# Python Interpreter found but Python\_EXECUTABLE not populated

**URL:** <https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398>\
**Category:** Code\
**Tags:** os:linux, gen:makefiles\
**Created:** [May 25, 2021, 8:22am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398 "2021-05-25T08:22:12Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![satrapes](https://discourse.cmake.org/user_avatar/discourse.cmake.org/satrapes/32/377_2.png) [@satrapes](https://discourse.cmake.org/u/satrapes)\
**Post date:** [May 25, 2021, 8:22am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/1 "2021-05-25T08:22:12Z")

</div>

Excerpt from my CMakeLists.txt

```
add_custom_command(
    OUTPUT ${PROJECT_SOURCE_DIR}/tests/bit_tests_lut.c
    COMMAND ${Python_EXECUTABLE} ${PROJECT_SOURCE_DIR}/python/generate_bit_lut_for_unit_tests.py
    DEPENDS ${PROJECT_SOURCE_DIR}/python/generate_bit_lut_for_unit_tests.py
)
add_custom_target(
    generated_bit_tests_lut
    DEPENDS ${PROJECT_SOURCE_DIR}/tests/bit_tests_lut.c
)

```

Output

```
-- The C compiler identification is GNU 11.1.0
-- The CXX compiler identification is GNU 11.1.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Python: /usr/bin/python3.9 (found version "3.9.5") found components: Interpreter 
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE  
-- Configuring done
-- Generating done
-- Build files have been written to: /home/satrapes/workspace/satrapes_bit_lib/cmake_bin
[7%] Generating ../../tests/bit_tests_lut.c
/bin/sh: line 1: ../../python/generate_bit_lut_for_unit_tests.py: Permission denied

```

So as you can see python interpreter executable has been found but it has not populated the relevant variable. (You notice that by the absence of python or /usr/bin/python3.9 after `/bin/sh: line 1:`

I have also tried ${Python2\_EXECUTABLE}, ${Python3\_EXECUTABLE} and ${PYTHON\_EXECUTABLE}

Any suggestions please?

---

<div class="post-metadata">

**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [May 25, 2021, 10:27am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/2 "2021-05-25T10:27:35Z")

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Can you show an [SSCCE](http://sscce.org/) of your CMake code? Namely, does the `find_package()` call come before the `add_custom_command()`, and is it in correct scope?

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<div class="post-metadata">

**Author:** ![satrapes](https://discourse.cmake.org/user_avatar/discourse.cmake.org/satrapes/32/377_2.png) [@satrapes](https://discourse.cmake.org/u/satrapes)\
**Post date:** [May 25, 2021, 10:49am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/3 "2021-05-25T10:49:19Z")

</div>

You are right about the sscce, I will show it later today when I get home if needed.

As far as I understand CMake does its introspection thing and detects python by itself. I don’t explicitly call `find_package()`. I assume that my commands run after that phase where I would have expected `Python_EXECUTABLE` to have been populated since it says it has found the Python Interpreter.

What I can tell you from memory is that the `add_custom_command` is in `root_dir/tests/CMakeLists.txt` and there is a line in `root_dir/CMakeLists.txt` that would `add_subdirectory(tests)`. The root CMakeLists.txt doesn’t do much apart from defining version and adding subdirectories.

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<div class="post-metadata">

**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [May 25, 2021, 12:35pm UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/4 "2021-05-25T12:35:03Z")

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> [@satrapes](#):
>
> As far as I understand CMake does its introspection thing and detects python by itself.

Not unless someone tells it to do so (your project, something you `find_package` for, your toolchain file(?)). A minimal CMake project does not look for Python. Locate who’s asking for it, and you may find something to help you solve the issue. CMake’s `--debug-find` command-line option might come in handy.

---

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**Author:** ![marc.chevrier](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/m/ecb155/32.png) [@marc.chevrier](https://discourse.cmake.org/u/marc.chevrier)\
**Post date:** [May 25, 2021, 2:00pm UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/5 "2021-05-25T14:00:30Z")

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`CMake` messages show that `Python` is effectively founded so `find_package(Python)` is called somewhere and `Python_EXECUTABLE` is defined. But, clearly, not in the scope (surely in some lower scope) where `add_custom_command` is used. This explain why `Python_EXECUTABLE` is not defined.

---

<div class="post-metadata">

**Author:** ![satrapes](https://discourse.cmake.org/user_avatar/discourse.cmake.org/satrapes/32/377_2.png) [@satrapes](https://discourse.cmake.org/u/satrapes)\
**Post date:** [May 31, 2021, 9:12am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/6 "2021-05-31T09:12:24Z")

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Thank you very much @Angew @marc.chevrier.  
You were right in everything and I am sorry that I haven’t replied earlier but it got busy at work and didn’t have any time.  
I was adding the subdirectory for googletests which looked for the python executable, prior to the custom command. My project is structured according to this site [GoogleTest · Modern CMake](https://cliutils.gitlab.io/modern-cmake/chapters/testing/googletest.html) (the submodule method).  
So I think that it has to do with the scope (which I only now realized partly how it works). So I guess that Python\_EXECUTABLE is set in the scope of the `add_subdirectory(googletest)` command and that is why it is not seen in the following command. Any thoughts on how to handle this?  
I have thought about maybe doing an `include` instead of an `add_subdirectory` and the other option is to do another find\_package(Python) in the parent scope.  
I don’t like the second choice much because you duplicate the effort.

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<div class="post-metadata">

**Author:** ![marc.chevrier](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/m/ecb155/32.png) [@marc.chevrier](https://discourse.cmake.org/u/marc.chevrier)\
**Post date:** [May 31, 2021, 9:18am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/7 "2021-05-31T09:18:31Z")

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Just add `find_package(Python...)` command where you need python! This is the normal way to use packages.

There is no real duplication because most of the costly work is stored in cached variables so subsequent calls of `find_package(Python..)` had nearly no cost.

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<div class="post-metadata">

**Author:** ![satrapes](https://discourse.cmake.org/user_avatar/discourse.cmake.org/satrapes/32/377_2.png) [@satrapes](https://discourse.cmake.org/u/satrapes)\
**Post date:** [May 31, 2021, 9:21am UTC](https://discourse.cmake.org/t/python-interpreter-found-but-python-executable-not-populated/3398/8 "2021-05-31T09:21:44Z")

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Thanks @marc.chevrier