[C++] Remove "python" from conda_env_cpp.txt

[asfimport]

I'm not sure why "python" is in this dependency file – if it is used to maintain a toolchain external to a particular Python environment then it confuses CMake like

CMake Warning at cmake_modules/BuildUtils.cmake:529 (add_executable):
  Cannot generate a safe runtime search path for target arrow-python-test
  because there is a cycle in the constraint graph:

    dir 0 is [/home/wesm/code/arrow/cpp/build/debug]
    dir 1 is [/home/wesm/miniconda/envs/arrow-3.7/lib]
      dir 2 must precede it due to runtime library [libcrypto.so.1.1]
    dir 2 is [/home/wesm/cpp-toolchain/lib]
      dir 1 must precede it due to runtime library [libpython3.7m.so.1.0]

  Some of these libraries may not be found correctly.
Call Stack (most recent call first):
  src/arrow/CMakeLists.txt:52 (add_test_case)
  src/arrow/python/CMakeLists.txt:139 (add_arrow_test)

Reporter: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #14072

_{Note: This issue was originally created as ARROW-6459. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I still find this to be a nuisance, @kszucs could you take a look at this?

[asfimport]

Krisztian Szucs / @kszucs:
Yep. I excluded it originally, but to build/test the libarrow_python (perhaps on travis) we added it.

[asfimport]

Wes McKinney / @wesm:
Do you want to do this for 1.0.0?

[asfimport]

Krisztian Szucs / @kszucs:
Not necessarily, moving to 2.0.0.

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
@AlenkaF now libarrow_python has moved to pyarrow, this should be doable?

[asfimport]

Alenka Frim / @AlenkaF:
It should, yes. Will make a draft PR and see what the CI thinks.

[asfimport]

Alenka Frim / @AlenkaF:
Additional question me and Joris have is about the need for Python for linting purposes in the Arrow C++.

There is still a part of the arrow/cpp/CMakeLists.txt code that needs Python for linting targets

arrow/cpp/CMakeLists.txt

Lines 179 to 192 in 47314c3

	# Needed for linting targets, etc.
	if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
	find_package(PythonInterp)
	else()
	# Use the first Python installation on PATH, not the newest one
	set(Python3_FIND_STRATEGY "LOCATION")
	# On Windows, use registry last, not first
	set(Python3_FIND_REGISTRY "LAST")
	# On macOS, use framework last, not first
	set(Python3_FIND_FRAMEWORK "LAST")
	find_package(Python3)
	set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
	endif()

There are several python files in arrow/cpp/build-support/
https://github.com/apache/arrow/tree/47314c3999d7b7a7f9167c6ed6793da756c411a1/cpp/build-support

One of them for example is called from a cmake target in cpp/CMakeLists.txt

arrow/cpp/CMakeLists.txt

Lines 259 to 278 in 47314c3

	if(${CLANG_FORMAT_FOUND})
	# runs clang format and updates files in place.
	add_custom_target(format
	${PYTHON_EXECUTABLE}
	${BUILD_SUPPORT_DIR}/run_clang_format.py
	--clang_format_binary
	${CLANG_FORMAT_BIN}
	${COMMON_LINT_OPTIONS}
	--fix
	${ARROW_LINT_QUIET})
	# runs clang format and exits with a non-zero exit code if any files need to be reformatted
	add_custom_target(check-format
	${PYTHON_EXECUTABLE}
	${BUILD_SUPPORT_DIR}/run_clang_format.py
	--clang_format_binary
	${CLANG_FORMAT_BIN}
	${COMMON_LINT_OPTIONS}
	${ARROW_LINT_QUIET})
	endif()

These files can still be useful for development in general. So the question is whether we want to keep Python included for the reason of linting cpp code?

[asfimport]

Alenka Frim / @AlenkaF:
cc @kszucs ^^

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
As Alenka mentioned above as well, at the moment you still need "a" Python available on your system when building Arrow C++, because the cmake files look for Python for the lint targets (and AFAIK this is always done, and you can't turn this off. But maybe we should change this and make this optional?)

As long as Python is required to build C++, I think it makes sense to keep Python in the conda env.

[github-actions]

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