Make Script Affect All Objects In Scene

This may be a really stupid question, but it has me stumped.

How do I get my script to affect all objects in a scene and not just the active one? I’m new to Python and am practicing script writing to make semi-useful addons for Blender users. The one I’m making now (below) is very basic. It is a simple material editing setup. It works, just not exactly what I wanted.

Can someone explain how to set it to affect all objects in the scene?

import bpy

"""This script is for converting all materials in a scene to render in a black and white ink style."""

for material in
    #This changes the diffuse color and type. It also affects the color ramp.
    material.diffuse_shader = 'TOON'
    material.diffuse_color[0] = 1
    material.diffuse_color[1] = 1
    material.diffuse_color[2] = 1
    material.diffuse_intensity = 1.0
    material.diffuse_toon_size = 1.7
    material.diffuse_toon_smooth = 0.0
    material.use_diffuse_ramp = True
    material.diffuse_ramp.elements[0].color = (0, 0, 0, 1)
    material.diffuse_ramp.elements[1].color = (1, 1, 1, 1)
    material.diffuse_ramp.elements[1].position = 0.5
    material.diffuse_ramp_input = 'RESULT'
    #This changes the specularity color and type. It also enables Receive Transparency.
    material.specular_intensity = 0.0
    material.specular_shader = 'TOON'
    material.specular_color[0] = 1
    material.specular_color[1] = 1
    material.specular_color[2] = 1
    material.specular_toon_smooth = 0.0
    material.specular_toon_size = 0.0
    material.use_transparent_shadows = True

Thank you to all that reply!

EDIT: Also, I know that this will effect all the materials, but I will be adding on to this script to effect mesh objects as well (Such as adding new materials by default). I am currently writing in additional code to add a UI Panel with buttons.

I found out how to do so through a tutorial by Jonathan Willamson