website - download the latest windows package and either unzip it to a directory on your existing path or to a new directory and add it to your path. Easiest, of course, is to unzip it to a new directory and then run your pip command in that directory.
What is swig anyway? Swig parses the interface definitions of code written in C/C++ and can output 'glue code' to allow code written in Python & a pile, currently 22, of other programming or scripting languages to use the C/C++ code transparently. Swig is free, open source and compatible with both open source and commercial use.
Should I keep it to hand? In most cases that I know of in python swig is only invoked during the installation or setup phase but since that setup phase will be used every time that any package built with swig is installed or upgraded there is no reason to get rid of it and quite a lot of reasons to have it somewhere on your path.

sudo apt-get remove swig
sudo apt-get install swig3.0
sudo ln -s /usr/bin/swig3.0 /usr/bin/swig
and then
pip install pymvpa2

Problems with installing Cython on a computer running Windows 7

I am trying to install Cython on a computer running Windows 7. I am using MinGW for my C-compiler. I've taken the following steps:
Installed MinGW.
Added C:\MinGW\bin to Path. Made sure Path included no empty spaces.
Created the file distutils.cfg in the distutils directory. It reads:
[build]
compiler=mingw
[build_ext]
compiler=mingw
I start the command prompt, got to the proper directory and type "python setup.py install". I get the following output:
running install
running build
running build_py
running build_ext
building 'Cython.Plex.Scanners' extension
error: Unable to find vcvarsall.bat
As far as I've understood, this is an error regarding setting MinGW as the C-compiler, but I've aldready included it into Path. What more can I do? Where do I go from here?

Open CMD and type:
SET VS90COMNTOOLS=%VS100COMNTOOLS%
Change the numbers for the edition value of Visual Studio you are running :)
Possibly stolen from here: error: Unable to find vcvarsall.bat
EDIT:
you can download the necessary packages from here:
http://go.microsoft.com/?linkid=7729279
It also sets VS90COMMNTOOLS for you :)

Why wont You use prebuilded packages from
Here
?
I also tried compiling Cython and also failed. This way is far easier :)

Error while installing CPLEX 12.6 for Python

I am trying to install CPLEX for python from the setup.py file existing in ILOG directory.
I am working on windows 8 32bit and python 2.7. I am using this command for installing:
python setup.py install
But I get this error,
error: could not create 'build': access is denied.
Is it due to the permission? How can I fix this?
thanks

I have no idea about CPLEX but processes in windows8 usually don't get write access to certain directories.
You could try to make your call "python setup.py install" in a cmd started as adminstrator, for example: windows key, search "cmd", right click, "run as adminstrator" ...

Installing (build) matplotlib in mac osx lion

I installed pynum and scipy (on osx Lion with python 2.7), but when I tried to build matplotlib
git clone https://github.com/matplotlib/matplotlib.git
cd matplotlib
python setup.py build
python setup.py install
I've got these errors:
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/ATS.framework/Headers/ATSTypes.h:242: error: declaration for parameter ‘FMFontDirectoryFilter’ but no such parameter
src/_macosx.m:5912: error: expected ‘{’ at end of input
lipo: can't open input file: /var/folders/qw/pr2f7vq91b3c3ngkxrrqplm8zkv09r/T//ccAgEklo.out (No such file or directory)
error: command 'llvm-gcc-4.2' failed with exit status 1
Could somebody please tell me what is the problem?
P.S.
At first I tried to install it in this way:
pip install -e git+https://github.com/matplotlib/matplotlib#egg=matplotlib-dev
but it does not worked for me

I always find this process more painful than it should be, but I've done it a few times now and I believe that these steps should get you set up:
Get Xcode 4.3.2, it's required for some of the later steps.
Download the latest version of python for OSX from python.org
Grab the Scipy superpack.
Uninstall any previous versions of numpy/matplotlib/scipy that you currently have. That includes doing cd /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/ and moving any numpy/matplotlib/scipy directories or eggs into a temp directory.
cd ~/Downloads(or wherever you downloaded the superpack script to) and run sh install_superpack.sh. Answer no to the question are you installing from a repository cloned to this machine or you'll be confused about why the script keeps failing.
That should be it! You should now be able to boot up the python console and import numpy, scipy, matplotlib.