Q:

How to restrict entire folders from os.path.walk? 

I have a little script that walks through a workspace and adds identical file names of type .asc to a list in a dictionary.   However, I would like to omit all .asc files in certain folders within the workspace (highlighted in blue).   How can I omit .asc files in the folders highlighted in blue, or alternatively, only include .asc files located in the subblock folders?   
 
import os, collections
from collections import defaultdict

workspace = r'C:\my\workspace'

# Get a list of all files in subfolders
rasters = defaultdict(list)
for root, dirs, files in os.walk(workspace):
    for file in files:
        if file.endswith(".asc"):
             rasters[file].append(os.path.join(root, file))

rasters = rasters.values()

A:

for current_dir, dirs, files in os.walk(workspace):
    if current_dir.endswith("IgnoreMe"):
        continue  #skip this folder

    for file in files:
        if file.endswith(".asc"):
             rasters[file].append(os.path.join(root, file))

more specifically for you 
for current_dir, dirs, files in os.walk(workspace):
    if "_" not in os.path.split(current_dir)[-1]: 
        #filter out all ".asc" files
       files = filter(lambda fname:not fname.endswith("asc"),files)
    for file in files:
         #these dont have any more ".asc" files

