Prevent files from being downloaded over and over again

cah.py

@@ -5,16 +5,17 @@ __maintainer__ = __author__
 __email__ = "info@klaus-uwe.me"
 __version__ = "0.1"
 
-import requests, bs4, PIL.Image, io
+import requests, bs4, PIL.Image, io, os
 
 def fileDownloader(url):
-    filename = url.split('/')[-1]
-    remote = requests.get(url, stream=True)
-    with open(filename, 'wb') as outfile:
-        for chunk in remote.iter_content(chunk_size=1024): 
-            if chunk:
-                outfile.write(chunk)
-                outfile.flush()
+ filename = url.split('/')[-1]
+ if not os.path.isfile(filename):
+  remote = requests.get(url, stream=True)
+  with open(filename, 'wb') as outfile:
+   for chunk in remote.iter_content(chunk_size=1024): 
+    if chunk:
+     outfile.write(chunk)
+     outfile.flush()
 
 def siteHandler(p = 15):
  suppe = bs4.BeautifulSoup(requests.get('http://explosm.net/comics/' + str(p)).text)