Python3: ImportError: No module named '_ctypes' when using Value from module multiprocessing

I am using Ubuntu and have installed Python 2.7.5 and 3.4.0. In Python 2.7.5 I am able to successfully assign a variable x = Value('i', 2), but not in 3.4.0. I am getting:

Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "/usr/local/lib/python3.4/multiprocessing/context.py", line 132, in Value
      from .sharedctypes import Value
   File "/usr/local/lib/python3.4/multiprocessing/sharedctypes.py", line 10, in <
module>
   import ctypes
   File "/usr/local/lib/python3.4/ctypes/__init__.py", line 7, in <module>
      from _ctypes import Union, Structure, Array
ImportError: No module named '_ctypes'

I just updated to 3.3.2 through installing the source of 3.4.0. It installed in /usr/local/lib/python3.4.

Did I update to Python 3.4 correctly?

One thing I noticed that Python 3.4 is installed in usr/local/lib, while Python 3.3.2 is still installed in usr/lib, so it was not overwritten.

Solution 1:

Installing libffi-dev and re-installing python3.7 fixed the problem for me.

to cleanly build py 3.7 libffi-dev is required or else later stuff will fail

If using RHEL/Fedora:

yum install libffi-devel

or

sudo dnf install libffi-devel

If using Debian/Ubuntu:

sudo apt-get install libffi-dev

Solution 2:

On a fresh Debian image, cloning https://github.com/python/cpython and running:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get install build-essential python-dev python-setuptools python-pip python-smbus
sudo apt-get install libncursesw5-dev libgdbm-dev libc6-dev
sudo apt-get install zlib1g-dev libsqlite3-dev tk-dev
sudo apt-get install libssl-dev openssl
sudo apt-get install libffi-dev

Now execute the configure file cloned above:

./configure
make # alternatively `make -j 4` will utilize 4 threads
sudo make altinstall

Got 3.7 installed and working for me.

SLIGHT UPDATE

Looks like I said I would update this answer with some more explanation and two years later I don't have much to add.

• this SO post explains why certain libraries like python-dev might be necessary.
• this SO post explains why one might use the altinstall as opposed to install argument in the make command.

Aside from that I guess the choice would be to either read through the cpython codebase looking for #include directives that need to be met, but what I usually do is keep trying to install the package and just keep reading through the output installing the required packages until it succeeds.

Reminds me of the story of the Engineer, the Manager and the Programmer whose car rolls down a hill.