Vjylku

NEW UPDATE

QGIS3 can now be installed from osgeo's homebrew tap, which saves mucking about with python symlinks... i.e.

brew tap osgeo/osgeo4mac

brew install qgis

Then type qgis in the terminal to launch, then pin the dock menu icon to simplify launching in future.

Note that the installation console logs will also provide an example for how to symlink qgis to your applications directory.

OLD UPDATE

(This method requires updating symlinks when versions change... i.e. not ideal)

For those who use homebrew, it is probably better to just use a symlink, otherwise all kinds of havoc could emerge. e.g. the official python binary will modify your ~./bash_profile so that python3 and pip3 no longer point to brew...

In my case I've now symlinked the homebrew version (along the lines of the discussion in the answer provided by @Evenfire):

sudo ln -s /usr/local/Cellar/python/3.6.5/Frameworks/Python.framework /Library/Frameworks/Python.framework

QGIS3 then complained that certain python packages were missing, which can be installed with pip3, e.g.

pip3 install owslib PyYaml psycopg2 jinja2 pygments plotly

It might also complain about gdal, in my case I simply used brew:

brew install gdal

ORIGINAL

(This method is not recommended for brew users!)

I've also got python3 installed via brew and am running into the same difficulty.

What seems to work is to install the official python binaries which install to /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.

From trial and error, this has to be done before the 1 Install GDAL Complete.pkg package is installed, which otherwise installs against the wrong python and QGIS 3 will complain about missing python packages.

Those with time to experiment could conceivably try symlinks to get around directly using the brew (or other) python3.

If you have installed Python3 with Homebrew, you can use symlinks to comply with the installer needs (as suggested by @shongololo).

It seems, the installer needs Python3 installed at (thanks @shongololo): /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.

(Install python3 using Homebrew to follow this part)

Find the Cellar directory on your machine. Mine is at: /usr/local/Cellar.
Then find this directory: /usr/local/Cellar/python3/3.x.y_z/Frameworks/Python.framework (where 3.x.y_z is the python3 version you want to use, for me it is 3.6.4_2). This is the directory we want the installer to find. We need to link to this directory from the directory the installer is looking at by making a Symbolic Link.

Using the ln command (ln -s TARGET_FILE_TO_LINK_TO LINK_NAME):
ln -s /usr/local/Cellar/python3/3.x.y_z/Frameworks/Python.framework /Library/Frameworks/Python.framework will do the trick. You certainly need to sudo.

This has worked for me.

(The "Cellar" is a place that all your "kegs" go. Homebrew installs packages to their own directory (in the Cellar) and then symlinks their files into /usr/local/. [from Ryan Kulla's blog])

I followed the link that came with the QGIS installer to https://www.python.org/downloads/ and installed python3 that way. The QGIS installer was then happy.

This seems to be contradicted by comments here:

"Because QGIS links the python library, you must install the python.org Python 3.6. You can't substitute another python 3 installation. Sorry, that's the way it is."

Posted at http://www.kyngchaos.com/blog/2018/20180315_qgis_3_must_use_python.org_python_3. If I knew how to contact William Kyngesbury, I would. He is performing a great service though.

A version using pyenv and brew:

brew install pyenv

env PYTHON_CONFIGURE_OPTS="--enable-framework CC=clang" pyenv install 3.6.5

sudo ln -s /Users/$USER/.pyenv/versions/3.6.5/Python.framework/Versions/3.6 /Library/Frameworks/Python.framework/Versions/3.6

sudo cp -R /Users/$USER/.pyenv/versions/3.6.5/Python.framework/Versions/3.6 /Library/Frameworks/Python.framework/Versions/3.6

sudo cp -r /Users/$USER/.pyenv/versions/3.6.5/lib/python3.6/lib-dynload/ /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/lib-dynload

sudo cp /Users/$USER/.pyenv/versions/3.6.5/lib/python3.6/lib-dynload/_struct.cpython-36m-darwin.so /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/lib-dynload/_struct.cpython-36m-darwin.so

# 

I faced the same issue when installing QGIS on mac.

It was failed, although I tried to softlink it with other python installed by anaconda, etc.

The way I solved the issue: download related python version in python.org and install.

Another version of pyenv and symlinking, using brew just for dependencies.

Recommended location to install pyenv is ~/.pyenv ($HOME/.pyenv), but could be any location.

1. 
   

   clone pyenv repository (maybe adapt ~/.pyenv to /path/to/pyenv)

   
   
   

   git clone https://github.com/pyenv/pyenv.git ~/.pyenv
   
   

   
   


2. 
   

   setup pyenv in ~/.bash_profile (maybe adapt $HOME/.pyenv to /path/to/pyenv, be aware, that the third line must stay the final one of .bash_profile, even after future updates)

   
   
   

   echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.bash_profile
   
   echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.bash_profile
   
   echo -e 'if command -v pyenv 1>/dev/null 2>&1; thenn  eval "$(pyenv init -)"nfi' >> ~/.bash_profile
   
   

   
   


3. 
   

   restart shell

   
   
   

   exec "$SHELL"
   
   

   
   


4. 
   

   install dependencies readline and xz the easy way with brew

   
   
   

   brew install readline xz
   
   

   
   


5. 
   

   install Python 3.6.x containing Python.framework (maybe adapt version number)

   
   
   

   env PYTHON_CONFIGURE_OPTS="--enable-framework" pyenv install 3.6.8
   
   

   
   


6. 
   

   always rehash after adding a new Python version to pyenv

   
   
   

   pyenv rehash
   
   

   
   


7. 
   

   symlink Python.framework

   
   
   

   sudo ln -s $(pyenv root)/versions/3.6.8/Python.framework /Library/Frameworks/Python.framework
   
   

   
   

Now the QGIS 3.6.0 installer from qgis.org should run fine plus macOS is well prepared for multiple Python versions, playing nice with system's Python.