String is ASCII or UTF-8?

Alf P. Steinbach alfps at
Tue Mar 9 18:02:29 CET 2010

* C. Benson Manica:
> Hours of Googling has not helped me resolve a seemingly simple
> question - Given a string s, how can I tell whether it's ascii (and
> thus 1 byte per character) or UTF-8 (and two bytes per character)?
> This is python 2.4.3, so I don't have getsizeof available to me.

Generally, if you need 100% certainty then you can't tell the encoding from a 
sequence of byte values.

However, if you know that it's EITHER ascii or utf-8 then the presence of any 
value above 127 (or, for signed byte values, any negative values), tells you 
that it can't be ascii, hence, must be utf-8. And since utf-8 is an extension of 
ascii nothing is lost by assuming ascii in the other case. So, problem solved.

If the string represents the contents of a file then you may also look for an 
UTF-8 represention of the Unicode BOM (Byte Order Mark) at the beginning. If 
found then it indicates utf-8 for almost-sure and more expensive searching can 
be avoided. It's just three bytes to check.

Cheers & hth.,

- Alf

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