python eval and string indexing
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 开发者_如何学Pythonaccounts for all four cases, I was just wondering if their was a more elegant way of doing this.
Thanks for your answers.
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.
How about:
string = '1234567890'
slice = '5:8'
sliceP = slice.split(':')
string[int(sliceP[0]):int(sliceP[1])]
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.
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