vim tab space

1. Your current setting(tab == 6-spaces-width && expand tab)

   :set ts=6 et
   
   #!/usr/bin/env python
   def fun():
   ······print "hello, world"

2. convet 6-spaces to tab

   :set noet
   :ret!
   
   
   #!/usr/bin/env python
   def fun():
   »·····print "hello, world"

3. reset tab to 4-space-width

   :set ts=4
   
   #!/usr/bin/env python
   def fun():
   »···print "hello, world"

4. convet tab to 4 spaces

   :set et
   :ret!
   
   #!/usr/bin/env python
   def fun():
   ····print "hello, world"