Python limiting character input

Question

I'm trying to only allow the letters a, b or c in an input for a .dat file in a Python program (code below), but I'm having difficulty getting the program to do this correctly.

varClass = "class" + input("Which class are you in? [A/B/C]: ").lower() + ".dat"
if not re.match("^[a-c]*$", varClass):
    print("Enter the correct class number")

This is what I have already, but it still continues to run even after an incorrect character has been entered.

varClass already contains "c" and "a" because you preppend "class" :X — Lachezar
– Lachezar, Commented Feb 26, 2015 at 9:54

vks · Accepted Answer · 2015-02-26 09:56:37Z

1

varClass = "class" + input("Which class are you in? [A/B/C]: ").lower() + ".dat"
if not re.match("^class[a-c]\.dat$", varClass):
    print("Enter the correct class number")

Your match will always return false as match matches from beginning and you have class at the beginning.

answered Feb 26, 2015 at 9:56

vks

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3 Comments

Johan Over a year ago

acctually it will always match true due to that "class" begins with c

vks Over a year ago

@Johan if input is c only then it will match.else it will fail

Johan Over a year ago

I'm talking about his code, it will always match true, @vks your code is correct

Padraic Cunningham · Accepted Answer · 2015-02-26 14:00:21Z

0

Take the input first, check it then add class and .dat, you have already added class and .dat before you check the input, adding then checking is doing things a bit backwards:

allowed = {"a","b","c"}
inp =  input("Which class are you in? [A/B/C]: ").lower()
if not allowed.issuperset(inp):
     print("Enter the correct class number")
else:
     var = "class{}.dat".format(inp)

If you have certain specific letter combinations you will have to add those in the set and check:

    inp =  input("Which class are you in? [A/B/C]: ").lower()
    if inp.lower() not in {"ab","ac"}:
         print("Enter the correct class number")
    else:
         var = "class{}.dat".format(inp)

edited Feb 26, 2015 at 14:00

answered Feb 26, 2015 at 9:54

Padraic Cunningham

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7 Comments

Jon Clements Over a year ago

While your not all logic matches the OPs, I don't think the OPs is actually correct....

Jordan Bolton Over a year ago

As the input is being written to a .dat file, how can I implement this as well?

Padraic Cunningham Over a year ago

@JordanBolton, the same way, if you only want abc in the class name then the logic is the same.

Jon Clements Over a year ago

@PadraicCunningham not sure a class of abababababcababa should really be valid... - think it should be that it's either A or B or C... one letter, that's it

Padraic Cunningham Over a year ago

@JonClements, True but I have no idea what the class names can be

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