Installing ArcGIS for Desktop with existing ActivePython install?

Alright, I figured out what was wrong. If you are using Python 2.7 and want ArcGIS to use your current Python install.

You need to download and install the following libraries before you install ArcGIS 10.1:

NumPy 1.6.1-

http://sourceforge.net/projects/numpy/files/NumPy/1.6.1/numpy-1.6.1-win32-superpack-python2.7.exe/download

Madplotlib 1.1.1-

http://sourceforge.net/projects/matplotlib/files/matplotlib/matplotlib-1.1.1/matplotlib-1.1.1.win32-py2.7.exe/download

If you do this, you will get a clean install.

EDIT:

The 10.1 documentation specifies that ArcGIS requires Python 2.7.x or Numpy 1.6.1 installed on the target computer. However, 10.3 documentation specifies Python 2.7.8 or Numpy 1.7.1, while 10.4 documentation specifies Python 2.7.10 and Numpy 1.9.2. When attempting to install ArcGIS 10.1 with Python 2.7.9+, ArcGIS will not see newer versions and will install a second Python 2.7 directory (as ArcGIS 10.1) within the existing Python27 directory.

I am not sure if this will work, but you can try creating a new text file named Desktop10.1.pth with the following content in your ActivePython installation's Lib\site-packages folder:

C:\Program Files (x86)\ArcGIS\Desktop10.1\bin
C:\Program Files (x86)\ArcGIS\Desktop10.1\arcpy
C:\Program Files (x86)\ArcGIS\Desktop10.1\ArcToolbox\Scripts

(Assuming a standard installation on a 64-bit OS).

There may be more plumbing required which is why I would say to just let ArcGIS do its thing...