Getting Error: "could not determine kind of name for C.PyEval_ReInitThreads"

[Voben]

I am trying to use the PyRun_SimpleString function for a print hello world however I am getting the error below:

../../go/pkg/mod/github.com/go-python/cpy3@v0.2.0/thread.go:53:2: could not determine kind of name for C.PyEval_ReInitThreads

This is the code that I am running.

package main

import "github.com/go-python/cpy3"

func main() {

defer python3.Py_Finalize()
python3.Py_Initialize()
python3.PyRun_SimpleString("print('hello world')")

}

I am using Python 3.7.15 and have a python3,pc file.

[christian-korneck]

what Python version are you using? This library currently only works with Python 3.7. This error usually happens when trying to use a newer version of Python.

It is possible to use newer version of Python if you remove some functions from go-python/cpy3 / datadog/go-python3 (bindings to stuff that has been removed from Python's C-API in newer version). For example, Python 3.8 works if you remove the bindings for PyEval_ReInitThreads, like I did here. More stuff might need to get removed for Python 3.9 and newer.

[Voben]

So when I run 'python --version' in the terminal, it outputs 'Python 3.7.15.' Do I need to be on an earlier version?

How would I delete the bindings. Should I download the zip of that commit or should I edit the import?

[christian-korneck]

what does pkg-config --modversion python3 say?

[Voben]

It returns 3.8 even though version is set to 3.7 and includedir to the python3.7m folder/directory that contains the Python.h file.

[christian-korneck]

not sure why that is but you can likely get it to work by:

• create a folder with a copy of the correct python3.pc file
• explicitly set env var PKG_CONFIG_PATH to this path (i.e. export PKG_CONFIG_PATH=/path/to/my-dir

make sure the env var is set when you install this Go module and when you run go build.
(also there's a chance that you might already have a "bad" binary in your Go cache from the previous installation attempt, here go clean -cache and go clean -modcache should help).

[christian-korneck]

btw what's your OS + OS version and how did you install Python 3.7?

[Voben]

So I edited the PKG_CONFIG_PATH var, I hadn't specified the pkg-config folder but just the directory it is in. Running pkg-config --modversion python3 now returns 3.7 as intended. After running the go clean -cache and go clean -modcache commands go build returns this error instead (which I have encountered before but thought I had solved by specifiying the includedir in the python3.pc file):

../../go/pkg/mod/github.com/go-python/cpy3@v0.2.0/boolean.go:11:10: fatal error: Python.h: No such file or directory

And I'm running it on a WSL Ubuntu 20.04 and I believe I had installed Python using the command sudo apt install python3.7.

[christian-korneck]

I just tried on a vanilla Ubuntu 20.04 container and this worked for me (can't think of anything that shouldn't also be working on WSL):

docker run --name ub1 --hostname ub1 -ti ubuntu:20.04

and in the Ubuntu 20.04 bash shell run (all as root):

apt update
apt install wget vim
apt install git #for go get/install
apt install build-essential #gcc for cgo
apt install pkg-config #for cgo to find python includes
apt install software-properties-common #for add-apt-repository


# install latest Go (distro packages are outdated)
cd $HOME
mkdir Downloads
cd Download
wget https://go.dev/dl/go1.19.3.linux-amd64.tar.gz
rm -rf /usr/local/go && tar -C /usr/local -xzf go1.19.3.linux-amd64.tar.gz
ln -s /usr/local/go/bin/go /usr/local/bin/go

# install py 3.7 from ppa (as no distro packages for Py 3.7)
add-apt-repository ppa:deadsnakes/ppa
apt install python3.7 python3.7-dev

# we explicitly need a "python3.pc" (not "python-3.7m.pc" or so)!
cp /usr/lib/x86_64-linux-gnu/pkgconfig/python-3.7m.pc /usr/lib/x86_64-linux-gnu/pkgconfig/python3.pc
# at this point a "pkg-config --list-all" should now show a record exactly named "python3"

# create a Go test program that uses go-python/cpy3:
cd $HOME
mkdir hello
cd hello
go mod init hello
vi main.go

contents of main go:

package main

import "github.com/go-python/cpy3"

func main() {

        defer python3.Py_Finalize()
        python3.Py_Initialize()
        python3.PyRun_SimpleString("print('hello from python in go')")

}

and back in the Ubuntu shell:

#this adds the go-python/cpy3 dep to go.mod and downloads it
go mod tidy

Now run the program:

go run .

Result:

$ root@ub1:~/hello# go run .
hello from python in go

[christian-korneck]

one more note: Instead of installing from the ppa deadsnakes repo you could also build Python 3.7 from source. Here's an example on Debian, it should be very similar on Ubuntu:
#18 (comment)

[Voben]

Quick question, is there an advantage to building Python 3.7 from source rather than using deadsnakes?

[christian-korneck]

technically probably not much difference. (More control and you don’t have to trust a third party package repo).

[Voben]

I'm setting up the docker container as you did, to as closely as possible trace your steps, and with the command cp /usr/lib/x86_64-linux-gnu/pkgconfig/python-3.7m.pc /usr/lib/x86_64-linux-gnu/pkgconfig/python3.pc, I am getting the error cp: cannot stat '/usr/lib/x86_64-linux-gnu/pkgconfig/python3.7m.pc': No such file or directory.

[christian-korneck]

did you install python3.7-dev ?

[Voben]

I found the issue, it was a stupid mistake in the directory that I missed. Continued with the rest of the commands and it worked!

Thank you so much for your help and time, I appreciate it so much!

[christian-korneck]

glad it works :)