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

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.

Plato

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

Gusty

I love the set subset approach :-) Care to take a timeit potshot at it? – Pandit

@MartijnPieters, not a chance - there are sooooo many ways to do this I don't want to spend half an hour organizing them all ;-) – Gusty

@user2357112, just because I can never remember what .issuperset() does, exactly, without looking it up :-( But I understand <= at once, and as Martijn says, they're the same thing in the end. .issuperset() is likely slower due to the method lookup expense. – Gusty

issuperset() is simply the inverse of issubset(); the >= to your <=. – Pandit

@MartijnPieters, sure, but with the method spellings I can never remember which one is <= and which >= without looking it up. So I avoid them. <= and >= are obvious to me. – Gusty

@MartijnPieters: issuperset would... wait, apparently it doesn't short-circuit. I thought it would do the test without building a set from the input_string, but when I tried {0}.issuperset(xrange(1000000000)) a few seconds ago, it started eating memory in a way that seems to indicate it turns the input into a set. I guess there's no performance advantage. – Binion

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>

Pandit

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
   

Afford

Sets have an issuperset method for this. – Binion

@Binion Please check my answer now, I have used issubset – Afford

That's not the syntax for turning a string into a set; you want set("1234567890^-+x"). (Why did I see the other thing first?) – Binion

@Binion That's called set comprehension. – Afford

No, it's not. It's a set literal with 1 element. – Binion

@TimPeters Ya, he is correct :) I always get confused with that notation :( – Afford

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
>>>

Cerotype

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.

Mesoderm

You can pass as None as the first argument to translate() instead. – Gusty

Does anyone even have a Python 2.5 anymore to check? LOL ;-) It works in all current Pythons :-) – Gusty

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

Drongo

min() is hardly the best function for this task. – Pandit

Agree that all would be the best choice. Hangover from other languages. – Drongo