How to check if a string in Python is in ASCII?

I think you are not asking the right question--

A string in python has no property corresponding to 'ascii', utf-8, or any other encoding. The source of your string (whether you read it from a file, input from a keyboard, etc.) may have encoded a unicode string in ascii to produce your string, but that's where you need to go for an answer.

Perhaps the question you can ask is: "Is this string the result of encoding a unicode string in ascii?" -- This you can answer by trying:

try:
    mystring.decode('ascii')
except UnicodeDecodeError:
    print "it was not a ascii-encoded unicode string"
else:
    print "It may have been an ascii-encoded unicode string"

def is_ascii(s):
    return all(ord(c) < 128 for c in s)

In Python 3, we can encode the string as UTF-8, then check whether the length stays the same. If so, then the original string is ASCII.

def isascii(s):
    """Check if the characters in string s are in ASCII, U+0-U+7F."""
    return len(s) == len(s.encode())

To check, pass the test string:

>>> isascii("♥O◘♦♥O◘♦")
False
>>> isascii("Python")
True

Vincent Marchetti has the right idea, but str.decode has been deprecated in Python 3. In Python 3 you can make the same test with str.encode:

try:
    mystring.encode('ascii')
except UnicodeEncodeError:
    pass  # string is not ascii
else:
    pass  # string is ascii

Note the exception you want to catch has also changed from UnicodeDecodeError to UnicodeEncodeError.