How to check if a string contains only characters from a given set in python

Question

I have a a user-inputted polynomial and I only want to use it if it only has characters in the string 1234567890^-+x.

How can I check if it does or not without using external packages? I only want to use built-in Python 2.5 functions.

I am writing a program that runs on any Mac without needing external packages.

Answer 1

Here are some odd ;-) ways to do it:

good = set('1234567890^-+x')

if set(input_string) <= good:
    # it's good
else:
    # it's bad

or

if input_string.strip('1234567890^-+x'):
    # it's bad!
else:
    # it's good

Answer 2

Use a regular expression:

import re

if re.match('^[-0-9^+x]*$', text):
    # Valid input

The re module comes with Python 2.5, and is your fastest option.

Demo:

>>> re.match('^[-0-9^+x]*$', '1x2^4-2')
<_sre.SRE_Match object at 0x10f0b6780>

Answer 3

1. You can convert the valid chars to a set, as sets offer faster lookup

2. Then you can use all function like this

   valid_chars = set("1234567890^-+x")  # Converting to a set
   if all(char in valid_chars for char in input_string):
       # Do stuff if input is valid
   

3. We can convert the input string also a set and check if all characters in the inputstring is in the valid list.

   valid_chars = set("1234567890^-+x")  # Converting to a set
   if set(input_string).issubset(valid_chars):
       # Do stuff if input is valid
   

Answer 4

What about just convert both the string into set and checking input_set is subset of good_set as below:

>>> good_set = set('1234567890^-+x')
>>> input_set1 = set('xajfb123')
>>> input_set2 = set('122-32+x')
>>> input_set1.issubset(good_set)
False
>>> input_set2.issubset(good_set)
True
>>>

Answer 5

Yet another way to do it, now using string.translate():

>>> import string
>>> all_chars = string.maketrans('', '')
>>> has_only = lambda s, valid_chars: not s.translate(all_chars, valid_chars)
>>> has_only("abc", "1234567890^-+x.")
False
>>> has_only("x^2", "1234567890^-+x.")
True

It is not the most readable way. It should be one of the fastest if you need it.

Answer 6

whitelist = '1234567890^-+x'

str = 'x^2+2x+1'
min([ch in whitelist for ch in str])
True


str='x**2 + 1' 
min([ch in whitelist for ch in str])
False