Develop Reference

How to change the default gcc compiler for python

I have installed gcc 4.9.2 in my computer (mac osx 10.10, 64 bit), while gcc version 4.2 is shown in Python environment. I don't know why this happened.
I encountered some issues when calling numpy.f2py function, I thought it may be related with the incompatible gcc compiler.
1.Python shell
▶ python
Python 3.5.5 | packaged by conda-forge | (default, Jul 23 2018, 23:45:11)
[GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
2.In terminal
▶ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-apple-darwin14.0.0/4.9.2/lto-wrapper
Target: x86_64-apple-darwin14.0.0
Configured with: ../gcc-4.9-20141029/configure --enable-languages=c++,fortran
Thread model: posix
gcc version 4.9.2 20141029 (prerelease) (GCC)
I thought this problem may be due to that the gcc compiler was installed after python. How to fix this issue, wish for your advice.

I believe that [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)] you get when you start the python interactive shell is the compiler that was used to compile the python interpreter, it has nothing to do with which compiler is used with f2py.

With Python 2.7.15 on a MAC console, virtualenv creates an environment with a different version of Python

Here is an extract of my console in a Mac, as you can see the Python on the console is 2.7.15 but on the environment it becomes 2.7.10 which is an older version... any help on why and how to fix it would be most welcomed!
dhcp-18-57:Wakeup6 **jbs$ python**
**Python 2.7.15** (default, May 1 2018, 16:44:14)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
dhcp-18-57:Wakeup6 jbs$ virtualenv env
New python executable in /Users/jbs/PycharmProjects/Wakeup6/env/bin/python
Installing setuptools, pip, wheel...done.
dhcp-18-57:Wakeup6 jbs$ source env/bin/activate
(env) dhcp-18-57:Wakeup6 **jbs$ python**
**Python 2.7.10** (default, Oct 23 2015, 19:19:21)
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()

If you installed python using Anaconda, then set up a new virtual environment for python 2.7.15 Then following directions in the Anaconda documentation, activate this virtual environment and you should be good to go.
If you didn't use Anaconda, just follow the directions for setting up a virtual environment on a Unix box. Then activate that environment and start working.
You can have multiple versions on your computer. Virtual environments allow you to run the version you want. I have virtual environments for python 2.7.13 and python 3.6.
If this doesn't work please let me know. I'll try to help.

Thank you all for your input. I finally figured out how to solve it. The difficult part was finding out how to locate the place where python 2.7.15 was installed which I'll leave intact in the response since I found it by trial and error looking in other places. I have not found a command that gives you the full path (perhaps an expert can tell us how to go from the laconic which python to the full path).
What I find interesting is that it does not really matter what the console uses when calling virtualenv. What matters is the version of Python used to install virtualenv which is the default version that will be used.
Essentially one has to use the -p option in virtualenv and make sure to find the right location where the desired version of python is.
dhcp-18--41:WakeUp8 jbs$ virtualenv -p /usr/local/Cellar/python#2/2.7.15/bin/python env
Running virtualenv with interpreter /usr/local/Cellar/python#2/2.7.15/bin/python
New python executable in /Users/jbs/PycharmProjects/WakeUp8/env/bin/python2.7
Also creating executable in /Users/jbs/PycharmProjects/WakeUp8/env/bin/python
Installing setuptools, pip, wheel...done.
dhcp-18-189-34-41:WakeUp8 jbs$ python
Python 2.7.15 (default, May 1 2018, 16:44:14)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
dhcp-18--41:WakeUp8 jbs$ pwd
/Users/jbs/PycharmProjects/WakeUp8
dhcp-18-189-34-41:WakeUp8 jbs$ source env/bin/activate
(env) dhcp-18-189-34-41:WakeUp8 jbs$ python
Python 2.7.15 (default, May 1 2018, 16:44:14)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

Python 3.x on python(x,y)?

I'm in the process of setting up a Matlab like environment so I downloaded the latest version of python(x,y) with all the modules that come with it and downloaded python 3.4.1. Does python(x,y) not run the latest version of python? I noticed because the python(x,y) shell doesn't auto calculate mathematical operations into floats which I read is a difference between python 2.x and 3.x. Do I just have to wait for a new release of (x,y) or am I missing something here?

