Checking if a vertex belongs to a vertex group in python

What you basically want is something like this:

import bpy ob = bpy.data.objects["Cube"] gi = ob.vertex_groups["Group"].index # get group index for v in ob.data.vertices: for g in v.groups: if g.group == gi: # compare with index in VertexGroupElement v.co[0] = 5 

v.groups is a collection of VertexGroupElements. VertexGroupElement contains group (as index) and weight (as float [0,1]).

find can not find anything, because v.groups does not contain any keys.

There is a quick yet counter-intuitive way to check whether a vertex with a certain index belongs to a group or not. Whenever you call your_object.vertex_groups[GROUP_NAME].weight(i) to check the weight of a vertex, if that vertex is not present in the mentioned group, this method raises a RuntimeError. You can use it. Here's the code:

import bpy GROUP_NAME="Group" obj=bpy.context.active_object data=obj.data verts=data.vertices for vert in verts: i=vert.index try: obj.vertex_groups[GROUP_NAME].weight(i) #if we get past this line, then the vertex is present in the group vert.co.x = 5 + offset # or whatever value you want except RuntimeError: # vertex is not in the group pass 

Basically, you iterate over all the vertices in your mesh, trying to get their weights (but here we don't save them to a variable, because for your particular task you don't need the values of weights). If a vertex is present in your group, the weight() returns its weight value, which is discarded immediately, and then it moves to the next line where it translates the vertex along the x axis. But, if a vertex is not present in the group, it raises RuntimeError exception, thus skipping the vertex translation part. pass means "do nothing", so it simply moves along to the next vertex.