python eval and string indexing

Question

Let's say I have a string

string = '1234567890'

and I want a slice of that string defined by another string

slice = '5:8'

This is easy to do with

>>>string[5:8]
'678'

However the slice is passed in through a file and changes on user input. Is their a way of doing something such as

>>>string[eval(slice)]
'678'

When I do this I get

    5:8
     ^
SyntaxError: invalid syntax

I have a function that accounts for all four cases, I was just wondering if their was a more elegant way of doing this.

Thanks for your answers.

Answer 1

You are getting the syntax error since 5:8 isn't a valid Python statement on its own; eval expects normal Python code, not just fragments.

If you really want to use eval , you can say:

string = '1234567890'
sliceInput = '5:8'
result = eval('string[' + sliceInput + ']')

However this is not at all secure if you're allowing user input. A safer way would be:

string = '1234567890'
sliceInput = '5:8'
sliceParts = sliceInput.split(':')
if len(sliceParts) != 2:
    # Invalid input -- either no ':' or too many
else:
    try:
        start, end = [ int(x) for x in sliceParts ]
    except ValueError:
        # Invalid input, not a number
    else:
        result = string[start : end]

Note that slice() is a built-in Python function, so it isn't considered good practice to use it as a variable name.

Answer 2

How about:

string = '1234567890'
slice = '5:8'
sliceP = slice.split(':')
string[int(sliceP[0]):int(sliceP[1])]

Answer 3

The slice syntax isn't permitted outside of brackets, so it will break if you try to eval it on its own. If you really want to eval input from a file, you can construct the complete call as a string, then eval it:

eval("string[" + slice + "]")

The usual caveats about eval apply: a malicious user can get your program to execute arbitrary code this way, so you might be better off trying to parse out the bounds rather than eval ing them.