For system Python or other cases where “site-packages” is a non-writable directory, the pip --user option is necessary to install a Python package under the user home directory. However, if using Python virtualenv (with or without conda) the pip --user option is invalid. Environment variables set by Python indicate when a virtualenv is being used by Python currently.

“pip” is important for locally installed packages, since pip via pyproject.toml will automatically use the latest setuptools. This is quite important as too many user systems have too-old setuptools. The project’s pyproject.toml file should contain at least:

pyproject.toml:

[build-system]
requires = ["setuptools>=61.0.0", "wheel"]
build-backend = "setuptools.build_meta"

Detect Anaconda environment by existence of environment variable CONDA_PREFIX

CMakeLists.txt

find_package(Python COMPONENTS Interpreter REQUIRED)

# detect virtualenv and set Pip args accordingly
if(DEFINED ENV{VIRTUAL_ENV} OR DEFINED ENV{CONDA_PREFIX})
  set(_pip_args)
else()
  set(_pip_args "--user")
endif()

To install a package (named in CMake variable _pypkg) from PyPI:

# install PyPI Python package using pip
execute_process(COMMAND ${Python_EXECUTABLE} -m pip install ${_pip_args} ${_pypkg})

To install a local package in development mode (live changes):

execute_process(COMMAND ${Python_EXECUTABLE} -m pip install ${_pip_args} -e ${CMAKE_CURRENT_LIST_DIR})

CMake internally in Modules/FindPython/Support.cmake detects Python virtualenv.