You can make Python 2 behave the same as Python 3 w.r.t. division with the following command;
from __future__ import division
Imports from __future__ should be the in the top of the file. There is probably a way to auto-load this expression (I know it is possible in IPython) but I'm not familiar with python(x,y).

For learning more about Python do the tutorials available via python.org. The latest version of Python3 is recommend.
Since you are in a transition process, take a look at SciPy (http://www.scipy.org) and Sage (http://www.sagemath.org/tour.html). These might be a better fit for the problems you need to solve.
If you do a lot of interactive work at the terminal, take a look at ipython (http://ipython.org).
Regarding the division operator is defaults to integer division in Python2, but will be just normal division in Python3. You can change this by using the -Q flag when starting the interpreter. (Do: python --help) For example:
$ python2.7 -Qnew
Python 2.7.6 (default, Nov 18 2013, 15:12:51)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/2
0.5
>>>
$ python2.7
Python 2.7.6 (default, Nov 18 2013, 15:12:51)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/2
0
>>>
$ python3.4
Python 3.4.1 (default, May 21 2014, 01:39:38)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/2
0.5
>>>

Having the feeling that the original question is epsecially regarding the python(x,y)-distribution and Python 3 my (long) answer to this is:
I used this distribution for many years and like it. But for me it seems that there are no plans to upgrade it to include a python 3 environment.
These days I would recommend the Anaconda distribution/project (https://www.anaconda.com/).
Very similar to the python(x,y)-idea but better maintained and supporting the "latest of everything".

How do I have two different pythons at the same place

Why does the same executable file report two different versions depending on the user/superuser status?
host:tmp$ /usr/bin/python
Python 2.7.5 (default, Mar 9 2014, 22:15:05)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ^D
host:tmp drf$ sudo bash
bash-3.2# /usr/bin/python
Python 2.6.8 (unknown, Mar 9 2014, 22:16:00)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ^D
bash-3.2# ls -la /usr/bin/python
-rwxr-xr-x 2 root wheel 58608 May 15 17:32 /usr/bin/python
bash-3.2# file /usr/bin/python
/usr/bin/python: Mach-O universal binary with 2 architectures
/usr/bin/python (for architecture x86_64): Mach-O 64-bit executable x86_64
/usr/bin/python (for architecture i386): Mach-O executable i386

/usr/bin/python in Mac OSX is a proxy program that selects between different versions of python based on environment variables. The python(1) manual page describes this in some detail. In short, the python interpreter version can be selected by:
defining the VERSIONER_PYTHON_VERSION environment variable to a system python version or
setting the Version property of the com.apple.versioner.python preference using defaults write
In either case, it will only select from the interpreters in /System/Library/Frameworks/Python.framework/Versions.

MacPorts on Snow Leopard: Python install seems to succeed but doesn't install a non-system Python

I've installed Python via MacPorts. According to this question, the files in /opt/local/bin should run the "correct" Python version. However, all those files are symlinks to:
/opt/local/Library/Frameworks/Python.framework/Versions/2.6/bin/
Running them directly from that folder (using no symlinks) runs an Apple build of Python!
Python 2.6.6 (r266:84292, Jan 6 2011, 13:25:25)
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
For comparison, running /usr/bin/python shows a slightly different version:
Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49)
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
They're both Apple builds! How do I install the correct version?

There are no "Apple builds" except the system Python (which is 2.6.1 on Snow Leopard, afai can deduce). The ports install seems to have worked as expected in this case. The reference to Apple is a reference to which GCC that was used.

I think it is running the build you want.
macports install every thing in /opt/local. So /opt/local/bin/python is a macport distribution.
/usr/bin/python is pre-install python distribution that comes with mac os x.
The following line, is an information about gcc build which is used for building macport python distribution.
[GCC 4.2.1 (Apple Inc. build 5664)]
If you have multiple versions of macport python installed, you can select the version using python-select
sudo port install python-select sudo
python-select python26
Also to ensure that you use macport python you could add the following to the PATH
export PATH=/opt/local/bin:$PATH