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Assign a unique value to a string s based on its lexicographic order

I want to come up with a function that assigns unique values to a string based on it's lexicographic order. For instance if my function is labelled as get_key(s), the function should take as input a string s and return a unique integer which will allow me to compare two strings based on those unique integers that I get , in O(1) time.
Some code for clarity:
get_key('aaa')
#Returns some integer
get_key('b')
#Returns another integer > output of get_key('aaa') since 'b' > 'aaa'
Any help would be highly appreciated.
Note: Cannot use python built in function id()

It's impossible.
Why? No matter what number you return for a string, I can always find a new string that's in between those two.
You would need an unlimited number of values, because there's an infinite amount of strings.

If I understand your problem clearly, one idea I come to is to convert the input to hex then from hex to int, this I believe would solve the problem, however, I guess it is impossible to solve it in O(1). The solution I provided (and every possible solution in my mind) needs O(n) since you don't have any specification on the input length and the function will operate depending on the length of the input.

Why can't a long integer be converted to an integer when inside a Python list?

I have seen many posts here, which gives ways of removing the trailing L from a list of python long integers.
The most proposed way is
print map(int,list)
However this seems not to work always.
Example---
A=[4198400644L, 3764083286L, 2895448686L, 1158179486, 2316359001L]
print map(int,A)
The above code gives the same result as the input.
I have noticed that the map method doesn't work whenever the number preceding L is large, and only when the numbers are in a list. e.g. Application of int() on 4198400644L does give the number without L, when out of the list.
Why is this occurring and more importantly, how to overcome this?
I think I really need to remove this L, because this is a small part of a program where I need to multiply some integer from this list A, with some integer from a list of non-long integers, and this L is disturbing.I could ofcourse convert the long integers into string,remove the L and convert them back to integer.But is there another way?
I am still using the now outdated Python 2.7.

Python has two different kinds of integers. The int type is used for those that fit into 32 bits, or -0x80000000 to 0x7fffffff. The long type is for anything outside that range, as all your examples are. The difference is marked with the L appended to the number, but only when you use repr(n) as is done automatically when the number is part of a list.
In Python 3 they realized that this difference was arbitrary and unnecessary. Any int can be as large as you want, and long is no longer a type. You won't see repr put the trailing L on any numbers no matter how large, and adding it yourself on a constant is a syntax error.

Constructing an object that's greater than any string

In Python 3, I have a list of strings and would find it useful to be able to append a sentinel that would compare greater than all elements in the list.
Is there a straightforward way to construct such an object?
I can define a class, possibly subclassing str, but it feels like there ought to be a simpler way.
For this to be useful in simplifying my algorithm, I need to do this ahead of time, before I know what the strings contained in the list are going to be (and so it can't be a function of those strings).

This is kind of a naïve answer, but when you're dealing with numbers and need a sentinel value for comparison purposes, it's not uncommon to use the largest (or smallest) number that a specific number type can hold.
Python strings are compared lexicographically, so to create a "max string", you'd simply need to create a long string of the "max char":
# 1114111 is the highest value that chr seems to accept
MAX_CHAR = chr(1114111)
# One million is entirely arbitary here.
# It should ideally be 1 + the length of the longest possible string that you'll compare against
MAX_STRING = MAX_CHAR * int(1e6)
Unless there's weird corner cases that I'm not aware of, MAX_STRING should now be considered greater than any other string (other than itself); providing that it's long enough.

How to change numbers in a number

I'm currently trying to learn python.
Suppose there was a a number n = 12345.
How would one go about changing every digit starting from the first spot and iterating it between (1-9) and every other spot after (0-9).
I'm sadly currently learning python so I apologize for all the syntax error that might follow.
Here's my last few attempts/idea for skeleton of the code.
define the function
turn n into string
start with a for loop that for i in n range(0,9) for i[1]
else range(10)
Basically how does one fix a number while changing the others?
Please don't give solution just hints I enjoy the thinking process.
For example if n =29 the program could check
19,39,49,59,69,79,89,99
and
21,22,23,24,25,26,27,28

Although you are new, the process seems far easy than you think.
You want to make that change to every digit of the number (let's say n=7382). But you cannot iterate over numbers (not even changing specific digits of it as you want to): only over iterables (like lists). A string is an iterable. If you get the way to do a int-str conversion, you could iterate over every number and then print the new number.
But how do you change only the digit you are iterating to? Again, the way is repeating the conversion (saving it into a var before the loop would make great DRY) and getting a substring that gets all numbers except the one you are. There are two ways of doing this:
You search for that specific value and get its index (bad).
You enumerate the loop (good).
Why 2 is good? Because you have the real position of the actual number being change (think that doing an index in 75487 with 7 as the actual one changing would not work well when you get to the last one). Search for a way to iterate over items in a loop to get its actual index.
The easiest way to get a substring in Python is slicing. You slice two times: one to get all numbers before the actual one, and other to get all after it. Then you just join those two str with the actual variable number and you did it.
I hope I didn't put it easy for you, but is hard for a simple task as that.

How to strip letters out of a string and compare values?

I have just learned Python for this project I am working on and I am having trouble comparing two values - I am using the Python xlwt and xlrd libraries and pulling values of cells from the documents. The problem is some of the values are in the format 'NP_000000000', 'IPI00000000.0', and '000000000' so I need to check which format the value is in and then strip the characters and decimal points off if necessary before comparing them.
I have tried using S1[:3] to get the value without alphabet characters, but I get a 'float is not subscriptable' error
Then I tried doing re.sub(r'[^\d.]+, '', S1) but I get a Typerror: expected a string or buffer
I figured since the value of the cell that is being returned via sheet.cell( x, y).value would be a string since it is alphanumeric, but it seems like it must be returned as a float
What is the best way to format these values and then compare them?

You are trying to get the numbers from the strings in the format shown? Like to get 2344 from NP_2344? If yes then use this
float(str(S1)[3:])
to get what you want. You can change float to int.

It sounds like the API you're using is returning different types depending on the content of the cells. You have two options.
You can convert everything to a string and then do what you're currently doing:
s = str(S1)
...
You can check the types of the input and act appropriately:
if isinstance(S1, basestring):
# this is a string, strip off the prefix
elif isinstance(S1, float):
# this is a float, just use it