Is this Boost::Python (Python 3.7) error “__init__() should return None, not 'NoneType'” a linking problem?

Question

Update

I\'m not going to add this as an answer, since I still haven\'t technically solved the problem. But since I\'ve now spent 2.5 days trying to get things to w

Answer 1

I am following a similar example and I adopt the Makefile from here. I have installed python 3.7.4 and boost-python via brew on macOS. To fix the NoneType issue, I follow the procedure below:

1. Check the Python Path
To check the python path, use

which python

If the output does not look like the following one (brew's python installation path)

/usr/local/opt/python/libexec/bin/python

set the PATH variable as

export PATH="/usr/local/opt/python/libexec/bin:$PATH"

Check if the Python path looks like the one above again.

2. Check the Compilation Flag
Below is the adopted Makefile. Note the LIB variable. If the boost-python flag is -lboost_python, change it to -lboost_python37.

CPP = clang++
PYLIBPATH = $(shell python-config --exec-prefix)/lib
# LIB = -L$(PYLIBPATH) $(shell python-config --libs) -lboost_python
LIB = -L$(PYLIBPATH) $(shell python-config --libs) -lboost_python37
OPTS = $(shell python-config --include) -O2

default: hello.so


hello.so: hello.o
    $(CPP) $(LIB) -Wl,-rpath,$(PYLIBPATH) -shared $< -o $@

hello.o: hello.cpp Makefile
    $(CPP) $(OPTS) -c $< -o $@

clean:
    rm -rf *.so *.o

.PHONY: default clean

Recompile the C++ code and run the python script. The NoneType issue should disappear.

Hope this helps.

Note
If you are using anaconda and want to restore the PATH variable after the above changes, try

export PATH="~/anaconda3/bin:$PATH"

Your anaconda's path may be different.

Credit
1. George's comment in How do I use brew installed Python as the default Python?
2. leiyc's comment in ld: library not found for -lboost_python on MacOS

Answer 2

Adding an answer here for those using the Anaconda or Conda-Forge Distribution:

The python interpreter statically links in the libpythonXY library. Which is why it makes the python binary different compared to other distributions.

The fix for the problem reported by OP is to use:

-undefined dynamic_lookup

Instead of:

-lpythonXY

You are creating a Python C/C++ extension, and not embedding the python interpreter. So you shouldn't be linking to the python library. Pybind11 handles this correctly.

See the following for more information:

• https://gitlab.kitware.com/cmake/cmake/issues/18100
• https://github.com/ContinuumIO/anaconda-issues/issues/9078

One a side note, python 3.8 has added an additional flag: --embed and only then it adds -lpythonXY in the output:

$ python3.8-config --libs
-ldl -framework CoreFoundation

$ python3.8-config --libs --embed
-lpython3.8 -ldl -framework CoreFoundation