Toggle between states with a single button The enum function allows a button to toggle between two states. Is there a way to do that in code with a normal single button whose state is momentary?
For example, I am trying to select faces of a model, then deselect with a standard button, not a toggle button.

I've only gotten a theoretical function:

for f in bm.faces: 
    if bm.faces[f.index].select == True:
        bm.faces[f.index].select = False
    else:
        bm.faces[f.index].select = True


This will test the first state, then the second which will make them both true. Is there a way to stop at the first state?
Pick the state to use before the loop.

set_state = bm.faces[0].select
for f in bm.faces:
    f.select = set_state


Note that within your loop f is the same as bm.faces[f.index]

Or if you want to loop through until you find the first selected, use break to stop the loop.

for f in bm.faces:
    print(f.index)
    if f.select:
        break


In blender's UI, pressing a button executes an operator. You can give an operator properties and set different values to be used when clicking each button in the UI. This also means you can have one button and choose which value it uses.

An operator with a boolean property -

class MySelector(bpy.types.Operator):
    bl_idname = 'mesh.my_select'
    bl_label = 'sample selecting'

    state = bpy.props.BoolProperty()

    def execute(self, context):
        obj = context.edit_object
        bm = bmesh.from_edit_mesh(obj.data)
        for f in bm.faces:
            f.select = self.state
        bmesh.update_edit_mesh(obj.data, True)

        return {'FINISHED'}


A panel to show the buttons -

class MySelectPanel(bpy.types.Panel):
    bl_idname = "OBJECT_PT_MySelector"
    bl_label = "Selecting"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'

    def draw(self, context):
        row = self.layout.row()
        row.operator(MySelector.bl_idname, text='Deselect').state = False
        row.operator(MySelector.bl_idname, text='Select').state = True


It is also possible to set multiple properties for one button

    b = row.operator(MySelector.bl_idname, text='Deselect')
    b.state = False
    b.value2 = 'Use this value'