Is a string pure ASCII?

is_ascii.py

def is_ascii(x):
    """Checks whether a string (unicode or bytes) is pure ASCII.

    Works on Python 2.6 and above, including Python 3.
    """
    if isinstance(x, bytes):
        x = x.decode('latin-1')    # This always succeeds.
    
    try:
        x.encode('ascii')
    except UnicodeEncodeError:
        return False
    return True