case insensitive - Python string.lower() not working? -

i trying create case-insensitive user input choice. think should correct isn't working. relevant snippet of code:

while true:      search = raw_input("choose search or b: ")     search = search.lower()      if search != {'a','b'}:         print "that not valid choice."      else:         if search == 'a':             searchafunction()         if search == 'b':             searchbfunction()         else:             print "search again." 

i want user able input 'a' or 'a'. @ moment working solution have this. doesn't seem pythonic?:

while true:      search = raw_input("choose search or b: ")      if search != {'a','a','b','b'}:         print "that not valid choice."  else:     if search in {'a','a'}:         searchafunction()     if search in {'b','b'}:         searchbfunction()     else:         print "search again." 

when include search = search.lower() stuck in loop of "search again". (in full program allows user choose search again or b after completing search). ideas?

two issues:

1) instead of

if search != {'a','b'}: 

... use

if search not in {'a', 'b'}: 

the != operator comparison, not set membership.

2) use search.upper() instead of search.lower() (or alternatively, use 'a' instead of 'a' in rest of code ...)