Re: byte count unicode string
- From: "John Machin" <sjmachin@xxxxxxxxxxx>
- Date: 20 Sep 2006 02:15:31 -0700
willie wrote:
willie wrote:>> Marc 'BlackJack' Rintsch:
>>
>> >In <mailman.313.1158732191.10491.python-l...@xxxxxxxxxx>, willie
wrote:
>> >> # What's the correct way to get the
>> >> # byte count of a unicode (UTF-8) string?
>> >> # I couldn't find a builtin method
>> >> # and the following is memory inefficient.
>> >> ustr = "example\xC2\x9D".decode('UTF-8')
>> >> num_chars = len(ustr) # 8
>> >> buf = ustr.encode('UTF-8')
>> >> num_bytes = len(buf) # 9
>> >That is the correct way.
>> # Apologies if I'm being dense, but it seems
>> # unusual that I'd have to make a copy of a
>> # unicode string, converting it into a byte
>> # string, before I can determine the size (in bytes)
>> # of the unicode string. Can someone provide the rational
>> # for that or correct my misunderstanding?
>You initially asked "What's the correct way to get the byte countof a
>unicode (UTF-8) string".
>
>It appears you meant "How can I find how many bytes there are in the
>UTF-8 representation of a Unicode string without manifesting the UTF-8
>representation?".
>
>The answer is, "You can't", and the rationale would have to be that
>nobody thought of a use case for counting the length of the UTF-8 form
>but not creating the UTF-8 form. What is your use case?
# Sorry for the confusion. My use case is a web app that
# only deals with UTF-8 strings. I want to prevent silent
# truncation of the data, so I want to validate the number
# of bytes that make up the unicode string before sending
# it to the database to be written.
# For instance, say I have a name column that is varchar(50).
# The 50 is in bytes not characters. So I can't use the length of
# the unicode string to check if it's over the maximum allowed bytes.
What is the database API expecting to get as an arg: a Python unicode
object, or a Python str (8-bit, presumably encoded in utf-8) ?
name = post.input('name') # utf-8 string
You are confusing the hell out of yourself. You say that your web app
deals only with UTF-8 strings. Where do you get "the unicode string"
from??? If name is a utf-8 string, as your comment says, then len(name)
is all you need!!!
*PLEASE* print type(name), repr(name) so that we can see what type it
is!!
If it says the type is str, then it's an 8-bit string, (presumably)
encoded in utf-8.
If it says the type is unicode, then please explain "web app that only
deals with UTF-8 strings" ...
# preferable
if bytes(name) > 50:
send_http_headers()
display_page_begin()
display_error_msg('the name is too long')
display_form(name)
display_page_end()
# If I have a form with many input elements,
# I have to convert each to a byte string
# before i can see how many bytes make up the
# unicode string. That's very memory inefficient
# with large text fields - having to duplicate each
# one to get its size in bytes:
They'd be garbage collected unless you worked very hard to hang on to
them. How large is "large"?
.
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