Python string.lower() not working?

Question

I am trying to create a case-insensitive user input choice. But what I think should be correct isn't working. This is the relevant snippet of code:

while True:

    search = raw_input("Choose search A or B: ")
    search = search.lower()

    if search != {'A','B'}:
        print "That was not a valid choice."

    else:
        if search == 'A':
            searchAfunction()
        if search == 'B':
            searchBfunction()
        else:
            print "Search again."

I want the user to be able to input 'a' or 'A'. At the moment this is the only working solution I have is this. It doesn't seem very Pythonic?:

while True:

    search = raw_input("Choose search A or B: ")

    if search != {'A','a','B','b'}:
        print "That was not a valid choice."

else:
    if search in {'A','a'}:
        searchAfunction()
    if search in {'B','b'}:
        searchBfunction()
    else:
        print "Search again."

When I include the search = search.lower() I just get stuck in a loop of "Search again". (In the full program this allows the user to choose to search again by A or B after completing a search). Any ideas?

If you use `lower()` your string will be converted to lower-case. You have to compare to lower-case characters then. — Klaus D., May 11 '15 at 14:37
`search = search.lower()` -> `search = search.upper()` and `if search != {'A','B'}:` -> `if search not in ['A','B']:` et voille. — RickyA, May 11 '15 at 14:40
also, you probably mean `search not in {'A','B'}` instead of `search != ['A','B']` — PeterE, May 11 '15 at 14:40
Also, `A` and `B` are UPPER CASE, `a` and `b` are lower case. — PM 2Ring, May 11 '15 at 14:41
@MartijnPieters what would `{'A', 'B'}` be anyways? Is it valid syntax? — RickyA, May 11 '15 at 14:41
`print("A in set" if 'A' in {'A', 'B'} else "A not in set")` -> `A in set` so it could work... — RickyA, May 11 '15 at 14:45
@RickyA: it is a set literal, defining a set with two values. You can efficiently test if another object is a member of the set with `otherobject in setobject`. — Martijn Pieters, May 11 '15 at 14:52

Emile · Answer 1 · 2015-05-11T15:08:54.730

3

two issues:

1) Instead of

if search != {'A','B'}:

... use

if search not in {'A', 'B'}:

The != operator is only for comparison, not for set membership.

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

edited May 11 '15 at 15:08

answered May 11 '15 at 14:41

Emile

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score 1 · Answer 2 · answered May 11 '15 at 14:41

1

You have two problems:

Firstly, you are converting the input to lower case with str.lower, but then trying to compare to upper case characters in the if statements.

Secondly, search != {'A', 'B'} should be search not in {'A', 'B'} instead.

Putting both together, you get if search not in {'a', 'b'}

answered May 11 '15 at 14:41

rlms

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Thank you so much, that makes a lot of sense. Don't know why it has taken me so long pouring over this to realise that .lower() of course needs lower case characters in the if statements! Thanks! – emiden May 11 '15 at 15:06
@emiden Glad to help! If one of these answers solved your problem, it would be good to accept it at some point. – rlms May 11 '15 at 19:03

score 1 · Answer 3 · answered May 11 '15 at 14:44

You'll need to compare against lower case literals, and use the appropriate comment comparator for testing the contents of your interable while True:

    search = raw_input("Choose search A or B: ")
    search = search.lower()

    if not search in {'a','b'}:
         print "That was not a valid choice."

    else:
         if search == 'a':
            searchAfunction()
         if search == 'b':
            searchBfunction()
         else:
            print "Search again."

Thank you so much! I don't know why I didn't realise that .lower() meant I needed to use lower case literals, it is so obvious! — emiden, May 11 '15 at 15:05

score 0 · Answer 4 · answered May 11 '15 at 14:42

You have a few things wrong with your code:

First you change your search variable to lower case, but then do all of your comparisions to upper case letters. Of course those will fail.

Try this instead:

search = search.upper()

Second, your != statement is wrong. Here you are comparing if the input is not equal to {'A', 'B'}.

Instead, you want to compare that the input is part of the list:

if search not in ['A','B']:

Python string.lower() not working?

4 Answers4