Add modifier to all objects and change modifier-property

I'm adding a decimate modifier to all objects in my scene and try to change the ratio-property afterwards on demand. I want all my objects to have a maximum of 1000 polygons, so my code for Blender looks as follows:

for obj in bpy.data.objects:
modDec = obj.modifiers.new("dec", type = "DECIMATE")
if (modDec.face_count > 1000):
modDec.ratio = 1000 / modDec.face_count


I actually thought this was pretty simple and straightforward but it won't work. All my objects get the modifier but the ratio remains "1". If I leave out the if-statement and just write something like modDec.ratio = 0.5 it works...the objects get the modifier and the ratio is actual 0.5. I don't get what's wrong with my code. Can someone please help me with this?

[EDIT] I forgot to mention that I work with Blender 2.78b. Sorry for that.

You have two problems.
First you are looping through all the objects, not just the mesh objects. So for example when the loop gets to a camera it can not add the decimate modifier, and the script brakes.

Second issue with the data not being available (modDec.face_count). This is yet another scene update issue.

Here is the changed code.

import bpy

for obj in bpy.data.objects:
if obj.type == "MESH":
modDec = obj.modifiers.new("Decimate", type = "DECIMATE")
bpy.context.scene.update()

while modDec.face_count >= 1000:
modDec.ratio *= 0.5
bpy.context.scene.update()


Notice the if obj.type == "MESH": simple if statement that is only working on the mesh objects.

The real change is the two bpy.context.scene.update() lines. You need two because of the while statement.
Last thing I changed is your if(modDec.face_count > 1000): line. You need to keep changing the ratio while the face_count is bigger then 1000.

The way you were setting the ratio it did not actually get the face count down below 1000.